Dose-Response Relationships Between Energy Availability and Bone Turnover in Young Exercising Women

Abstract

To help refine nutritional guidelines for military servicewomen, we assessed bone turnover after manipulating the energy availability of 29 young women. Bone formation was impaired by less severe restrictions than that which increased bone resorption. Military servicewomen and others may need to improve their nutrition to avoid these effects. Introduction:We determined the dose-response relationship between energy availability (defined as dietary energy intake minus exercise energy expenditure) and selected markers of bone turnover in 29 regularly menstruating, habitually sedentary, young women of normal body composition. Materials and Methods: For 5 days in the early follicular phase of two menstrual cycles separated by at least 2 months, subjects expended 15 kcal/kgLBM/day in supervised exercise at 70% of aerobic capacity and consumed controlled amounts of a clinical dietary product in balanced (45 kcal/kgLBM/day) and one of three restricted (either 10, 20, or 30 kcal/kgLBM/day) energy availability treatments in random order. Blood was sampled at 10-minute intervals, and urine was collected for 24 h. Samples were assayed for plasma osteocalcin (OC), serum type I procollagen carboxy-terminal propeptide (PICP), and urinary N-telopeptide (NTX). Results: NTX concentrations (p Ͻ 0.01) and indices of bone resorption/formation uncoupling (Z NTX-OC and Z NTX-PICP ; both p Ͻ 10

Cross-sectional evidence of suppressed bone mineral accrual among female adolescent runners

“…(31) The combined effects of these factors on runners' energy intake and expenditure levels therefore may lead to an energy deficit or to a condition known as low energy availability, which has been documented to alter metabolic and hypothalamic hormones that regulate energy balance, growth, menstrual function, and bone turnover. (32) In support of this, field studies have documented that elite and recreational youngadult endurance runners exhibit inadequate energy intake during training and leading up to a competition, with one study reporting a 76% prevalence of an ''energy deficit'' in their sample of 33 young-adult women. (33) Furthermore, BMI and percent body fat, which typically appear higher among those with greater energy status levels, were lower among the runners aged 16 years and/or 17 to 18 years than the nonrunners.…”

Low energy availability in exercising men is associated with reduced leptin and insulin but not with changes in other metabolic hormones

“…The present controlled study was designed to assess whether low energy availability has similar metabolic effects in exercising men as those previously reported in sedentary women (Hilton & Loucks, 2000;Ihle & Loucks, 2004;Loucks & Heath, 1994;Loucks & Thuma, 2003;Loucks et al, 1998). We chose to reduce energy availability to 15 kcal · kg −1 FFM based on Loucks' experiments, which demonstrated that effects were more pronounced when energy availability was reduced to 10-20 kcal · kg −1 FFM · day −1 when compared to 30 kcal · kg −1 FFM · day −1 (Hilton & Loucks, 2000;Ihle & Loucks, 2004;Loucks & Heath, 1994;Loucks & Thuma, 2003;Loucks et al, 1998).…”

“…We chose to reduce energy availability to 15 kcal · kg −1 FFM based on Loucks' experiments, which demonstrated that effects were more pronounced when energy availability was reduced to 10-20 kcal · kg −1 FFM · day −1 when compared to 30 kcal · kg −1 FFM · day −1 (Hilton & Loucks, 2000;Ihle & Loucks, 2004;Loucks & Heath, 1994;Loucks & Thuma, 2003;Loucks et al, 1998). In our control condition, energy availability was 40 kcal · kg −1 FFM · day −1 , which was lower than the 45 kcal · kg −1 FFM · day −1 that had been operationally defined as balanced energy availability by Loucks and colleagues (Hilton & Loucks, 2000;Ihle & Loucks, 2004;Loucks & Heath, 1994;Loucks & Thuma, 2003;Loucks et al, 1998). However, mock participants who underwent preliminary testing reported fullness, bloating, constant pressure to eat and weight concerns when consuming the energy intake needed to reach an energy availability of 45 kcal · kg −1 FFM · day −1 , particularly during the C + EX condition.…”

“…It is notable that controlled experiments on the metabolic effects of low energy availability have only been conducted in sedentary women (Hilton & Loucks, 2000;Ihle & Loucks, 2004;Loucks & Heath, 1994;Loucks & Thuma, 2003;Loucks et al, 1998); no controlled experiments have utilized men, and particularly exercising men. Reductions in T3, IGF-1 and testosterone have been reported in field studies in male soldiers exposed to various levels of energy deficiency during military training (Friedl et al, 2000;Kyrolainen et al, 2008;Nindl et al, 2007), which suggests that key metabolic hormones may also be suppressed in severely energy-deficient exercising men.…”

Less-than-expected weight loss in normal-weight women undergoing caloric restriction and exercise is accompanied by preservation of fat-free mass and metabolic adaptations

“…20 Primary outcomes included body weight, body composition, RMR and metabolic hormones linked to energy conservation and future weight regain, including leptin, 16,21,22 ghrelin, 21,23 triiodothyronine (T3) 24,25 and insulin-like growth factor-1 (IGF-1). 26 We hypothesized that 3 months of caloric restriction combined with exercise would result in significant weight loss, but that exercise would preserve lean mass and promote loss of fat mass such that actual weight loss would be less than predicted. We further hypothesized that women exposed to a greater energy deficit would demonstrate more pronounced metabolic adaptations indicative of energy conservation when compared with women in a smaller energy deficit, and that these metabolic adaptations would further attenuate actual weight loss.…”