Deviations from diets presumed to be natural for domestic cats have been suggested to cause chronic diseases in cats.1 Conclusive evidence to support or refute this supposition is difficult to obtain because long-term studies would be impractical and the composition of presumed natural diets of domestic cats is not readily known. With respect to dietary fat, deviation from a supposed ancestral diet is suggested to impact human health.2,3 Substitution of vegetable for animal fat, with a few common exceptions (eg, flaxseed and canola oils), has favored increased consumption of vegetable oils with a high content of n-6 series polyunsaturated fatty acids relative to n-3 series polyunsaturated fatty acids.4,5 Although beneficial when substituted for saturated fat, high dietary content of n-6 polyunsaturated fatty acids has been regarded as potentially detrimental to humans.6–8
Domestic cats typically consume diets made of commodities in the human food supply, such as corn and soy beans and meat from livestock reared on these seed crops, which are high in n-6 polyunsaturated fatty acid content. Consequently, the types and amounts of fatty acids ingested by pet cats probably differ from those of cats prior to domestication. In support of this are reports9 that the fatty acid composition of wild game differs from that of domestic livestock. Wild carnivores appear able to assimilate fatty acids of their wild prey, and for some fatty acids, enrichments in the carnivore predator are greater than those in the prey.
The carcasses of large carnivores, compared with those of their herbivorous prey, contain greater concentrations of DHA, which is a highly unsaturated, long-chain, n-3 polyunsaturated fatty acid.10 Carnivores were once suggested to readily synthesize DHA from the shorter-chain n-3 polyunsaturated fatty acid α-linolenic acid, which is abundant in wild game.11 However, evidence in support of such synthesis is lacking in domestic cats.12 In adult cats fed only vegetable oil as a fat source, α-linolenic acid could be elongated but not desaturated to form longer-chain derivatives. A similar observation regarding elongation and desaturation was made of the n-6 polyunsaturated fatty acid linoleic acid, which is nutritionally essential in cats.13
The activity of Δ6 fatty acid desaturase appears to be low in adult cats and somewhat greater but still low in neonates.14 As a result, the most common polyunsaturated fatty acids of plant sources are not substantive precursors of long-chain polyunsaturated fatty acids of the n-3 series (ie, DHA and EPA) or the n-6 series (ie, arachidonic acid). Although not well synthesized in cats, long-chain polyunsaturated fatty acids are a necessity, particularly in tissues of the nervous system, where DHA is the principal long-chain n-3 polyunsaturated fatty acid.15 Carnivores have more nervous tissue mass than do their prey. Hence, the greater amount of long-chain polyunsaturated fatty acids in carnivores is suggested to reflect accumulation rather than synthesis, which is slow even in species well endowed with Δ6 fatty acid desaturase.16
The extent and impact of variations in dietary fatty acid content on the health of domestic cats are not well understood beyond that which is known about requirements for the n-6 polyunsaturated fatty acids linoleic acid and arachidonic acid. Findings in a few reports15,17,18 suggest that extreme variations in dietary polyunsaturated fatty acid content may adversely affect reproduction and neurologic and retinal development in cats. Other findings in cats and other species indicate that variation in dietary polyunsaturated fatty acid composition may have a considerable effect on immunity, inflammation, antioxidant status, and eventual adiposity.19–21
The objectives of the study reported here were to infer the polyunsaturated fatty acid composition of a presumed natural diet of cats and compare the findings with the polyunsaturated fatty acid composition of diets commonly fed to pet cats. To make this inference, the aim was to measure fatty acid fractions in extracts of adipose tissues collected from feral cats trapped in regions remote from human activity. Such fractions were assumed to reflect dietary proportions of polyunsaturated fatty acids on the basis of validations performed in previous investigations in cats and other species.22,23 The estimated dietary composition of fatty acids of the feral cats was hypothesized to be qualitatively different from that in commercially available feline extruded diets. Specifically, the presumed natural diet was postulated to have a greater abundance of long-chain n-3 polyunsaturated fatty acids among dietary fatty acids and a lower ratio of n-6 to n-3 polyunsaturated fatty acids. Classic observations10,16 regarding bodily fatty acid composition of wild herbivores and carnivores were the basis of our hypothesis. The overall purpose was to provide information useful for formulation of nutritious diets for cats and for further characterization of their essential fatty acid requirements, which have been incompletely elucidated.24
Materials and Methods
Animals—Cadavers of 7 feral adult domestic cats (3 sexually intact males and 4 sexually intact females) were used in the study. The cats had been captured in kill trapsa baited with a dehydrated rabbit polymerb during government animal control efforts in 2 wildlife reserves remote from human residences and agriculture activities in New Zealand (Boundary Stream Mainland Island and the Whanganui River watershed region). After capture, the carcasses were kept frozen (−20°C) until later thawed for adipose tissue sample collection and examination of stomach contents. The trapping, which was not conducted by the investigators, was in accordance with current best practice developed for feral cat monitoring and control by the New Zealand National Possum Control Agencies. The humaneness of the traps was assessed in accordance with guidelines of the National Animal Welfare Advisory Committee.25
Inspections of the cats' stomach contents confirmed that their diet consisted of prey (small rabbits, mice, birds, lizards, skinks, beetles, wetas, spiders, and flies). Immediately after thawing, the adipose samples were sealed in airtight vials and kept frozen until later analysis of fatty acid composition of extracted total lipids, triacylglycerols, and phospholipids.
Diets—Three bags of 7 feline dry (extruded) diets,c each produced by a different manufacturer, were purchased at retail stores between April and July of 2009 in western (Sacramento, Calif), central (Columbia, Mo), and eastern (Alexandria, Va) regions of the United States. Each bag was labeled with a unique code or best-by date. Market survey resultsd were used to identify the brands of the evaluated diets as ones that were commonly purchased for pet cats. The brands represented 68% of the market share of dry diets sold during 2006, where dry diets accounted for 60% of the retail value of feline food products sold. The specific product for each brand that was analyzed was the most commonly sold product in the brand as reported by all but 1 manufacturer during telephone conversations with company representatives.
