Body composition at high altitude: a randomized placebo-controlled trial of dietary carbohydrate supplementation.

BACKGROUND: Body mass loss is inevitable with chronic hypoxic exposure. However, the exact body-composition changes, their causes, and possible treatments remain unknown.

OBJECTIVE: The objective was to investigate body composition during a high-altitude expedition by using non-empirically derived methods, experimentally manipulating energy intake, and investigating the influence of initial body composition.

DESIGN: Forty-one participants completed a 21-d expedition in the Himalayas. Energy intake was manipulated with a double-blind, placebo-controlled, randomized trial of carbohydrate energy supplementation. Body composition was assessed before and after the expedition by using a 4-component model including fat mass, total body water, bone mineral mass, and residual mass (principally protein and glycogen). Data were analyzed by repeated-measures analysis of variance.

RESULTS: Participants allocated to receive carbohydrate were given an additional 15,058 +/- 6211 kcal over the 21-d expedition (>6 kcal x kg(-1) x d(-1)). Nevertheless, the functionally important residual mass decreased in both groups by 6% (main effect of time: P = 0.021), with no effect of allocation (interaction effect: P = 0.116). Similar decreases were observed for fat mass (11%) and total body water (3%), which were also unabated by allocation. Furthermore, high initial fat mass (by median split) did not preserve residual mass (high-fat compared with low-fat participants: residual loss = 5% compared with 8%; P = 0.990).

CONCLUSIONS: High-altitude exposure decreased body mass, including the functionally important residual component. These losses were not abated by increasing energy intake or an initially high fat mass. Factors other than negative energy balance must contribute to body-composition changes with chronic hypoxia. This trial was registered at clinicaltrials.gov as NCT00731510.