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. 2025 Apr 12;39(3):e70089. doi: 10.1111/jvim.70089

The Seasonality of Serum Insulin Concentrations in Equids and the Association With Breed, Age, and Sex

Ana Lopes 1,✉, Laura Huber 2, Andy E Durham 1
PMCID: PMC11992590  PMID: 40219807

ABSTRACT

Background

Current laboratory reference values for serum insulin concentrations do not account for seasonal fluctuations and differences associated with breed, sex, and age.

Hypothesis

We hypothesized that serum insulin concentrations would differ with the season, breed, sex, and age.

Animals

Laboratory records from 21 236 cases.

Methods

Cases were included where basal serum insulin concentration (INS) or serum insulin concentration after administration of Karo light syrup (INSpk) was measured, along with plasma glucose, breed, age, and sex. Multivariate analysis was used to investigate a seasonal effect on serum insulin concentrations, alongside the effect of breed, age, and sex.

Results

Basal serum insulin concentration in winter was significantly higher than in the other seasons (p < 0.001). Serum insulin concentration following administration of Karo light syrup in winter was significantly higher than in summer and fall (p < 0.001). The breed effect was assessed in the 9 most prevalent breeds and the donkey. Shetland ponies had significantly higher INS and INSpk than all breeds except Welsh ponies(p < 0.01). Welsh ponies had significantly higher INS than all other breeds except Shetland ponies (p < 0.01). Welsh ponies had significantly higher INSpk than all breeds (all p < 0.001), except Arabians, New Forest ponies, and Shetland ponies. Females had significantly higher INS than males (p < 0.001) and there was a positive and significant association between age and INS (est = 0.02; SE = 0.002, p < 0.001).

Conclusions

Insulin concentrations are influenced by season, breed, age, and sex. This information is essential for better understanding and management of insulin dysregulation.

Keywords: endocrine, equine, laboratory samples, season

Abbreviations

GLU

glucose concentration

ID

insulin dysregulation

INS

basal serum insulin concentration

INSpk

serum insulin concentration following administration of Karo light syrup

PPID

pituitary pars intermedia dysfunction

1. Introduction

Insulin dysregulation (ID) is a consistent feature of equine metabolic syndrome (EMS) and is also a common finding in horses with pituitary pars intermedia dysfunction (PPID) [1]. Hyperinsulinemia induces laminitis in horses [2, 3] and laboratory testing of blood insulin concentrations is the basis of diagnosis, monitoring, and prevention of ID and hyperinsulinemia‐associated laminitis. Interpretation of insulin values of equids generally uses diagnostic cutoffs that are the same throughout the year [4]. However, many seasonal variables including quantity and quality of diet, temperature, photoperiod, and inherent physiologic variability might influence insulin concentrations.

Previous studies evaluating the seasonality of serum insulin have included relatively small numbers of animals and have found quite variable results without clear consensus [5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]. Furthermore, there are breed‐related differences in insulin dynamics [16], which are not related to seasonality. Sex affects insulin sensitivity in humans [17, 18]. Similarly, female donkeys may be more insulin dysregulated than males [19] as is the case in other equids [20]. Stallions are 8 times less likely than mares to develop hyperinsulinaemia‐associated laminitis [21].

This study was designed to examine a large dataset, comprising basal serum insulin concentrations and serum insulin response to oral sugar challenge, in samples submitted for investigation and monitoring of ID in equids from practices throughout the United Kingdom. The primary hypothesis was that serum insulin concentrations would differ depending upon the time of year, as well as breed, sex, and age.

2. Materials and Methods

This study involved a retrospective analysis of laboratory records from the Liphook Equine Hospital between January 2012 and September 2023, all subject to owner consent for the use of such data. The available data from 57 701 submissions was collected from the laboratory database. These submitted requests were for analysis of resting serum insulin concentration (INS) and/or serum insulin concentration measured once between 60 and 90 min after oral administration of 0.45 mL/kg Karo Light Syrup (INSpk). Values of INSpk were only available from 2017 until 2023, as the Karo Light Syrup test was not performed in our laboratory prior to this date. Cases were included where INS or INSpk, date of sample collection, age, breed, sex, and serum glucose concentration (GLU) were available. The seasons were defined as winter (December, January, February), spring (March, April, May), summer (June, July, August) and fall (September, October, November). To assess breed effect, animals were divided into 11 groups, including the 9 most prevalent breeds or types: Arabians (breed 1), Cobs (breed 2), Connemara ponies (breed 3), Irish Sport Horses (breed 5), New Forest ponies (breed 6), Shetland ponies (breed 7), Thoroughbreds (breed 8), Warmbloods (breed 9), and Welsh ponies (of all sections A to D; breed 10). Data available for donkeys was also included (breed 4) and data submitted for horses of other breeds was grouped into an extra group (breed 11). Geldings and stallions were combined in the same group (males) because the forms for sample submission to the laboratory did not always reliably distinguish between the two.