Samples weighing 200 g each were obtained from each bag and ground to fine particles. Portions of the samples were analyzed for proximate contents at a university laboratory.e Described methods were used to extract lipids from the samples via reflux in ether.26 Fatty acid composition of the lipid fraction was determined through gas chromatography of methyl esters.27
Fatty acid analyses—Samples (approx 1 g) of subcutaneous (inguinal) and intra-abdominal (mesenteric) adipose tissue were obtained from thawed cadavers. One sample was collected from each site. One subsample of each adipose sample (approx 100 mg) was thawed, cut into small pieces, and homogenized for 30 to 60 seconds in 5 mL of ice-cold aqueous buffer (Tris[hydroxymethyl]aminomethane hydrochloridef [50 mmol/L] and Na2EDTA [1 mmol/L]; pH, 7.4), to which internal standards were added (5 mg of triacylglycerols [17:0] and 0.1 mg of phosphatidylcholine [17:0]). The homogenate was diluted to 10 mL with the buffer, and the extraction process was initiated with 40 mL of a mixture of chloroform, methanol, and acetic acid (2:1:0.015 [vol/vol/vol]). The resulting lower phase was removed and dried with nitrogen gas, and the residual total lipid extract was reconstituted in 0.1 to 0.2 mL of a mixture of chloroform and methanol (2:1 [vol/vol]).
Phospholipid and triacylglycerol portions of the reconstituted lipid mixture were obtained via thin-layer chromatography fractionation. Reconstituted aliquots and phospholipid and triacylglycerol standards were applied to silica gel plates.g The plates were developed for 20 minutes in a mixture of hexane, diethyl ether, and glacial acetic acid (80:20:3 [vol/vol/vol]), dried in air, stained with fluorescent dye, and scraped to remove silica gel that was identified to contain phospholipids and triacylglycerols.
Methyl esters of fatty acids in the lipid extracts and diet samples and plate scrapings containing phospholipids and triacylglycerols were generated by heating for 60 minutes at 80°C in 1.7 mL of methanol and 0.1 mL of acetyl chloride. Hexane (2 mL) and water (0.75 mL) were added after cooling. Following mixing, the resulting upper phase was separated and run through a sodium sulfate column and dried with nitrogen gas. The residue was reconstituted in heptane (0.15 or 0.5 mL), and its fatty acid methyl esters were quantified by gas chromatography. Fatty acid fractions were determined to a minimum of 0.1% of the total of fatty acids.
Fatty acids in adipose extracts of total lipid were also quantified by high-performance liquid chromatography to confirm an unexpectedly high abundance of 18:3n–3 among the adipose fatty acids that was indicated in gas chromatographic determinations. For this, 10 μL of extract in a mixture of chloroform and methanol (2:1 [vol/vol]) from 6 adipose samples was heated to 90°C for 25 minutes in 1 mL of potassium hydroxide (0.2 mol/L) in methanol.28 The mixtures were cooled to room temperature (approx 22°C), mixed with 1 mL of water, and acidified with 6 mol of hydrochloric acid/L. Fatty acids released in the saponification process were extracted 3 times with 1 mL of diethyl ether. The ether extracts were pooled, applied to a sodium sulfate column, dried with nitrogen gas, and reconstituted in 1 mL of 2-propanol.
Fluorescent adducts of freed fatty acids were produced and separated via high-performance liquid chromatography as described elsewhere.29 For this, 50 μL of the 2-propanol solution was promptly dried with nitrogen gas. The resulting residue was redissolved in 50 μL of acetonitrile and mixed with 50 μL of 4-bromomethyl-7-methoxycoumarinh in acetonitrile (10 g/L) and 50 μL of a 1.0:0.1:1.0 solution of 18-crown-6 etheri (5.3 g in 1 L of acetonitrile), potassium carbonate (9.6 g in 1 L of water), and acetonitrile. After the mixture was heated at 60°C for 15 minutes and potential particulates were removed by centrifugation (16,100 × g for 10 minutes), 5 μL of the mixture was injected at 0.6 mL/min on a heated (45°C) reverse-phase columnj equilibrated with a mixture of acetonitrile and water (75:25). Fluorescent adducts were detectedk as they were eluted with increasing acetonitrile concentration (a linear increase [75% to 100%] from 15 to 35 minutes and 100% from 35 to 40 minutes).
Dietary fatty acid estimation—Each fatty acid identified in lipid extracts obtained from the diet samples was expressed as a fraction of the total of identified fatty acids. The dietary concentration of each fatty acid was calculated as the product of the fatty acid fraction multiplied by the dietary fat concentration indicated by the ether extract mass of the proximate analysis of the corresponding diet.
The fractions of DHA, EPA, linoleic, and α-linolenic acids in the dietary fat of the feral cats were inferred from fractions of these fatty acids in the total lipid fraction of subcutaneous adipose tissue samples. The fractions were estimated with mathematical transformations, with slope and intercept terms of regression equations derived from observations regarding colony cats fed defined diets.22 Briefly, the proportion of dietary fatty acids each cat had consumed was taken to be equal to the cat's adipose fatty acid proportion minus an intercept term (−0.43, −0.02, 2.89, and 0.38 for DHA, EPA, linoleic, and α-linolenic acids, respectively), with the difference divided by a slope term for the same fatty acids (0.95, 0.43, 0.87, and 0.72, respectively).22
Dietary intake of DHA, EPA, linoleic, and α-linolenic acids by the feral cats was also estimated on an ME basis. For this estimation, it was assumed that the dietary fatty acid proportion was linearly correlated to the subcutaneous adipose fatty acid proportion,22,30 that the amount of dietary linoleic acid was 80% of the amount of dietary n-6 polyunsaturated fatty acids,31 and that dietary fat contributes to 47% of the ME intake when whole prey is consumed. The latter assumption was based on the mean of proximate compositions reported for whole rats, mice, and chickens.32 The ME was calculated by use of standard Atwater factors, with the assumption that fat mass of the feral cats was not excessive (ie, 14% to 18% of body weight).
