Influence of lifestyle behaviours on bone mineral density among young healthy women: A two-year study. A Tentative Salutary Model

Sammanfattning: The overall aim of this prospective observational study with a salutogenic approach was to investigate bone mineral density (BMD) and BMD changes in the heel bone, in a group of healthy women, in relation to physiological factors and lifestyle behaviours over a two-year period as well as investigating the women’s view of their lifestyle behaviours. Data were collected in 1999 (T1) and 2001 (T2). Healthy young women (n=152) filled in a structured questionnaire, BMD measurements were performed by a heel bone scanner (DEXA), and deoxypyridinoline (U-DPD) was measured. Data were analysed by means of simple and multiple linear regression and logistical regression. Data were collected by means of qualitative interviews with eleven of the informants, and grounded theory was used as the method of analysis. Of the participants, 62% had decreased/unchanged bone density, and 38% had increased their bone density over the 2 years. Use of oral contraceptives (OC) and alcohol consumption were associated with an increased risk of negative BMD changes. Self-reported poor health was associated with decreased BMD. Smokers had lower BMD although OC seemed to moderate the negative impact of smoking probably related to bone turnover. Irrespective of smoking and OC, self-reported sleep satisfaction explained 3% of the variability in BMD change. Time spent outdoors was associated with BMD change irrespective of smoking and OC use. Women with OC induced regular menstruation had higher BMD than women with naturally regular menstruation. OC use in combination with smoking was linked to high alcohol consumption and higher frequency of self-reported body weight reduction. There was a positive association between a high level of physical activity, body weight and BMD. When the sample was divided according to underweight (BMI <19), normal weight (BMI 19-24) and overweight (BMI >24), cross-sectional body weight was of no significance for the prediction of BMD (T2). Among underweight women, DPD (T1) explained 46.3% of the variability in BMD (T2), and a high level of physical activity had a negative impact on BMD. Among overweight women, the difference in time spent outdoors during winter between T1 and T2 was the single most important factor for BMD levels, and self-reported sleep satisfaction was associated with increased BMD. The women’s (N=11) views on lifestyle behaviours were characterised by a number of interrelated dimensions; motivation, goals, actions and strategies. Women with a relaxed outlook on life had increased BMD while women with a rigid outlook on life had decreased their BMD irrespective of smoking and physical activity. Respondents who had a rigid outlook on life viewed actions such as lifestyle habits as a means to an end, where the goal of, for example, physical activity was a way of staying slim. For respondents with a relaxed outlook on life, the enacted lifestyle behaviours, such as physical activity, were a goal in themselves. It was hypothesised that women with a relaxed outlook on life were more satisfied with their sleep, and the whole quantitative sample (n=152) was divided into two groups according to self-reported sleep satisfaction. The result from the comparative analysis showed that women who were more satisfied with their sleep were significantly more likely to have healthier lifestyle behaviours, better physical and psychological subjective health, and were more likely to have increased BMD. In conclusion, time spent outdoors may moderate the negative influence of smoking on BMD, while motivation and sleep may be salutary factors associated with improved BMD. A tentative bio-psychosocial salutary model of the association between motivation, outlook on life, sleep, lifestyle behaviours and BMD is presented.