2.1. Sample Analysis

Blood samples were collected into vacutainer tubes (plain for serum insulin and sodium fluoride/potassium oxalate for plasma glucose) and sent to the Liphook Equine Hospital Laboratory. Samples were received and analyzed within 1 day of collection. Serum insulin concentrations were measured on a Siemens Immulite 2000 xpi chemiluminescent analyzer, which has been previously validated [22] and serum glucose concentrations were run on a Beckman Coulter AU480, validated in our laboratory.

2.2. Statistical Analysis

Statistical analysis was performed using RStudio (version 2023.12.1 + 402). Breed and season interactions were initially visualized by plotting plasma INS, INSpk, and glucose concentration against each breed for every season of the year. The distribution of the data was visually inspected, and a normality test was performed (Shapiro–Wilk test), and when necessary, data was log transformed to meet assumptions of normality. A stepwise regression with backward elimination was used to find the best model fit using log‐transformed INS and INSpk as the outcomes in 2 separate models and the breed (11 categories), age, season, sex (male including castrated and intact and female), glucose, and their interactions as main independent variables using the R package lme4 [23]. Model fit was evaluated and compared by the adjusted R squared and by diagnostic plots visualization. For the outcome INS, the best model fit included the effects of glucose, age, sex, season, and breed, and no interaction terms were included. For INSpk, the final model included season and breed, and no interaction terms. The INS and INSpk concentrations recorded originally as upper or lower limit of quantitation were substituted by a randomly generated number (n = 100) using the limit number and the mean and standard deviation of 1–10, depending on whether it was a lower or upper limit. This modified method was used as previously [24], to avoid censoring biologically relevant results (4103 values fell in the upper limit and 4403 values fell in the upper limit of detection). Posthoc multiple pairwise comparisons of mean INS or INSpk levels between categorical variables (season, breed, and sex) were performed using Tukey's multiple comparison (to compare seasons) test with Bonferroni adjustment (to compare breeds) using the R package multcomp version 1.4–25 [25]. Significance level was set as p < 0.05, and all results from multiple pairwise comparisons are presented as the estimate (est) and standard errors (SE; Supporting Information).

3. Results

After inclusion criteria were applied, there were 15 140 cases where basal serum insulin concentration was measured along with plasma glucose and with records of breed, age, and sex. There were 3051 samples collected in winter, 4079 in spring, 4762 in summer, and 3248 in fall. The breed groups included: Arabians (breed 1, n = 290), Cobs (breed 2, n = 1706), Connemaras (breed 3, n = 638), donkeys (breed 4, n = 88), Irish Sport Horses (breed 5 n = 359), New Forest ponies (breed 6, n = 418), Shetland ponies (breed 7, n = 1581), Thoroughbreds (breed 8, n = 381), Warmbloods (breed 9, n = 883), and Welsh ponies (breed 10, n = 2647). An extra category (breed 11) included all the animals of all other breeds, and there were 6149 animals in this group. The animals included had a median age of 16 years (interquartile range, 12–20 years old) and there were 6454 females and 8686 males.

From the original set of animals, there were 6096 samples with serum insulin and plasma glucose concentrations measured following the administration of Karo light syrup. These samples were only available from 2017 onwards, as this test was not performed at our laboratory before then. Samples comprised 1012 collected in winter, 1479 in spring, 2161 in summer, and 1444 in fall. There were 89 Arabians, 771 Cobs, 269 Connemaras, 41 donkeys, 168 Irish Sport Horses, 250 New Forest ponies, 324 Shetland ponies, 129 Thoroughbreds, 328 Warmbloods, and 920 Welsh ponies, and 2807 horses of other breeds. Age and sex data was only present for 5445 out of the 6096 animals. For these animals, the median age was 15 years old (interquartile range, 10–19 years old) and there were 2339 females and 3106 males. Sex and age did not enter the model for INSpk because it reduced the number of cases in the model, did not improve the fit, and was not significantly associated with the outcome.