Statistical analysis—Fatty acid fractions in extracts of adipose and diet samples are expressed as molar fractions of the total fatty acid content. Statistical analyses were conducted with commercially available software.l Values for measured fatty acid fractions were logit-transformed prior to the statistical analyses. The significance of fatty acid proportion differences among the total, phospholipid, and triacylglycerol extracts was determined via general linear ANOVA modeling.m The subcutaneous and intra-abdominal fatty acid values for each lipid extract were treated as repeated measures in the ANOVA. With this model, the finding of a significant time effect indicated a difference in fatty acid fractions between the sites from which adipose samples had been collected. Bonferroni adjustment was used for post hoc analyses when significant differences were identified. Wilcoxon 2-sample testsn were used to determine the significance of differences between fatty acid fractions in the diet samples and those estimated to be in the diets of the cats from which the adipose samples were obtained. Spearman rank correlation (ρ) analysiso was used to determine the significance of linear relationships between fatty acid fractions in total lipid extracts of subcutaneous and intra-abdominal adipose tissue. Values of P ≤ 0.05 were considered significant.
Results
Adipose fatty acids—The fatty acids in subcutaneous and intra-abdominal adipose tissue samples from feral cats that were most abundant in the total, phospholipid, and triacylglycerol fractions were mixtures of saturated fatty acids (16:0 [palmitic acid] and 18:0 [stearic acid]), monounsaturated fatty acid (18:1n–9 [oleic acid]; Table 1), and polyunsaturated fatty acids (18:2n–6 [linoleic] and 18:3n–3 [α-linolenic acid]; Table 2). These fatty acids constituted a mean of 84% to 90% of the total amount of fatty acids. Nine other fatty acids were identified in lesser abundance. These fatty acids comprised between 6.6% and 0.1% of the total amount of fatty acids.
Median (range) relative fractions (mol %) of saturated fatty acids and monounsaturated fatty acids in total, triacylglycerol, and phospholipid fractions of lipid extracts from samples of subcutaneous and intra-abdominal adipose tissue from 7 feral domestic cats.
| Total lipid | Triacylglycerol | Phospholipid | ||||
|---|---|---|---|---|---|---|
| Fatty acid | Subcutaneous | Intra-abdominal | Subcutaneous | Intra-abdominal | Subcutaneous | Intra-abdominal |
| 14:0 | 3.1 (2.2–5.5) | 2.9 (2.1–4.6) | 2.5 (2.3–6.0) | 2.9 (2.3–6.6) | 2.9 (1.2–4.8) | 4.0 (1.6–5.1) |
| 16:0 | 24.0 (23.2–28.2) | 24.5 (20.7–26.0) | 24.0 (23.3–34.2) | 25.1 (17.6–25.9) | 28.0 (25.3–42.1) | 23.0 (13.2–38.4) |
| 18:0 | 14.2 (9.8–17.2) | 14.9 (10.3–19.0) | 14.4 (9.9–16.3) | 16 (10.8–22.8) | 18.5 (14.4–39.4) | 14.2 (10.2–45.9) |
| Total SFA | 38.5 (32.1–44.0) | 37.4 (33.0–43.6) | 35.6 (32.8–41.9) | 39.9 (33.5–45.9) | 46.1 (35.9–57.2) | 42.2 (33.8–52.9) |
| 14:1 | 0.1 (≤ 0.1) | 0.1 (< 0.1–0.2) | 0.1 (0.1–0.8) | 0.1 (< 0.1–0.2) | 0.2 (< 0.1–0.9) | 0.2 (0.1–0.3) |
| 16:1 | 0.9 (0.7–1.8) | 0.9 (0.8–1.7) | 0.9 (0.7–1.5) | 0.9 (0.5–2.1) | 0.7 (0.3–2.2) | 0.9 (0.3–2.1) |
| 18:1n–7 | 1.3 (0.9–1.5) | 1.3 (1.0–1.6) | 1.1 (0.9–1.5) | 1.3 (0.9–1.7) | 1.1 (< 0.1–1.7) | 1.2 (0.4–1.6) |
| 18:1n–9 | 17.2 (14.9–23.3) | 20.1 (14.4–21.2) | 16.7 (15.0–21.7) | 19.5 (14.1–22.6) | 15.4 (0.3–27.3) | 19.2 (< 0.1–24.1) |
| Total MUFA | 25.6 (24.9–29.3) | 25.5 (22.4–27.1) | 25.5 (24.3–35.0) | 26.0 (18.7–27.2) | 29.3 (26.3–43.8) | 24.0 (15.6–43.8) |
| Total PUFA | 35.2 (30.4–43.0) | 35.5 (32.8–42.7) | 38.9 (26.5–42.8) | 36.7 (28.2–40.4) | 25.8 (9.5–37.9) | 37.7 (3.3–45.1) |
| Total SFA:total PUFA | 1.1 (0.7–1.4) | 1.2 (0.9–1.3) | 0.9 (0.8–1.5) | 1.2 (0.8–1.6) | 1.8 (0.9–5.2) | 1.2 (0.8–2.4) |
MUFA = Monounsaturated fatty acids. PUFA = Polyunsaturated fatty acids. SFA = Saturated fatty acids.
Median (range) relative fractions (mol %) of polyunsaturated fatty acids in total, triacylglycerol, and phospholipid fractions of lipid extracts from the adipose tissue samples in Table 1.
