Diurnal rhythm and effects of feeding, exercise and recombinant equine growth hormone on serum insulin concentrations in the horse.
Authors: Noble G K, Sillence M N
Journal: Equine veterinary journal
Summary
# Editorial Summary: Diurnal Insulin Rhythms and Growth Hormone Effects in Horses Noble and Sillence's 24-hour sampling protocol across 12 geldings revealed important patterns in equine insulin metabolism: despite identical meals, horses mounted significantly higher insulin peaks following their morning feed (577 ± 108.3 pmol/l at 09.30 h) compared to their afternoon meal (342.4 ± 75.7 pmol/l at 17.30 h), with nadir concentrations occurring overnight between midnight and 07.30 h. A 3-week exercise programme alone did not alter basal insulin concentrations, but recombinant equine growth hormone administration markedly elevated fasting insulin levels (P = 0.01), rising from baseline 44.4 ± 15.3 pmol/l to 320.9 ± 238.2 pmol/l by day 12—a concerning seven-fold increase that proved variable between individuals and only resolved upon cessation of treatment. For practitioners managing obese or insulin-dysregulated horses, these findings substantially limit growth hormone's therapeutic utility: whilst the hormone could theoretically support lean mass development, it reliably induces problematic hyperinsulinaemia that exercise cannot mitigate. This work underscores the importance of pre-prandial sampling timing for accurate insulin assessment and reinforces that growth hormone is contraindicated in insulin-resistant equids where maintaining insulin sensitivity is paramount.
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Practical Takeaways
- •Do not use recombinant equine growth hormone to treat obesity in horses, particularly those with insulin resistance, as it consistently elevates insulin levels and worsens metabolic status
- •Exercise alone does not reduce basal insulin concentrations, so weight management in insulin-resistant horses requires dietary intervention as a primary strategy
- •Single pre-feeding morning blood samples provide reliable basal insulin measurements for monitoring metabolic status in individual horses
Key Findings
- •Horses showed diurnal insulin rhythm with peaks of 577 ± 108.3 pmol/l at 09:30 h and 342.4 ± 75.7 pmol/l at 17:30 h following feeding, with nadir between midnight and 07:30 h
- •Exercise alone had no effect on basal insulin concentrations prior to growth hormone administration
- •Recombinant equine growth hormone increased serum insulin concentrations from 44.4 ± 15.3 pmol/l to 320.9 ± 238.2 pmol/l by day 12 (P = 0.01), causing variable hyperinsulinaemia that resolved after treatment cessation
- •Growth hormone therapy is contraindicated for obesity management in insulin-resistant equids due to induced hyperinsulinaemia