3.1. The Effect of Season on Insulin Concentrations

When all other variables were accounted for, INS in winter was significantly higher than in spring (est, 0.205; SE, 0.031), summer (est, 0.180; SE, 0.031), and fall (est, 0.230; SE, 0.033; all p < 0.001). Estimates and standard errors related to comparisons of INS between seasons are presented in Table S3.

Similarly, INSpk in winter was significantly higher than in summer (est, 0.130; SE, 0.040) and fall (est, 0.245; SE, 0.042; p < 0.001). Serum insulin concentration following administration of Karo light syrup was significantly lower in fall when compared to spring (est, 0.206; SE, 0.04), summer (est, 0.115; SE, 0.036), and winter (est, 0.245; SE, 0.042; p < 0.001; Figure 1). Estimates and standard errors related to comparisons of INSpk between seasons are presented in Table S5.

FIGURE 1.

FIGURE 1

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Effect of season on INS (left panel) and INSpk (right panel). The central dot represents the estimated marginal mean, and error bars show the 95% confidence intervals. Seasons with different letters are significantly different (p < 0.05) according to Tukey's multiple pairwise comparisons, with Bonferroni corrections for multiple testing. INS: Serum insulin concentration; INSpk: Serum insulin concentration after administration of Karo light syrup.

3.2. The Effect of Breed on Insulin Concentrations

When breed was compared while controlling other variables, Welsh ponies had significantly higher INS than all other breeds except Shetland ponies (all p < 0.001 except New Forest ponies where p < 0.01). Shetland ponies had significantly higher INS than all breeds except the Welsh ponies (p < 0.001), and Connemara ponies had significantly higher INS than Irish Sport Horses (p < 0.05), Thoroughbreds, and Warmbloods (both p < 0.001). Thoroughbreds had significantly lower INS than all other breeds except donkeys (all p < 0.001 except vs. Irish Sport Horses where p < 0.01; Figure 2). Estimates and standard errors related to comparisons of INS between breeds are presented in Table S1.

FIGURE 2.

FIGURE 2

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Effect of breed on INS (left panel) and INSpk (right panel). The central dot represents the estimated marginal mean, and error bars show the 95% confidence intervals. Breeds with different letters are significantly different (p < 0.05) according to Tukey's multiple pairwise comparisons, with Bonferroni corrections for multiple testing. INS: Serum insulin concentration; INSpk: Serum insulin concentration following administration of Karo light syrup. Breed codes: 1. Arabians, 2. Cob, 3. Connemara, 4. Donkey, 5. Irish Sport Horse, 6. New Forest pony, 7. Shetland pony, 8. Thoroughbred, 9. Warmblood, 10. Welsh pony, 11. All other breeds. These figures represent all insulin concentrations without regard to seasons.

When considering breed comparisons for INSpk, Shetland ponies had significantly higher INSpk than all breeds except Welsh ponies (all p < 0.001 except vs. Arabians p = 0.003 and New Forest ponies p = 0.001). Welsh ponies had significantly higher INSpk than all breeds (all p < 0.001), except Arabians, New Forest ponies, and Shetland ponies. Thoroughbreds had significantly lower INSpk than all other breeds except donkeys, Irish Sport Horses, and Warmbloods (p < 0.001 except vs. Arabians where p < 0.05, Cobs where p < 0.01; Figure 2). Estimates and standard errors related to comparisons of INSpk between breeds are presented in Table S4.

3.3. The Effect of Sex and Age on Basal Insulin Concentrations

Females were found to have significantly higher INS than males (est = 3.17; SE = 0.02, p < 0.001; Figure 3) and there was a positive and significant association between age and INS (est = 0.02; SE = 0.002, p < 0.001; Figure 4). Estimates and standard errors related to comparisons of INS between males and females are presented in Table S2.

FIGURE 3.

FIGURE 3

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Effect of sex on INS. The central dot represents the estimated marginal mean, and error bars show the 95% confidence intervals. Sex with different letters is significantly different (p < 0.05) according to Tukey's posthoc comparison test. Females have significantly higher INS than males (p < 0.001). INS: Serum insulin concentration. This figure represents all insulin concentrations without regard to seasons.

FIGURE 4.