| Total lipid | Triacylglycerol | Phospholipid | ||||
|---|---|---|---|---|---|---|
| Fatty acid | Subcutaneous | Intra-abdominal | Subcutaneous | Intra-abdominal | Subcutaneous | Intra-abdominal |
| 18:2n–6 | 19.5 (15.0–28.2) | 20.1 (16.0–28.2) | 20.9 (12.9–27.8) | 19.5 (16.3–27.2) | 13.7 (6.0–25.9) | 20.1 (0.3–29.0) |
| 18:3n–3 | 10.8 (5.4–18.7) | 10.7 (4.5–17.9) | 10.7 (5.9–18.7) | 9.9 (3.4–17.1) | 8.2 (< 0.1–14.5) | 8.0 (< 0.1–18.6) |
| 18:3n–6 | 0.4 (0.3–0.7) | 0.4 (0.2–0.7) | 0.4 (0.3–0.6) | 0.4 (0.3–0.8) | 0.5 (< 0.1–1.0) | 0.4 (0.2–6.1) |
| 20:2n–6 | 0.4 (0.4–0.5) | 0.5 (0.4–0.6) | 0.4 (0.3–0.5) | 0.4 (0.4–0.6) | 0.3 (< 0.1–4.9) | 0.4 (< 0.1–2.0) |
| 20:3n–3 | 0.3 (< 0.1–0.5) | 0.3 (< 0.1–0.5) | 0.4 (< 0.1–0.5) | 0.3 (< 0.1–0.5) | < 0.1 (< 0.1–0.4) | 0.2 (< 0.1–0.5) |
| 20:3n–6 | 0.3 (0.2–0.6) | 0.4 (0.2–0.5) | 0.4 (0.2–0.4) | 0.3 (< 0.1–0.5) | 0.3 (< 0.1–0.7) | 0.4 (< 0.1–0.6) |
| 20:4n–6 | 1.2 (0.9–5.0) | 1.1 (0.8–1.6) | 1.2 (0.8–1.5) | 0.9 (0.7–1.5) | 1.6 (< 0.1–1.9) | 1.3 (< 0.1–3.3) |
| 20:5n–3 | < 0.1 (< 0.1–0.2) | < 0.1 (≤ 0.1) | < 0.1 (< 0.1–0.5) | < 0.1 (< 0.1) | < 0.1 (< 0.1–0.6) | < 0.1 (< 0.1) |
| 22:5n–3 | 2.2 (< 0.1–3.2) | 2.5 (0.9–5.5) | 2.0 (< 0.1–3.2) | 2.3 (< 0.1–4.8) | 0.7 (< 0.1–2.3) | 1.4 (< 0.1–4.0) |
| 22:6n–3 | 0.8 (0.6–1.6) | 0.8 (0.6–1.7) | 1.1 (0.4–2.9) | 0.7 (0.5–1.3) | 0.8 (< 0.1–4.5) | 1.0 (0.8–2.4) |
| Total n-3 | 13.4 (9.4–23.0) | 13.2 (9.8–21.8) | 13.2 (9.9–20.5) | 12.5 (8.0–20.9) | 10.0 (1.0–15.9) | 14.3 (2.4–23.4) |
| Total n-6 | 21.0 (17.1–29.6) | 21.9 (18.1–29.5) | 22.3 (14.8–29.7) | 20.6 (17.6–28.1) | 15.8 (7.3–27.9) | 21.7 (0.9–30.7) |
| Total n-6: total n-3 | 1.6 (0.8–2.2) | 1.7 (0.8–2.5) | 1.3 (0.9–2.3) | 1.7 (0.8–2.5) | 2.1 (1.1–8.7) | 1.5 (0.4–3.1) |
Relative fractions of a few fatty acids varied somewhat between the sample collection sites. Fractions of the polyunsaturated fatty acids linoleic acid and DPA (22:5n–3) in the intra-abdominal adipose samples were significantly (P < 0.05) greater than those in subcutaneous samples by 11% and 46%, respectively, whereas the proportion of the saturated fatty acid palmitic acid in the subcutaneous adipose samples was significantly (P = 0.017) greater than that in the intra-abdominal adipose samples by 11%.
Among the polyunsaturated fatty acids across all lipid fractions, the predominant fatty acid was linoleic acid, which was present at a mean of 14% to 21% of the total amount of fatty acids (Table 2). The next most abundant polyunsaturated fatty acid was α-linolenic acid, which constituted 8% to 11% of the total amount. The n-6 polyunsaturated fatty acids in the total lipid fraction were present in the following order of abundance: linoleic acid, 20:4n–6 (arachidonic acid), and, with approximately equal abundance, 18:3n–6, 20:2n–6, and 20:3n–6. The n-3 polyunsaturated fatty acids in the total lipid fractions in order of abundance were α-linolenic, DPA, DHA, and 20:3n–3. The n-3 polyunsaturated fatty acid EPA constituted < 0.1% of fatty acids in the total lipid fraction. The ratio of n-6 to n-3 fatty acids ranged from means of 1.3 to 2.1 among all lipid fractions.
Considerable between-cat differences were evident in the adipose fatty acid profiles. Across cats, the most abundant fatty acid in total lipid fractions was palmitic (5/7 cats) or linoleic (2/7 cats) acid. These fatty acids together with α-linolenic acid accounted for approximately half of all fatty acids in the total lipid fractions. In 1 cat, the percentage of α-linolenic acid was as high as 19%, marginally exceeding the amount of linoleic acid. Across cats, the proportion of α-linolenic acid increased linearly with decreasing proportion of the sum of the 18-carbon fatty acids, linoleic, oleic, and stearic acids in subcutaneous (ρ = −0.75; P < 0.052) and intra-abdominal (ρ = −0.96; P < 0.001) adipose samples. No significant linear relationships with the proportion of α-linolenic acid in both sample collection sites were identified between single or combined fractions of 4 of the 5 most abundant fatty acids (ie, palmitic, stearic, oleic, and linoleic acids).
The proportion of the n-3 polyunsaturated fatty acid 20:3n–3 in the total lipid extract increased linearly with the α-linolenic acid proportion in the subcutaneous (ρ = 0.82; P = 0.023) and intra-abdominal (ρ = 0.95; P < 0.001) samples of adipose tissue (Figure 1). The fractions of other n-3 polyunsaturated fatty acids identified in the samples were not linearly correlated with the proportion of α-linolenic acid. For the n-6 polyunsaturated fatty acids detected in the total lipid extract, a positive correlation was identified between fractions of linoleic acid and 20:2n–6 in the subcutaneous (ρ = 0.86; P = 0.014) but not intra-abdominal (ρ = 0.61; P = 0.148) adipose samples but not between linoleic acid and 20:3n–6 (γ-linolenic) or arachidonic acid in either sample type.
Relationship between fatty acid fractions of 18:3n–3 and 20:3n–3 in total lipid extracts from samples of subcutaneous (white circles) and intra-abdominal (black circles) adipose tissue from 7 feral domestic cats. The lines are plots of linear functions derived from least squares analyses of the subcutaneous (dashed line; proportion of 20:3n–3 = 0.0314 × [proportion of 18:3n–3] −0.0137; ρ = 0.82; P = 0.023) and intra-abdominal (solid line; proportion of 20:3n–3 = 0.0275 × [proportion of 18:3n–3 + 0.0386]; ρ = 0.95; P < 0.001) observations.
