Objective—To develop proxies calculated from basal
plasma glucose and insulin concentrations that predict
insulin sensitivity (SI; L·min–1·mU–1) and beta-cell
responsiveness (ie, acute insulin response to glucose
[AIRg]; mU/L·min–1) and to determine reference quintiles
for these and minimal model variables.
Animals—1 laminitic pony and 46 healthy horses.
Procedure—Basal plasma glucose (mg/dL) and insulin
(mU/L) concentrations were determined from blood
samples obtained between 8:00 AM and 9:00 AM.
Minimal model results for 46 horses were compared
by equivalence testing with proxies for screening SI
and pancreatic beta-cell responsiveness in humans
and with 2 new proxies for screening in horses (ie, reciprocal
of the square root of insulin [RISQI] and modified
insulin-to-glucose ratio [MIRG]).
Results—Best predictors of SI and AIRg were RISQI
(r = 0.77) and MIRG (r = 0.75) as follows: SI =
7.93(RISQI) – 1.03 and AIRg = 70.1(MIRG) – 13.8,
where RISQI equals plasma insulin concentration–0.5
and MIRG equals [800 – 0.30(plasma insulin concentration
– 50)2]/(plasma glucose concentration – 30).
Total predictive powers were 78% and 80% for RISQI
and MIRG, respectively. Reference ranges and quintiles
for a population of healthy horses were calculated
Conclusions and Clinical Relevance—Proxies for
screening SI and pancreatic beta-cell responsiveness
in horses from this study compared favorably with
proxies used effectively for humans. Combined use
of RISQI and MIRG will enable differentiation
between compensated and uncompensated insulin
resistance. The sample size of our study allowed for
determination of sound reference range values and
quintiles for healthy horses. (Am J Vet Res
Objective—To evaluate genetic and metabolic predis-positions and nutritional risk factors for development of pasture-associated laminitis in ponies.
Design—Observational cohort study.
Procedures—A previous diagnosis of laminitis was used to differentiate 54 ponies (PL group) from 106 nonlaminitic ponies (NL group). Pedigree analysis was used to determine a mode of inheritance for ponies with a previous diagnosis of laminitis. In early March, ponies were weighed and scored for body condition and basal venous blood samples were obtained. Plasma was analyzed for glucose, insulin, triglycerides, nonesterified fatty acids, and cortisol concentrations. Basal proxies for insulin sensitivity (reciprocal of the square root of insulin [RISQI]) and insulin secretory response (modified insulin-to-glucose ratio [MIRG]) were calculated. Observations were repeated in May, when some ponies had signs of clinical laminitis.
Results—A previous diagnosis of laminitis was consistent with the expected inheritance of a dominant major gene or genes with reduced penetrance. A prelaminitic metabolic profile was defined on the basis of body condition, plasma triglyceride concentration, RISQI, and MIRG. Meeting ≥ 3 of these criteria differentiated PL-from NL-group ponies with a total predictive power of 78%. Determination of prelaminitic metabolic syndrome in March predicted 11 of 13 cases of clinical laminitis observed in May when pasture starch concentration was high.
Conclusions and Clinical Relevance—Prelaminitic metabolic syndrome in apparently healthy ponies is comparable to metabolic syndromes in humans and is the first such set of risk factors to be supported by data in equids. Prelaminitic metabolic syndrome identifies ponies requiring special management, such as avoiding high starch intake that exacerbates insulin resistance.
Objective—To characterize the effects of pregnancy on insulin sensitivity (SI) and glucose dynamics in pasture-maintained mares fed supplemental feeds of differing energy composition.
Animals—Pregnant (n = 22) and nonpregnant (10) healthy Thoroughbred mares.
Procedures—Pregnant and nonpregnant mares underwent frequently sampled intravenous glucose tolerance tests at 2 times (period 1, 25 to 31 weeks of gestation; period 2, 47 weeks of gestation). Following period 1 measurements, mares were provided a high-starch (HS; 39% starch) or high-fat and -fiber (14% fat and 70% fiber) supplemental feed. From a subset of mares (n = 12), blood samples were collected hourly for 24 hours to assess glycemic and insulinemic response to feeding while pastured. The minimal model of glucose and insulin dynamics was used to estimate SI, glucose effectiveness, and acute insulin response to glucose from tolerance testing data.
Results—Pregnant mares during period 1 had a lower SI and glucose effectiveness and higher acute insulin response to glucose than did nonpregnant mares. The SI value decreased in nonpregnant but not pregnant mares from periods 1 to 2. Pregnant mares fed HS feed had a greater glycemic and insulinemic response to feeding than did any other group.
Conclusions and Clinical Relevance—Pregnant mares had slower glucose clearance and greater insulin secretion at 28 weeks of gestation than did nonpregnant mares. Glucose and insulin responses to meal feeding, particularly with HS feed, were greater in pregnant mares, indicating that pregnancy enhanced the postprandial glycemic and insulinemic effects of starch-rich feed supplements.