Testosterone Status Following Short-term, Severe Negative Energy Balance Predicts Fat-free Mass Loss in U.S. Marines

Abstract

Male US military personnel exposed to periods of severe negative energy balance often experience marked reductions in circulating total testosterone, and it is not uncommon for concentrations to fall below normal levels (<300 ng/dL). However, testosterone fluctuations following severe negative energy balance are variable, and little is known about the metabolic and physiological differences between males who experience low testosterone (LT) and those who maintain normal testosterone (NT). PURPOSE: To determine metabolic and physiological differences between males with LT versus those with NT following 8 d of strenuous military training. METHODS: Male US Marines (n = 68) were dichotomized based on testosterone concentration (< or ≥ 300 ng/dL) following 8 d of severe negative energy balance incurred during military training. Body composition (DEXA) and whole-body protein turnover (15N alanine) were measured and blood and urine samples collected before (PRE) and after (POST) training. Linear mixed models were used to assess the effects of testosterone status, time, and their interaction on outcomes. RESULTS: Testosterone concentrations decreased PRE (505 ± 135 ng/dL) to POST (284 ± 115 ng/dL, P < 0.01). When volunteers were dichotomized by POST testosterone status (NT, n = 24: 407 ± 97 vs. LT, n = 44: 217 ± 52 ng/dL, P < 0.01), PRE BMI, total fat mass, and testosterone were greater in NT compared to LT (P < 0.05). LT lost more fat-free mass (FFM, -3.4 ± 1.3 kg) and less fat mass (-2.4 ± 1.2 kg) compared to NT (-2.4 ± 2.0 and -3.2 ± 1.6 kg, respectively; P-interaction < 0.03). Insulin (+5.55 ± 9.35 μU/mL) and norepinephrine (+0.23 ± 0.29 ng/mL) increased from PRE to POST in LT, whereas no changes occurred in NT (+1.03 ± 3.75 μU/mL and +0.04 ± 0.30 ng/mL, respectively), resulting in significant differences between groups (P-interaction < 0.02). Independent of time, LT had greater whole-body protein synthesis, breakdown, and flux (P-status < 0.01), but not net balance (P-status = 0.2), compared to NT. CONCLUSION: Military personnel susceptible to operational stress-induced LT may be predisposed to greater FFM loss during periods of negative energy balance incurred during short-term, strenuous military training. Supported by US Army Medical Research and Materiel Command; authors’ views not official US Army or DoD policy.