Citation: American Journal of Veterinary Research 74, 4; 10.2460/ajvr.74.4.589
Relationship between fatty acid fractions of 18:3n–3 and 20:3n–3 in total lipid extracts from samples of subcutaneous (white circles) and intra-abdominal (black circles) adipose tissue from 7 feral domestic cats. The lines are plots of linear functions derived from least squares analyses of the subcutaneous (dashed line; proportion of 20:3n–3 = 0.0314 × [proportion of 18:3n–3] −0.0137; ρ = 0.82; P = 0.023) and intra-abdominal (solid line; proportion of 20:3n–3 = 0.0275 × [proportion of 18:3n–3 + 0.0386]; ρ = 0.95; P < 0.001) observations.
Citation: American Journal of Veterinary Research 74, 4; 10.2460/ajvr.74.4.589
Relationship between fatty acid fractions of 18:3n–3 and 20:3n–3 in total lipid extracts from samples of subcutaneous (white circles) and intra-abdominal (black circles) adipose tissue from 7 feral domestic cats. The lines are plots of linear functions derived from least squares analyses of the subcutaneous (dashed line; proportion of 20:3n–3 = 0.0314 × [proportion of 18:3n–3] −0.0137; ρ = 0.82; P = 0.023) and intra-abdominal (solid line; proportion of 20:3n–3 = 0.0275 × [proportion of 18:3n–3 + 0.0386]; ρ = 0.95; P < 0.001) observations.
Citation: American Journal of Veterinary Research 74, 4; 10.2460/ajvr.74.4.589
Diet composition—The proximate contents of the 7 evaluated commercial feline diets were similar on a weight basis, with a few exceptions. Modest between-diet variation was greatest for crude fat and least for crude protein (Table 3). Although energy densities were similar across diets, moderate differences were evident in their macronutrient energy distribution. The amount of energy in crude fat varied by 100%, providing 18% to 37% of ME, as estimated via modified Atwater factors of 8.5, 3.5, and 3.5 kcal/g for fat, protein, and carbohydrate, respectively24 More consistent across diets were the amounts of energy in protein (29% to 36% of ME) and of energy in carbohydrate, as inferred from the nitrogen-free extract content (34% to 48% of ME).
Results of proximate analysis of 7 commercial extruded feline diets.*
| Diet | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Variable | 1 | 2 | 3 | 4 | 5 | 6 | 7 | Mean | Range (%)† |
| Crude protein‡ | |||||||||
| g/kg | 323 | 325 | 323 | 339 | 343 | 357 | 325 | 333 | 10.4 |
| Percentage ME§ | 32.8 | 34.0 | 33.2 | 33.9 | 33.8 | 35.7 | 29.0 | 33.2 | 20.2 |
| Nitrogen-free extract‖ | |||||||||
| g/kg | 422 | 459 | 432 | 382 | 390 | 378 | 385 | 407 | 19.9 |
| Percentage ME§ | 42.8 | 48.0 | 44.4 | 38.2 | 38.5 | 37.8 | 34.4 | 40.6 | 33.6 |
| Crude fat¶ | |||||||||
| g/kg | 99 | 71 | 90 | 115 | 115 | 109 | 169 | 110 | 89.4 |
| Percentage ME§ | 24.4 | 18.0 | 22.4 | 27.9 | 27.7 | 26.5 | 36.6 | 26.2 | 71.1 |
| Ash (g/kg) | 73 | 67 | 66 | 80 | 64 | 70 | 50 | 67 | 44.8 |
| Crude fiber (g/kg) | 14 | 15 | 13 | 15 | 11 | 10 | 7 | 12 | 65.0 |
| Moisture (g/kg) | 70 | 64 | 76 | 71 | 78 | 75 | 65 | 71 | 19.6 |
| ME (MJ/kg) | 14.4 | 14.0 | 14.2 | 14.6 | 14.8 | 14.6 | 16.4 | 14.7 | 16.2 |
Mean of 3 analyses, each of which involved a sample from a bag with a unique code or best-by date.
Calculated as (maximum – minimum)/mean × 100.
Nitrogen content × 6.25.
Estimated via modified Atwater factors −3.5, 8.5, and 3.5 kcal/g of diet for crude protein, crude fat, and nitrogen-free extract, respectively.
Calculated by difference.
Measured via ether extraction.26
Diet fatty acids—The monounsaturated fatty acid oleic acid was the most abundant fatty acid in all diets, and its proportion varied little among them (Table 4). Oleic acid together with palmitic acid accounted for approximately half (50% to 62%) of the fatty acids in each of the diets. The dietary monounsaturated fatty acids, of which oleic acid was the greatest contributor, were of abundance similar to that of the polyunsaturated fatty acids in the adipose samples (Table 2). The n-3 polyunsaturated fatty acids fractions in the diets were significantly (P < 0.01) lower than those in the total lipid extract of subcutaneous and intra-abdominal adipose samples of the feral cats. The ratios of n-6 to n-3 polyunsaturated fatty acids across the diet samples (median, 16.7) were significantly (P < 0.01) greater than the ratios of these fatty acids among total lipid extracts of subcutaneous (mean, 1.6) and intra-abdominal (mean, 1.7) adipose samples. Concentrations of polyunsaturated fatty acids varied substantially across the diets, ranging from 9 to 36 g/kg of diet for linoleic acid, < 0.1 to 0.5 g/kg for arachidonic acid, 0.4 to 4.4 g/kg for α-linolenic acid, < 0.1 to 0.7 g/kg for DPA, and < 0.1 to 0.5 g/kg for DHA.
