The effect of diet-induced weight loss on bone and muscle strength in overweight and obese individuals

Abstract

The increasing prevalence of overweight and obesity is an alarming global issue. Not only do elevated body weights influence individual health and mortality, it also has an impact on a larger scale by placing financial burden on society. While diet-induced weight loss is the cornerstone treatment for overweight or obesity, some but not all studies have suggested that it has a harmful effect on bone and muscle strength. Bone and muscle strength are known in unison as the musculoskeletal system and have a parallel relationship, whereby factors affecting one system also tend to affect the other. Negative effects on bone and muscle strength independently, or together, results in an increased risk of disease states such as osteoporosis, dynaopenia and sarcopenia, and thus adversely affect overall health. The research presented in this thesis focuses on elucidating potential harmful implications that diet-induced weight loss may have on bone (Chapter 2) and muscle strength (Chapter 3), explored through two systematic reviews and meta-analyses using a random effects model. In Chapter 2, included data were from 41 publications of overweight or obese but otherwise healthy adults who followed a dietary weight loss intervention, and which examined total hip, lumbar spine or total body bone mineral density (BMD) via dual energy x-ray absorptiometry, or serum or urinary concentrations of markers of bone turnover, at the start and end of the intervention. There were significant decreases in total hip BMD with dietary interventions of 6, 12 or 24 (but not 3) months’ duration. No significant changes in BMD occurred in the lumbar spine or total body following dietary weight loss interventions ranging in duration from 3 to 24 months, except for a significant decrease in total body BMD after dietary interventions lasting 6 months. No significant changes occurred in the serum concentrations of the marker of bone turnover, N-terminal propeptide of type I procollagen. Interventions of 2 or 3 months in duration (but not of 6, 12 or 24 months’ durations) induced significant increases in serum concentrations of the bone turnover markers osteocalcin, C-terminal telopeptide of type I collagen or N-terminal telopeptide of type I collagen, indicating an early effect of diet-induced weight loss to promote bone breakdown. While results from individual studies varies, this meta-analysis shows a clear effect of diet-induced weight loss to significantly reduce total hip BMD in overweight and obese individuals, consistent with the observed increases in circulating levels of bone turnover markers. Since the hip is the gold-standard site for diagnosing osteoporosis and assessing fracture risk, it is now important to determine the long-term effects that diet-induced weight loss may have on fracture risk in overweight and obese adults. In Chapter 3, the aim was to identify how diet-induced weight loss in adults with overweight or obesity impacts on muscle strength. 27 publications, including 33 interventions, most of which were 8-24 weeks in duration, were included. Meta-analysis of knee extensor strength as measured by isokinetic dynamometry found a significant decrease following diet-induced weight loss, by 7.5% from baseline values. Meta-analysis of handgrip strength showed a non-significant decrease with dietary restriction for weight loss. Due to variability in methodology and muscles tested, no other data could be meta-analyzed, and qualitative assessment of the remaining interventions revealed mixed results. Despite varying methodologies, diets and small sample sizes, these findings suggest a potential adverse effect of diet-induced weight loss on muscle strength. While these findings should not act as a deterrent against weight loss in people with overweight or obesity, due to the known health benefits of losing excess weight, they call for strategies to combat strength loss – such as weight training and other exercises – during diet-induced weight loss. From the two studies presented in this thesis, it can be seen that diet-induced weight loss can negatively impact on bone and muscle strength in overweight and obese populations. Bone and muscle are organs that are fundamental to the maintenance of health. They have a dual relationship whereby factors that affect one organ will also affect the other. This parallel regulation of bone and muscle was reflected in the findings from this thesis, where dietary weight loss interventions resulted in decreases in BMD, as well as reductions in muscle strength. Therefore weight loss can induce adverse effects, and this could conceivably increase the risk of problems such as osteoporosis and dynaopenia, but future research is needed to determine relative benefits, as well as the possible risks, of treating overweight and obesity with diet-induced weight loss. Some of the mechanisms that may contribute to this dual detrimental effect of diet-induced weight loss on bone and muscle strength have been proposed in Chapter 4, however future research is necessary to determine the factors contributing to this potential adverse effect, as well as interventions to curb the possible negative consequences.