Exercise and oral contraceptive use suppress the normal age-related increase in bone mass and strength of the femoral neck in women 18-31 years of age

Women who exercise during their second and third decades may increase their peak bone mass and lower their eventual risk for postmenopausal fracture, However, the effects of exercise in younger women can be modulated by the u se of oral contraceptives, which may prevent the normal accretion of bone m ass that would otherwise occur, We hypothesized that exercise intervention in young adult women would significantly increase both bone mass and the be nding rigidity of the femoral neck. We further hypothesized that exercise i ntervention in the presence of oral contraceptive use would have a negative effect on bone mass and bending rigidity. Women 18-31 years of age (n = 12 3) were classified by oral contraceptive use (OC, NOC) and age (18-23, 24-3 1 years), and then randomized into exercise or nonexercise groups. The exer cise protocol consisted of three sessions/week of aerobic and nonaerobic ex ercises, and continued for 2 years. Each 6 months, the femoral neck of each subject was scanned using a Lunar dual-energy X-ray absorptiometry (DEXA) scanner, and bone mineral content, density and geometric information were u sed to calculate estimated stresses and bending rigidity at the hip. Percen t changes from baseline were analyzed using two-way analysis of variance (A NOVA) at 6, 12, 18, and 24 months. Women who neither exercised nor took ora l contraceptives (NE/NOC) had the greatest percentage increases in cross-se ctional area (4.98 +/- 2.29%), cross-sectional moment of inertia (9.45 +/- 2.37%), total bone mineral density (2.07 +/- 2.09%), fracture index (8.03 /- 2.03%), and safety factor (20.03 +/- 5.79%) over the 24 month exercise p rogram. Women who exercised and did not take oral contraceptives (E/NOC) de clined on most variables related to femoral strength and bone mass, whereas those women who took oral contraceptives were usually intermediate between NE/NOC and E/NOC, whether they exercised or not. These data show that eith er exercise or OC use is associated with a suppression of the normal increa se in bone mass and mechanical strength in the femoral neck in women 18-31 years old, but the combination of exercise and OC use appears to have a les s suppressive effect. (C) 2000 by Elsevier Science Inc. All rights reserved .