FIGURE 4

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Predicted effect of age on INS derived from a linear mixed effects regression model. The plot shows the predicted values of INS (y‐axis) across the range of age (x‐axis), with shaded regions representing 95% confidence intervals. Age is positively and statistically significantly associated with INS (est per year = 0.02; SE = 0.002, p < 0.001). INS: Serum insulin concentration. This figure represent all insulin concentrations without regard to seasons.

3.4. The Effect of Glucose on Insulin Concentrations

When controlling for other variables, plasma glucose was positively and significantly associated with INS (est = 0.205; SE = 0.006, p < 0.001; Figure 5).

FIGURE 5.

FIGURE 5

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Predicted effect of GLU on INS derived from a linear mixed‐effects regression model. The plot shows the predicted values of INS (y‐axis) across the range of GLU (x‐axis), with shaded regions representing 95% confidence intervals. GLU is positively and statistically significantly associated with INS (est = 0.205; SE = 0.006, p < 0.001). INS: Serum insulin concentration; GLU: Blood glucose concentration.

4. Discussion

This study demonstrates the relevance of several variables to better inform the diagnostic application of serum insulin concentrations in equids. It was found that there were significant effects of glucose, season, breed, age, and sex on insulin concentrations, which should be considered when interpreting laboratory results. The current study found serum insulin concentration to be highest in the winter, when both basal concentrations and concentrations after oral carbohydrate challenge were examined, although the latter was not significantly different when comparing winter and spring.

Previous studies, evaluating the seasonality of insulin in smaller numbers of animals, have reported conflicting results, with higher basal insulin concentrations in spring [9, 11], fall [8, 13, 14, 15], and winter [5], while others found no significant seasonal variation [7, 12]. A recent study found higher basal insulin concentrations in fall compared to spring [26] although no samples were assessed in winter or summer. Two studies assessed the seasonality of insulin concentrations after an oral sugar challenge, and while one reported higher results in fall [6], the other found no seasonal variation [10]. Differences in study results may be due to differences in feeding regimes, husbandry systems, or the population used. Additionally, small samples of specific breeds that might not be representative of the general population of horses were used in most studies. Most studies had a duration of 1 year, although insulin responses may vary from year to year [7], and a study with a longer duration might give more reliable results.

Perhaps, the simplest and most direct explanation for seasonal differences in insulin concentrations might be a direct association with diet; for example, some horses may be fed greater amounts of sugar and starch in supplementary feed due to a perceived need for additional calories in winter. The additionally demonstrated increased insulin response to oral sugar challenge (Figure 1b) could also fit with this, given previous studies showing changes in insulin sensitivity resulting from high sugar and starch feeds [27, 28]. Similarly, it might be that fall is associated with less supplementary feeding. Unfortunately, this study design was not able to investigate these possibilities.

Alternatively, seasonal changes in serum insulin concentrations could be driven by endogenous mechanisms, including variations in peripheral insulin sensitivity and pancreatic responsiveness, as part of a circannual metabolic cycle. The thrifty gene hypothesis was first introduced more than 60 years ago which proposed that individuals with a more rapid or prolonged insulin secretory response would have a survival advantage when food sources were scarce and only variably present [29]. Additionally, the concepts of insulin resistance (or “anti‐insulins”) with compensatory insulin secretion were also proposed. Seasonal fluctuations in the metabolic activity of horses have been suggested previously [30, 31] and fluctuations in insulin sensitivity have been described as biological adaptations that allow for the storage of body fat in preparation for challenging conditions and scarcity of food [32]. Seasonal metabolic variations in horses have been discussed in previous studies where Przewalski horses were found to have a decreased metabolic rate in winter, which preceded changes in the food supply [30]. Another study found reductions in heart rate and body temperature during winter in domesticated Shetland ponies [31]. In pregnant mares, lower metabolic activity in winter caused fetal size to be reduced when the final growth phase coincided with the winter months [33].