Relative percentage of brand market share and fatty acids content of the diets in Table 3.*
| Diet | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Variable | 1 | 2 | 3 | 4 | 5 | 6 | 7 | Median | Range (%)† |
| Market share (%) | 5.8 | 5.0 | 12.3 | 4.2 | 15.9 | 11.9 | 12.3 | — | — |
| Fatty acid (mol %) | |||||||||
| 14:0 | 2.5 | 1.2 | 2.5 | 0.7 | 0.8 | 2.6 | 1.4 | 1.4 | 134 |
| 16:0 | 22.4 | 21.5 | 22.0 | 18.0 | 23.2 | 22.4 | 24.8 | 22.4 | 30.3 |
| 18:0 | 16.4 | 8.9 | 12.2 | 5.3 | 4.9 | 13.5 | 10.9 | 10.9 | 105 |
| Total SFA | 37.3 | 38.6 | 29.1 | 24.5 | 36.8 | 31.7 | 41.4 | 36.8 | 46.0 |
| 14:1 | 0.4 | 0.2 | 0.2 | 0.1 | 0.2 | 0.4 | < 0.1 | 0.2 | 178 |
| 16:1 | 2.3 | 4.5 | 2.5 | 4.0 | 6.2 | 2.7 | 2.9 | 2.9 | 133 |
| 18:1n–7 | 1.4 | 1.6 | 1.4 | 1.6 | 1.9 | 1.5 | 2.4 | 1.6 | 66.0 |
| 18:1n–9 | 34.9 | 36.1 | 36.3 | 31.9 | 36.6 | 36.5 | 36.7 | 36.3 | 13.1 |
| Total MUFA | 42.8 | 41.3 | 45.1 | 38.0 | 40.6 | 42.7 | 39.3 | 41.3 | 17.4 |
| 18:2n–6 | 9.1 | 20.1 | 10.3 | 31.1 | 20.3 | 7.9 | 18.0 | 18.0 | 129 |
| 18:3n–6 | < 0.1 | 0.1 | < 0.1 | < 0.1 | 0.2 | 0.1 | < 0.1 | < 0.1 | 433 |
| 18:3n–3 | 0.4 | 0.9 | 0.5 | 3.8 | 0.9 | 0.5 | 0.7 | 0.7 | 483 |
| 20:2n–6 | < 0.1 | 0.1 | < 0.1 | < 0.1 | 0.2 | < 0.1 | 0.6 | < 0.1 | 1,260 |
| 20:3n–6 | < 0.1 | < 0.1 | < 0.1 | < 0.1 | 0.2 | < 0.1 | 0.1 | < 0.1 | 359 |
| 20:4n–6 | < 0.1 | 0.3 | 0.1 | 0.2 | 0.5 | 0.2 | 0.3 | 0.2 | 199 |
| 20:5n–3 | < 0.1 | < 0.1 | < 0.1 | 0.6 | 0.4 | 0.2 | < 0.1 | < 0.1 | 1,290 |
| 22:6n–3 | < 0.1 | < 0.1 | < 0.1 | 0.5 | 0.4 | 0.2 | < 0.1 | < 0.1 | 1,830 |
| Total PUFA | 19.9 | 9.1 | 23.2 | 36.3 | 11.1 | 21.5 | 9.6 | 19.9 | 137 |
| Total SFA:total PUFA | 1.9 | 4.3 | 1.3 | 0.7 | 3.3 | 1.5 | 4.3 | 1.9 | 196 |
| Total n-3 | 0.4 | 0.9 | 0.6 | 4.9 | 1.8 | 0.8 | 0.7 | 0.8 | 529 |
| Total n-6 | 9.1 | 20.6 | 10.5 | 31.5 | 21.4 | 8.2 | 19.2 | 19.2 | 121 |
| Total n-6: Total n-3 | 21.2 | 22.9 | 18.5 | 6.5 | 11.9 | 9.8 | 26.5 | 18.5 | 108 |
— = Not applicable.
See Tables 1 and 3 for remainder of key.
Significantly (P < 0.01) greater fractions of α-linolenic acid and DHA were estimated to be among the fatty acids of the hypothetically consumed, natural feral cat diets, compared with fractions in the commercial extruded diets evaluated (Figure 2). The inferred linoleic acid proportion in diets of the feral cats did not appear to differ significantly from that in the commercial diets. The EPA proportion consumed by the feral cats could not be estimated because EPA was detectable in only 1 of the 7 subcutaneous and none of the intra-abdominal samples of adipose tissue obtained from the feral cats.
Mean ± SD percentages of selected fatty acids measured in fat extracted from 7 commercially available dry-type (extruded) feline diets and fractions estimated to be in the fat of hypothetical diets consumed by feral domestic cats (7). Amounts of various fatty acids in the hypothetical diet were inferred from values in samples of subcutaneous and intra-abdominal adipose tissue obtained from the feral cats with correction through linear regression coefficients correlating adipose with dietary fatty acid fractions.22 *Indicated value differs significantly (P < 0.05) between the commercial and hypothetical diets. Note that for 20:5n3 and 22:6n3, values displayed are actual percentage multiplied by 10.
Citation: American Journal of Veterinary Research 74, 4; 10.2460/ajvr.74.4.589
Mean ± SD percentages of selected fatty acids measured in fat extracted from 7 commercially available dry-type (extruded) feline diets and fractions estimated to be in the fat of hypothetical diets consumed by feral domestic cats (7). Amounts of various fatty acids in the hypothetical diet were inferred from values in samples of subcutaneous and intra-abdominal adipose tissue obtained from the feral cats with correction through linear regression coefficients correlating adipose with dietary fatty acid fractions.22 *Indicated value differs significantly (P < 0.05) between the commercial and hypothetical diets. Note that for 20:5n3 and 22:6n3, values displayed are actual percentage multiplied by 10.
Citation: American Journal of Veterinary Research 74, 4; 10.2460/ajvr.74.4.589
Mean ± SD percentages of selected fatty acids measured in fat extracted from 7 commercially available dry-type (extruded) feline diets and fractions estimated to be in the fat of hypothetical diets consumed by feral domestic cats (7). Amounts of various fatty acids in the hypothetical diet were inferred from values in samples of subcutaneous and intra-abdominal adipose tissue obtained from the feral cats with correction through linear regression coefficients correlating adipose with dietary fatty acid fractions.22 *Indicated value differs significantly (P < 0.05) between the commercial and hypothetical diets. Note that for 20:5n3 and 22:6n3, values displayed are actual percentage multiplied by 10.
Citation: American Journal of Veterinary Research 74, 4; 10.2460/ajvr.74.4.589
Discussion
The present study had 2 major objectives: to estimate amounts of nutritionally essential polyunsaturated fatty acids in presumed natural diets of feral cats and to compare these amounts with those determined in diets commonly purchased for cats in the United States. Polyunsaturated fatty acids intake was estimated by use of a validated method involving measurement of polyunsaturated fatty acids fractions in adipose tissue. In a study22 involving nearly weaned kittens, diets with varying enrichments of 4 polyunsaturated fatty acids (linoleic acid, α-linolenic acid, EPA, and DHA) were fed for 5 months. As reported in other species,23 the investigators in that study22 found that fractions of each of these fatty acids in subcutaneous adipose tissue were significantly linearly correlated with dietary polyunsaturated fatty acids fractions. With respect to linoleic acid, which is nutritionally essential to cats,13 another study29 revealed a positive linear correlation in linoleic acid fractions between subcutaneous adipose tissue and diet. The same study found that the mean ± SD linoleic acid fraction of adipose fatty acids in a large number of European cats (n = 154) given commercial diets was 16 ± 3%, which is within the range of linoleic acid fractions found in the adipose samples of the feral cats of the present study (Table 2). Hence, it would appear that the linoleic acid fraction of fatty acids in diets of the feral cats varies widely (15% to 28%) but coincides with fractions in many commercial feline diets used in Europe and in the United States (9% to 31%; Table 4).
