Assessment of body fat in the pony: Part II. Validation of the deuterium oxide dilution technique for the measurement of body fat

Reasons for performing the study: Excessive accumulations or depletions of body fat have been associated with increased morbidity and mortality in horses and ponies. An objective, minimally-invasive method to accurately quantify body fat in living animals is required to aid nutritional management and define welfare/performance limits. Objectives: To compare deuterium oxide (D ₂O) dilution-derived estimates of total body water (TBW) and body fat with values obtained by 'gold standard' proximate analysis and cadaver dissection. Hypothesis: D ₂O dilution offers a valid method for the determination of TBW and body fat in equids. Methods: Seven mature (mean±s.e. 13±3 years, 212±14kg, body condition scores 1.25-7/9), healthy, Welsh Mountain pony mares, destined for euthanasia (for nonresearch purposes) were used. Blood samples were collected before and 4h after D ₂O (0.11-0.13g/kg bwt, 99.8 atom percent excess) administration. Plasma was analysed by gas isotope ratio mass spectrometry following filtration and zinc reduction. After euthanasia, white adipose tissue (WAT) mass was recorded before all body tissues were analysed by proximate chemical analyses. Results: D ₂O-derived estimates of TBW and body fat were strongly associated with proximate analysis- and dissection-derived values (all r ²>0.97, P≤0.0001). Bland-Altman analyses demonstrated good agreements between methods. D ₂O dilution slightly overestimated TBW (0.79%, limits of agreement (LoA) -3.75-2.17%) and underestimated total body lipid (1.78%, LoA -0.59-4.15%) and dissected WAT (0.72%, LoA -2.77-4.21%). Conclusions and potential relevance: This study provides the first validation of the D ₂O dilution method for the minimally-invasive, accurate, repeatable and objective measurement of body water and fat in living equids.