In this study, basal insulin was significantly higher in winter than in all other months, and INSpk was higher in winter than in summer and fall. Thus, higher circulating insulin concentrations in the winter are consistent with the above evolutionary hypotheses in at least 2 respects. First, augmented insulin secretory responses in the winter would promote storage of food when available; and secondly, the presence of relative insulin resistance in the winter would facilitate mobilization of energy stores when food was unavailable as well as compensatory insulin secretion. In contrast, insulin concentrations tended to be lowest in fall (significant for INSpk only) which would be consistent with highest insulin sensitivity at that time of year promoting an anabolic status in preparation for winter. In addition to higher serum insulin concentrations in winter months (Figure 1), this study also implied greater pancreatic secretory responses to glucose in the winter compared with most other seasons, which supports these findings arising from endogenous metabolic changes rather than simply changes in dietary management. As an additional consideration, the winter‐associated increase in basal serum insulin concentration could be promoted by low ambient temperatures that could lead to increased circulating cortisol and consequent insulin resistance and compensatory insulin secretion [34].

Alternatively, the variations in insulin could reflect a physiologic adaptation to increasing sugar ingestion through the grazing season, being associated with a diminishing β‐cell response and increased insulin resistance in the months following the grazing season [27]. It has been suggested that seasonal changes in β‐cell sensitivity to glucose, inversely related to day length, may be responsible for higher amounts of insulin being produced in winter to ensure rapid cellular glucose uptake and maximal utilization of the limited nutritional energy available [5]. In humans, seasonal variations in insulin sensitivity have been shown, with lower values during the winter and higher during the summer [35].

Breed‐related differences in insulin dynamics have been shown [16] but not related to seasonality. Ponies and Andalusians have greater postprandial hyperinsulinemia than Standardbreds, associated with differences in innate insulin sensitivity [16]. Our results showed that breed had a significant effect on serum insulin concentrations. Shetland and Welsh ponies had higher concentrations of insulin, and Thoroughbreds and Warmbloods had lower values. This may partially reflect differences in management, as the breeds with lower insulin concentrations are generally used for disciplines that require higher exercise intensity, and exercise improves insulin sensitivity [36, 37]. Curiously, donkeys did not appear to follow the same seasonal pattern observed in horses, and both INS and INSPK were higher in spring and summer (data available in annex). This difference in results in the donkey cohort suggests the need for further studies in this species and possibly further validation of laboratory methods or reference intervals for insulin concentration in this species. The assay used in this study has no published validation for the measurement of insulin concentrations in donkeys.

A positive and statistically significant association between age and insulin was found. Age is associated with insulin resistance [38, 39] and hyperinsulinemia [20, 40]. Additionally, there is an increased probability of PPID developing with increased age, with possible associated ID [41]. Mares have significantly higher insulin concentrations, consistent with previous reports [20]. In humans, there is an influence of age and gender, and different reference limits for different populations are considered [42].

This study did not include fasting insulin results, as this has no pathophysiologic relevance when assessing insulin dysregulation. Therefore, the authors considered basal insulin samples that were collected at any point of the day, to represent the response to normal dietary intake. This analysis may provide more relevant information related to diet‐associated insulin dynamics.

The main limitation of this study was that it was based on laboratory submissions and therefore the cases included were likely to be biased toward endocrinopathy and could be managed differently from other horse populations. Horses with a diagnosis of insulin dysregulation may have dietary restrictions, including limited spring and summer grazing, to prevent laminitis by restricting exposure to grass during perceived high‐risk periods. This study was further limited by the lack of knowledge of pre‐test feeding for many resting insulin values. However, in the group of horses where insulin was measured following an oral sugar test, the glycaemic challenge was similar between all animals and the results found were similar to the group where basal insulin was measured. Furthermore, information such as breed, age, sex, and pre‐analytical sample handling was provided by referring veterinary surgeons and not further verified. However, occasional errors should have little effect on the results of the study due to the large datasets. Furthermore, it is possible that other variables that were not considered in our analysis, including the EMS status of individual horses or varying weather conditions of different years, could have influenced our results.

Our study analyzed a large dataset with limited information on the individual animal and, although confounders may be missed, it has the advantage of strong statistical power and broader population representation, providing useful information for the identification of trends and patterns.

5. Conclusions

This study provided further information on insulin concentration and how it is influenced by season, breed, age, and sex and provided evidence of physiological seasonal variations in blood insulin concentrations and responses to glucose. Improved understanding of laboratory interpretation of insulin concentration is fundamental for the appropriate management of insulin dysregulation.

Disclosure

Authors declare no off‐label use of antimicrobials.

Ethics Statement

Authors declare no Institutional Animal Care and Use Committee or other approval was needed. Authors declare human ethics approval was not needed.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Data S1.

JVIM-39-e70089-s001.docx (24.7KB, docx)

Funding: The authors received no specific funding for this work.

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