It’s no secret: Equine obesity levels are on the rise. While all overweight horses need to drop a few pounds, you might want to pay special attention to the particularly chubby broodmares gobbling up grass in your pastures—especially if you’re hoping for a foal this spring. Researchers have confirmed that obesity appears to have negative effects on mares’ follicles and oocytes.

Dawn Sessions-Bresnahan, MS, PhD, an assistant professor in the Berry College Department of Animal Science, in Mount Berry, Georgia, worked with colleagues at Colorado State University, in Fort Collins, to determine if increased body weight affected reproduction.

The researchers used 16 nonlactating mature mares with body conditions of either 5.1 (10.5% body fat) or 7.9 (16.2% body fat) on the 1 to 9 scale. The former mares were considered controls and the latter ones were classified as obese. When the researchers detected a follicle greater than 35 millimeters with uterine edema (fluid swelling indicative of impending ovulation), they administered a gonadotropin-releasing hormone analog to help the follicle mature.

Approximately 22 to 24 hours after administration, the team collected the oocytes (immature egg cells), granulosa cells (which surround developing ovarian follicles), and follicular fluid (which is released during ovulation) from each follicle. On the same day, the team collected blood samples prior to morning feeding to evaluate mares’ levels of glucose, insulin, leptin, adiponectin, and other metabolites.

They found that:

  • Obese mares had higher follicular fluid leptin levels than did control mares. In women undergoing in vitro fertilization, lower follicular fluid leptin levels are considered a marker for successful reproduction.
  • Obese mares showed elevated linoleic acid levels, as well as lower glutathione peroxidase gene expression in oocytes. In cows, high linoleic acid concentrations reduce oocyte maturity rates and gene expression for glutathione peroxidase (an antioxidant enzyme).
  • Obese mares experienced elevated steric acid levels in follicular fluid compared to control mares. In dairy cows and humans, elevated levels of this lipid appear to decrease cell survival rate.
  • Obese mares showed an increase in follicular fluid triglycerides (which store fat for energy) and genes specific to endoplasmic reticulum (an organelle in plant and animal cells responsible for protein and lipid synthesis) stress. In mice, follicular fluid high in triglycerides can increase endoplasmic reticulum stress and negatively impact embryo development.
  • Obesity altered the expression of genes responsible for mitochondrial function in granulosa cells. In mice, a high-fat diet can affect mitochondrial function by reducing the production of antioxidants important in cell defense.
  • Obese mares had 35% more DNA methylation in granulosa cells compared to control mares. In humans, obesity and insulin resistance affect DNA methylation in white blood cells, potentially impairing DNA’s repair abilities.

Then, the team evaluated the oocytes for lipid content. The obese mares’ oocytes contained 38 different lipids when compared to control mares, most of which play an important role in the stability of cellular structure and triglycerides containing the fatty acids linoleic and stearic acid. Researchers believe these triglycerides could protect the oocytes against cold temperatures and serve as an energy source. However, they also believe excess lipid content in oocytes could lead to intercellular structural function and stress.

Additionally, the researchers looked for correlations between serum and follicular fluid metabolite concentrations, hoping to identify a potential marker of obesity-related changes. Obese mares’ insulin and leptin levels correlated in both serum and follicular fluid, similar to the correlation observed in women undergoing in vitro fertilization. This suggests practitioners could use serum insulin levels to identify obesity-related follicular changes, the researchers said.

Simply put, the sum of those findings suggest that obesity could have a negative impact on mares’ follicles and oocytes and, thus, overall fertility.

Next, researchers hope to examine how maternal obesity impacts the fetus.

"We hope that this research and future work on how the disrupted follicular environment potentially affects offspring metabolic health will yield helpful insights for improved equine health," Sessions-Brensnahan said.

The study, “Effect of Obesity on the Preovulatory Follicle and Lipid Fingerprint of Equine Oocytes,” was published Biology of Reproduction.