Reflections on obesity,exercise,and musculoskeletal health

Obesity is a disease that has become an epidemic in economically developed countries,and manifests itself at increasing rates in economically developing countries. Obesity is associated with a variety of comorbidities, such as metabolic disease,diabetes,cardiovascular diseases,and musculoskeletal disorders, all resulting in tremendous costs to health care systems around the world, a reduced capacity for work, and reduced quality of life for people with obesity, coupled with physical inactivity,thereby producing a vicious circle of inactivity-induced diseases that enhances obesity.

In this special topic of the Journal of Sport and Health Science(JSHS),4 leading authorities and their collaborators provide insights into the complex issues associated with obesity,and how diet and exercise might contribute to offset some of the risk factors for diseases associated with obesity.

Griffin and colleagues and Collins and colleagues present the most recent evidence of the effects of diet-induced obesity on musculoskeletal systems in a mouse and a rat model,respectively. Griffin et al. chose a high-fat diet to induce obesity in mice and, following obesity induction, added aerobic exercise (wheel running) to study the effects of diet and exercise on knee osteoarthritis. Collins et al. used a high-fat combined with a high-sucrose diet to study the effects of dietinduced obesity on knee osteoarthritis in rats exposed to this diet in childhood and in adulthood. Griffin et al. found little diet and exercise effects on knee osteoarthritis in their mouse model, but found distinct differences in the metabolic and inflammation profiles of mice exposed to a normal diet and a high-fat diet, and mice exposed to aerobic exercise and not exposed to exercise.They argue that there might be metabolic and inflammation profiles before osteoarthritis development,and that these pre-osteoarthritis profiles might be used as biomarkers and clinical indicators of risk for knee osteoarthritis development in populations with obesity.

The results reported by Collins and colleagues were surprising in the fact that rats exposed to the high-fat/highsucrose diet following weaning (at age of 3 weeks) for 14 weeks,and rats exposed to the same diet starting at the age of 12 weeks for 4 weeks had distinctly different outcomes.Interestingly, the adult animals that were exposed to the diet for only 4 weeks had more severe knee osteoarthritis scores than the rats exposed to the high-fat/high-sucrose diet for 14 weeks that included the weeks of exposure of the adult rats.Since the metabolic and inflammation profiles in the adult and childhood group rats were similar, and body fat percentage was also similar, it was concluded that the age of exposure, rather than the duration of exposure to the high-fat/high-sucrose diet was a crucial factor for the onset of knee osteoarthritis in this model. Rats exposed to the high-fat/high-sucrose diet in childhood seemed able to cope better with the metabolic disturbance than the rats raised on a normal diet until adulthood,and then exposed for a short time to the obesity-inducing diet. This result may have profound implications for human obesity, if direct translation is possible. This result also needs further and more long-term follow-up into the rate of progression of musculoskeletal disorders as adulthood progresses, as previous studies indicate that obesity in childhood has severe, and some irreversible,effects for health outcomes in adulthood.1

Klancic and Reimer provide an excellent overview of the effects of obesity on the gut microbiota:the community of all microbes in the intestinal tract. Disturbances of the gut microbiota have been shown to have tremendous effects on the health of the host animal.Disruption of the gut microbiota can cause obesity,and the most powerful regulator of the gut microbiota is diet. High-fat and high-sucrose diets causing obesity are strong negative modulators of the gut microbiota,while diets rich in prebiotic fibers are positive regulators of the gut microbiota.Klanic and Reimer provide a summary of the many diseases directly associated with the disturbance of the gut microbiota, and review the current literature on the effects of diet (specifically diets rich in prebiotic fiber) and exercise on the gut microbiota. They also reserve a chapter for the effects of the gut microbiota on musculoskeletal health, thereby making a connection to the papers by Griffin et al. and Collins et al., who discuss the effects of aerobic exercise interventions as a potentially powerful tool to combat obesity. Recent work from our laboratory demonstrated that 5 days of moderate running exercise prevented high-fat/high-sucrose-induced knee osteoarthritis in rats,surprisingly without reducing obesity (body fat percentage), but by normalizing the metabolic and inflammatory profiles of the exercise-exposed rats.2This study suggests that body fat percentage, or the amount of obesity, is not necessarily an appropriate indicator of metabolic health and, thus, risk of musculoskeletal degeneration. Rather, obese individuals can be metabolically healthy or unhealthy and, when healthy,they seem to be protected from musculoskeletal degeneration to a great extent.

Finally,Steele and colleagues provide insights into the effects of obesity on community-dwelling women. They demonstrated in their large-scale study that obesity in women was associated with larger breast size, which in turn was associated with increased loading of the thoracic spine,resulting in increased thoracic kyphosis(spine curvature)and greater musculoskeletal pain in the upper spine,shoulders,and neck area,compared with agematched women who were not obese. They also found that women with obesity were spending less time performing physical activity,thus increasing risk factors associated with physical inactivity.They concluded their work with the urgent appeal to study obesity more in the context of women, and associated musculoskeletal problems,as demonstrated here for upper back and neck problems associated with increased breast size, and further call for increased research on the effect of intervention protocols aimed at preventing obesity in women and improving outcomes for women suffering from obesity.

It is the hope that this special topic of JSHS on, obesity,exercise, and the gut microbiota generates more research on obesity. Obesity is an epidemic of grand proportion, costing immeasurable pain and suffering, including social and mental problems not discussed here, and is associated with severe comorbidities that affect the quality of life of millions of people. Groups,nations,and social structures that can combat obesity effectively and reduce the rates of obesity in a segment of the population or a country substantially,will have tremendous advantages in all aspects of life,society,and community,compared with those who cannot.

Competing interests

The author declares that he has no competing interests.