On an ME basis, dietary linoleic acid concentrations estimated for the feral cats (8% to 18% of ME) were greater than linoleic acid concentrations calculated for the evaluated commercial diets (2.1% to 6.7% of ME), with the exception of 1 diet,g which contained 8.7% of ME as linoleic acid (Tables 3 and 4). For some of the feral cats, the estimated dietary linoleic acid concentrations were greater than a dietary safe upper limit recommended for health maintenance of cats (approx 12% of ME).24 The health impact of increasing linoleic acid in diets formulated for pet cats is not clear. Requirements and safe upper limits for linoleic and α-linolenic acids in other species vary with relative abundances of these fatty acids. The safe upper limit for dietary linoleic acid in cats might increase with increasing α-linolenic acid proportion. In cats, plasma and liver accumulation of the fatty acid 20:2n–6, which is an elongation product of linoleic acid, is an inferred indicator of excess dietary linoleic acid, although 20:2n–6 is not known to have adverse effects.24 In the adipose tissue of the feral cats of the present study, the proportion of 20:2n–6 did increase linearly with that of linoleic acid, and the highest abundance of 20:2n–6 was only 0.6%.
High dietary linoleic intake has been cautioned against because of suggested proinflammatory and neoplastic potential.33,34 Limits of 5% to 10% of dietary ME as n-6 polyunsaturated fatty acids are suggested for humans,35 in which linoleic acid is the most abundant of the n-6 polyunsaturated fatty acids in diets.5 However, the hypothesis that ingestion of a high amount of linoleic acid has unhealthy consequences is not consistently supported. Indeed, anti-inflammatory effects have been associated with linoleic acid and its derivative arachidonic acid.20 Linoleic and arachidonic acids are not produced or poorly synthesized in cats,12,14 but they are plentiful in lean meats, such as liver, that cats may eat.16 As for whether the amount of linoleic acid influences the risk of cancer development, no association has been detected between the amount of ingested linoleic acid and mammary gland cancer in cats.29
A particularly noteworthy finding of the present study is the high inferred intake of n-3 polyunsaturated fatty acids by the feral cats. A high percentage of α-linolenic acid in adipose tissue appeared principally responsible for this finding. Docosahexaenoic acid contributed to a lesser extent, whereas EPA was detectable in only a few adipose samples. Although EPA does not concentrate well in adipose tissue,36 the EPA observation suggests that EPA was not abundant in diets of the feral cats. Given that the proportion of EPA in adipose tissue and diets of cats is linearly correlated,22 it would appear that EPA constituted < 0.8% of fatty acids in the feral cat diets.
The fatty acids in the presumptive natural diets of the feral cats were estimated to be 7% to 25% α-linolenic acid and 1.1% to 2.1% DHA. These fractions, with a few exceptions, were strikingly higher than those found among the analyzed commercial diets. In the commercial feline dry-type diets evaluated in other studies,37,38 the proportion of n-3 polyunsaturated fatty acids in adipose tissue and commercial diets relative to the proportion in a presumptive natural diet is also low (0% to 3% α-linolenic acid and 0% to 1% DHA). After the described method for estimation of linoleic acid intake by the feral cats was applied, intakes of α-linolenic acid by the cats would appear to have been between 3.3% and 12% of dietary ME. Relative to intake of α-linolenic acid, intake of DHA would appear to have been much lower (between 0.5% and 1.0% of dietary ME), and intake of EPA would appear to have been even lower (< 0.4% of dietary ME). With 1 exception, intake of α-linolenic acid and DHA by the feral cats would be many times as great as the intake that might occur with the evaluated commercial diets, where α-linolenic acid and DHA were calculated to contribute between 0.4% and 3.8% and ≤ 0.05% of dietary ME, respectively.
As the estimated fatty acid intake among the feral cats indicated, ratios of dietary n-6 to n-3 polyunsaturated fatty acids appeared to be low, perhaps as low as 1:1 to 2:1, presuming that the ratios were similar to the ratios of n-6 to n-3 polyunsaturated fatty acids identified in total lipid extracts of adipose tissue. Such ratios would be quite low, compared with the ratios of 6:1 to 27:1 found in the analyzed commercial diets. They would also be lower than the ratio of 5:1 to 10:1 recently investigated for its beneficial effects on inflammation and immune function in cats.19,39 The ratios of n-6 to n-3 polyunsaturated fatty acids in feral cat diets would mostly reflect extraordinarily high dietary content of n-3 polyunsaturated fatty acids, principally α-linolenic acid. Consumption of a diet high in α-linolenic acid content in a habitat remote from human contact is consistent with classic findings.40 Food selection patterns by wild herbivores, compared with domestic herbivores, include more leaf than seed material. Consequently, wild herbivores consume greater amounts of α-linolenic acid than do domestic herbivores because of a higher proportion of this fatty acid in leaf lipids, compared with oils in seeds of livestock feeds. Because α-linolenic acid is integral in structural and storage lipids of tissues, carnivores that consume wild game are predictably more replete in α-linolenic acid than carnivores fed meat from livestock16
The high proportion of α-linolenic acid in adipose tissue of the feral cats was positively correlated with the proportion of the longer-chain n-3 polyunsaturated fatty acid 20:3n–3 but not with DHA. These findings were consistent with cats having substantial activities of fatty acid elongase but not Δ-6 fatty acid desaturase, which are metabolic attributes that would appear common to carnivores.41,42 The inferred high DHA intake by the feral cats probably reflected the presence of DHA in prey consumed, particularly in the retinal, brain, and other nervous system tissue. Because of their geographic locations, the cats were unlikely to have had access to cold-water marine animals, which are typically abundant in DHA and other long-chain n-3 polyunsaturated fatty acids.
