Effect of Weight Change on Markers of Bone Turnover and Phosphorus Excretion

•Weight loss was correlated with a biomarker of bone resorption•Compared to weight gain, weight loss tended to increase fecal P excretion•Urinary P did not substantially contribute to whole body P excretion•Phosphorus homeostasis in adult horses is maintained via the digestive tract There is little information about how weight change in horses impacts bone turnover and the metabolism of minerals associated with bone. This study evaluated weight change in mature horses as a factor that could alter bone turnover and fecal P output. Fifteen horses (555 ± 8 kg) were assigned to three treatments: weight loss (LO; n = 5), weight maintenance (MA; n = 5), and weight gain (GA; n = 5). Diets contained 75%, 100%, and 145% of maintenance digestible energy requirements for the three treatments, respectively, but contained similar amounts of protein and minerals. At the end of the weight change period (27 ± 6 d), blood samples were analyzed for bone biomarkers and a 5-day total fecal collection was conducted to measure fecal mineral output. Horses fed the MA diet had an average daily weight change that was not different from either the GA or LO treatments, while weight change was different between the GA group and the LO group (0.49 kg/d vs. -1.16 kg/d; P = .017). Weight change was negatively correlated with cross-linking C-terminal telopeptides of type-I collagen, a biomarker of bone resorption (r = -0.62; P = .014) and tended to be positively correlated with bone alkaline phosphatase, a biomarker of bone formation (r = 0.48; P = .068). Also, fecal P output tended to be lower in GA than in LO horses (P = .085), while MA was intermediate and not different, suggesting that weight loss was increasing bone resorption, resulting in a tendency for higher P loss from the body. Weight change in horses can influence bone metabolism as well as mineral excretion.