Aged bone displays an increased responsiveness to low-intensity resistance exercise.

The ability of bone to respond to increased loading as a function of age was tested by use of three-point bending and histomorphometry. The hindlimbs of male Fischer 344 rats of three age groups (young = 4 mo, adult = 12 mo, and old = 22 mo; n = 10 per age group) were progressively overloaded by training the rats to depress a lever high on the side of a cage while wearing a weighted backpack. This squatlike movement required full extension of the hindlimbs. Exercised (Exer) rats performed 50 repetitions three times per week for 9 wk. Pack weight was gradually increased to 65% of body weight. Controls (n = 10 per age group) performed the same exercise without additional weight. Neither the mechanical properties of the femur nor histomorphometry in the proximal tibia was significantly affected in young or adult rats. However, old Exer rats were found to have significantly smaller medullary areas and a decreased trabecular spacing than their age-matched controls. These results suggest a greater sensitivity to increased loading in aged rats.

Bone structural and mechanical properties are affected by hypotransferrinemia but not by iron deficiency in mice.

Hypotransferrinemia is a genetic defect in mice resulting in <1% of normal plasma transferrin (Tf) concentrations; heterozygotes for this mutation (+/hpx) have low circulating Tf concentrations. We used this mutant mouse in conjunction with dietary iron deficiency to study the influence of Tf and iron on bone structural and mechanical properties. Twenty-one weanling wild-type BALB/cj +/+ mice and 21 weanling +/hpx mice were fed iron-deficient or iron-adequate diets for 8 weeks. Twelve hpx/hpx mice were fed the iron-adequate diet. Hypotransferrinemia resulted in increased tibia iron and calcium concentrations, lower femur failure load, and extrinsic stiffness. Because the femurs of the hpx/hpx mice were disproportionately small, these bones actually had increased tissue material properties (ultimate stress [US] and modulus of elasticity) than those of wild-type mice. This is the first report on the effect of dietary iron deficiency on bone structural and mechanical properties. Dietary iron deficiency in +/+ and +/hpx mice decreased tibia iron concentrations but had no effect on tibia calcium and phosphorus concentrations or femur structural or mechanical properties. Because the bones of the hpx/hpx mice were small, but had superior tissue mechanical properties, we conclude that Tf is important for normal bone mineralization.

Unexplained disturbance in body weight regulation: diagnostic outcome assessed by doubly labeled water and body composition analyses in obese patients reporting low energy intakes.

SUBJECTS: Ten patients who had long-term disturbances in body weight regulation, were referred over a 3-year period for obesity evaluation, and reported low energy intakes (< 1,200 kcal/day). OBJECTIVE: To ascertain whether these patients had a low energy expenditure and thus reduced energy requirement, and/or whether they were misreporting their energy intake. DESIGN: Comparison of outcome measures in referred patients and in obese control patients who did not report low energy intakes and disturbances in body weight regulation. MAIN OUTCOME MEASURES: Low energy expenditure was evaluated with serum thyroid hormone levels, resting metabolic rate (RMR), thermic effect of food (TEF), and total energy expenditure (TEE) by doubly labeled water technique. Misreporting of energy intake was evaluated by comparing patients' self-reported energy intake with energy intake estimated by doubly labeled water and body composition analyses over a 14-day period. STATISTICAL ANALYSES PERFORMED: Low energy expenditure was considered present in a patient if RMR or TEE was more than 15% below predicted values according to results from the control group. Patient group TEF was compared with TEF results observed in the control group. RESULTS: All patients had normal serum thyroid hormone levels. Eight patients had RMR and TEE values within 15% of predicted values and were substantially underreporting their energy intake. One patient had low TEE (-19%) and a normal RMR, a finding that implies a low level of physical activity. This patient also underreported energy intake as estimated by the doubly labeled water technique during the study (-38%). The 10th patient had a low RMR (-23.2%) and TEE (-25.0%), the mechanism of which was uncertain. This patient's reported food intake over the 14-day period was accurate but was less than her long-term intake over months or years as suggested by doubly labeled water TEE estimates. The TEF response in patients was not significantly different from that observed in the control group. CONCLUSIONS: Underreporting of energy intake from foods is a frequent finding in patients with disturbances in body weight regulation who are referred for obesity evaluation. Severe underreporting may be detectable by means of screening measures available to most dietitians. Low energy expenditure, due either to physical inactivity or to metabolic factors, is also observed. Modern evaluation methods provide new insights into patients with weight regulatory disturbances and at the same time stimulate important new research questions.

Body composition and two-compartment model assumptions in male long distance runners.

Long distance running increases bone mineral mass, skeletal muscle weight, and extracellular fluid volume. Each of these changes may have an impact on classic two-compartment body composition methods that assume a constant fat-free body mass (FFM) density (1.100 g.cc-1), potassium content (65 mmol.kg-1 FFM), and hydration (0.73 kg H2O.kg-1 FFM). The aims of the present study were: to use newly developed multi-component methods to evaluate the validity of two-compartment methods in white male recreational long-distance runners (N = 10); and to compare the body composition of these runners to sedentary controls (N = 10) of similar age, body weight, and body mass index. Runners had a significantly smaller fraction of body weight as fat (P = 0.001) and a larger fraction of FFM as lower extremity skeletal muscle (P = 0.045) and bone (P = 0.049). Although FFM constituted a larger proportion of body weight in the runners, the fractional contributions of water, protein, and mineral were similar to those in the control group. There were no significant differences between the two groups in density of FFM, total body potassium/FFM, and total body water/FFM. New methods thus allow in-depth analyses of body composition in athletes, with results suggesting that classic two-compartment methods are valid in white recreational long-distance runners.