Weight Loss Is Associated With Loss of Hip BMD in Women

Abstract

The bone density of women is closely associated with body weight. However, a quantifiable expected change of bone density in relation to weight change is not widely known. PURPOSE: The aim of this study was to quantify the relationship between weight changes (WC) and changes in hip bone mineral density (BMD) in a 4-yr prospective cohort study of 170 middle-aged women. A secondary purpose was to determine the extent differences in baseline weight, age, physical activity (PA), calcium intake, and menopausal status affected the relationship between weight change and hip BMD change. METHODS: All subjects were non-smokers and had BMI's less than 30 at baseline. Average age of the sample was 41.5 ± 2.9 years. BMD was measured at the left hip using DEXA (Hologic 4500W). Body weight was measured using a computerized scale accurate to within 10 grams. Dietary calcium intake was assessed using 7-day weighed food records. Food records were entered into a data base by a registered dietitian and ESHA Research software was used to analyze the diets. Subjects were divided into quartiles based on WC and the middle-two quartiles were collapsed forming three groups classified as Weight Loss (WL) [mean weight change: −5.0 ± 3.4 kg], Low Weight Gain (LWG) [mean weight change: 1.1 ± 1.5 kg], and High Weight Gain (HWG) [mean weight change: 6.8 ± 3.7 kg]. Regression analysis using the general linear model (GLM) technique was employed to quantify the relationship between WC and BMD. RESULTS: Mean weight at baseline was 65.8 ± 10.5 kg and mean WC over the four years was 1.04 ± 5.0 kg. At baseline, mean calcium intake was 773.8 ± 322.3 mg per day. Baseline mean hip BMD was 0.938 ± 0.114 g/cm3 and at follow-up, mean hip BMD was 0.932 ±0.118 g/cm3, a mean change of-0.005 ± 0.036 g/cm3. Changes in hip BMD across the 4 yrs differed as a function of the weight change group (F=7.0, P=0.0012). The WL group had a mean change in BMD of −0.02 g/cm3, whereas the LWG and HWG groups had BMD changes of 0.0001 g/cm3 and 0.0014 g/cm3, respectively. On average, women in the WL group lost 2.5% in hip BMD. For each change in body weight of 10 kg, there was a corresponding change of hip BMD of 0.016 g/cm3 (F=8.5, P=0.004). Adjusting for age and menopause status weakened the relationship. Controlling for PA and calcium intake strengthened the associations. After controlling for all of the confounders together, the association between WC and hip BMD change was significant (F = 6.84, P = 0.0014). CONCLUSIONS: As women lose weight, they also lose BMD at the hip. In the present study, women who lost an average of 5 kg over 4 yrs lost about 2.5% in hip BMD.