The evidence of a strong inverse relationship between physical activity and risk of colon cancer seems unequivocal enough to justify the public health message that modest increases in physical activity of the population will help reduce the incidence of colon cancer.
The risk of endometrial cancer is associated with increased weight and body size (27). There are several case-control studies showing that inactivity is associated with an increased risk of endometrial cancer even when BMI is taken into account (Table 3). However, confounding by diet and other factors hamper the interpretation of these data.
Overweight is a risk factor for prostate cancer. The findings concerning the role of physical activity are controversial. Some case-control (97) and cohort studies (26,78) in which allowance has been made for BMI show no relationship between physical activity and prostate cancer, whereas some cohort studies (1,48,67) show a protective effect. A protective effect of exercise against benign prostatic hyperplasia has been observed in the U.S. Health Professionals Study, but the results were not controlled for weight (70).
Most epidemiological studies of cancer, obesity, and physical activity to date are beset with major methodological weaknesses, including problems in assessing physical activity and inadequate control of confounding factors. Because of the complex interrelationships between lifestyle variables, large long-term cohort studies are needed with careful designs, including proper definition and size of the study population, maintenance of high response rates, and improved and standardized methods of measuring factors linked to lifestyle and behavior.
It is not known whether the type, intensity, and timing in life cycle are important determinants of the protective effect of physical activity. There is a need for better assessment of the mode, intensity, and timing of physical activity in both observational and intervention studies.
There has been little research on the effects of physical activity and weight reduction on the progression of hormone-dependent cancers and colon cancer. Long-term RCT of exercise and dietary intervention in patients with these cancers are needed.
The protective effect of exercise against gallstones has been documented in the U.S. male population. Large-scale, high-quality epidemiological studies on well-defined patterns of physical activity (see section on research priorities in cancers) are needed to evaluate the effects of exercise in women and in other populations.
Also, RCT are needed to assess whether physical activity can prevent the formation of gallstones in obese people during and after weight reduction.
Some studies suggest that vigorous physical activity may predispose to osteoarthritis by the means of mechanical insult to the joint (9,16). An increased risk has been associated with strenuous sports (41) and with competitive but not recreational running (43). A beneficial effect of physical exercise in the prevention of osteoarthritis has not been demonstrated in any study thus far. Both physical exercise therapy (11) and weight reduction (58) may alleviate the symptoms of arthritis, but few studies have addressed the simultaneous effects of these modalities. One small uncontrolled study combining exercise with weight reduction (10) suggested that physical exercise may help preserve lean body mass during weight loss in obese patients with rheumatoid arthritis, but no effect of symptoms was observed in this study.
Obesity and low physical activity are among the proposed risk factors of low back pain, but the findings are controversial (30,49,100). While many RCT show therapeutic benefits of exercise (17,65,66) and deleterious effect of bed rest (55) in the treatment of back pain, the evidence of benefits of physical activity in back pain is equivocal (13,55).
The evidence of benefits of physical activity, beyond the effect on body weight regulation, in the prevention and treatment of osteoarthritis and back pain is inconclusive. Given the public health importance of these conditions and the theoretical benefits of weight control and physical exercise in these conditions, more research in this area is needed.
The research priorities concerning the role of physical activity in obesity-related musculosceletal disorders are as follows:
Obesity is associated with reproductive disorders including polycystic ovary syndrome, menstrual disorders, infertility, miscarriage, and pregnancy complications. These problems are ameliorated by weight reduction (64). We identified no studies that have specifically examined the relationship between these conditions and physical activity in obese patients.
RCT are needed to test whether exercise alone or combined with weight reduction by diet is effective in the treatment of sleep apnea in obese subjects and in normalizing reproductive abnormalities in obese women.
Obese persons often have limitations in psychological and physical well-being and in other aspects of the day-to-day life, often summarized under the multidimensional concept of health-related quality of life (HRQL) (29,75,86). Weight reduction appears to improve some dimensions of HRQL (76), but the few data are difficult to interpret, especially in less severe obesity (76).
RCT are needed to determine whether physical activity improves the various aspects of health-related quality of life beyond its effect of weight loss. Valid assessment of HRQL should be included in all intervention studies of obese persons.
Also, RCT are needed to study the effects of physical activity in binge eating disorder.
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