The proportion of DPA was about twice as great as that of DHA in the adipose tissue of the feral cats. The predominant long-chain n-3 polyunsaturated fatty acid in meats is DPA, rather than DHA, which predominates in fish.43 Hence, the feral cats probably ate more meat than fish, and most of the DHA in the cats was probably synthesized from α-linolenic acid in terrestrial prey possessing substantial Δ-6 fatty acid desaturase activity. The DHA may have been more abundant in the feral cats than in their prey that produced it. Among dietary fatty acids, DHA appears to be more concentrated in the tissues of carnivores than in tissues of their herbivorous prey.10,16
Although α-linolenic acid and DHA appeared abundant in diets of the feral cats, α-linolenic acid does not appear to be an essential fatty acid in cats and a nutritionally required amount of DHA has not been clearly established.24 The Association of American Feed Control Officials recommendations do not compel pet food manufacturers to include these fatty acids in the manufacture of commercial feline diets. Nonetheless, a high dietary amount of α-linolenic acid may beneficially modulate dermal hypersensitivity reactions in cats and bone metabolism in disease conditions in humans and animals.37,44 In overweight cats, a high dietary proportion of long-chain n-3 polyunsaturated fatty acids proportion reportedly improves glucose tolerance.45 In such cats, an increase in the fatty acid proportion of short-chain n-3 polyunsaturated fatty acids (ie, α-linolenic acid) may improve rate of body weight loss with energy restriction. To a lesser extent, α-linolenic acid and linoleic acid are more readily oxidized than stored, compared with saturated and monounsaturated fatty acids.46
Substantial adipose tissue reserves of n-3 as well as n-6 polyunsaturated fatty acids may be beneficial to overweight cats during periods of lower food intake, when risk of developing hepatic lipidosis increases. Liver lipids in cats with hepatic lipidosis appear to contain less polyunsaturated fatty acids, compared with healthy cats.47 In the feral cats of the present study, the n-3 polyunsaturated fatty acids fractions in intra-abdominal adipose (which were similar to those of subcutaneous adipose) appeared greater than those that have been reported for cats with hepatic lipidosis.47 Accumulation of lipids in the liver is suggested to reflect a dietary deficiency of n-3 polyunsaturated fatty acids in some but not all rodents in which hepatic steatosis is induced.27,48
Limitations of the present study were that the ingestion of polyunsaturated fatty acids by the feral domestic cats were not directly measured. Fatty acid fractions in adipose tissues might be influenced by amount of food consumed, maintenance energy needs, and body condition. Also, a small number of cats were used in the study, which did not include a control group. Furthermore, inferences about natural diets of cats were made on the basis of data from cats in only 2 locations in 1 geographic region, and genetic composition relative to that in the pet cat population was not characterized. Nevertheless, the study findings importantly indicate that the fatty acid fractions present in common commercial diets may deviate substantially from those that cats might ingest through consumption of prey instead. The findings should provide justification for investigation of health outcomes of wild carnivores, the diets of which contain a high amount of polyunsaturated fatty acids containing a high proportion of n-3 polyunsaturated fatty acids, particularly α-linolenic acid.9,49 In contrast, the evaluated commercial diets were low in α-linolenic acid content and, in general, long-chain polyunsaturated fatty acids. One evaluated diet appeared devoid of any long-chain polyunsaturated fatty acids.
The findings of the present study supported the hypothesis that fractions of fatty acids in diets of cats freely living in unpopulated areas substantially differ from those of cats fed commonly purchased extruded feline diets. The feral cats' diets were inferred to contain greater fractions of 18-C and long-chain n-3 polyunsaturated fatty acids and a lower ratio of n-6 to n-3 polyunsaturated fatty acids than the commercially produced diets contained. The health implications of this difference are not clear but warrant additional investigation because dietary polyunsaturated fatty acids content is known to modulate many physiologic processes. The optimal fatty acid content may differ between pet and feral cats because of differences in genetic constitution, traits determining survival, or environmental pressures. Nonetheless, further characterization of tolerated ranges of dietary fatty acid fractions for cats is warranted and may lead to a clearer understanding of nutritional requirements for optimal health maintenance.
ABBREVIATIONS
| DHA | Docosahexaenoic acid |
| DPA | Docosapentaenoic acid |
| EPA | Eicosapentaenoic acid |
| ME | Metabolizable energy |
Super X220, Belisle Traps, Blainville, QC, Canada.
Connovation Erayz, Connovation Ltd, Manukau, New Zealand.
Friskies Seafood Sensations, Nestlé Purina PetCare, St Louis, Mo; Special Kitty Gourmet, Wal-Mart Stores, Bentonville, Ark; Meow Mix Original Choice, Del Monte Pet Products, Pittsburgh, Pa; Nutro Natural Choice Complete Care Adult, Nutro Products, Industry, Calif; Iams ProActive Health Original, Iams Co, Cincinnati, Ohio; Purina Cat Chow Complete Formula, Nestlé Purina PetCare, St Louis, Mo; Science Diet Optimal Care, Hill's Pet Nutrition, Topeka, Kan.
Pet Food and Pet Care Products: Euromonitor from trade sources/national statistics, Euromonitor International, Chicago, Ill.
University of Missouri Agricultural Experiment Station Chemical Laboratory, Columbia, Mo.
Trizma HCl, Sigma-Aldrich, St Louis, Mo.
Partisil HPK, 60 Å, 200 μm, 10 × 10 cm, Whatman International, Maidstone, Kent, England.
Santa Cruz Biotechnology, Santa Cruz, Calif.
Sigma-Aldrich, St Louis, Mo.
Ultrasphere ODS, 5 μm, 2.0 × 250 mm, Beckman, San Ramon, Calif.
RF-535, 398-nm emission, 325-nm excitation, Shimadzu, Kyoto, Japan.
SAS, version 9.1.3, SAS Institute Inc, Cary, NC.
PROC GLM, SAS, version 9.1.3, SAS Institute Inc, Cary, NC.
PROC NPAR1WAY, SAS, version 9.1.3, SAS Institute Inc, Cary, NC.
PROC CORR, SAS, version 9.1.3, SAS Institute Inc, Cary, NC.
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