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ScienceWeek
MEDICAL BIOLOGY: OBESITY AND PHYSICAL ACTIVITY
The following points are made by S.N. Blair and T.S. Church (J. Am. Med. Assoc. 2004 292:1232):
1) The modern living environment in developed countries is characterized by low daily energy expenditure and an abundant and inexpensive food supply, making positive energy balance common. The result is a rightward skewing of the body mass index (BMI) distribution and an increasing prevalence of obesity.
2) Indisputable evidence links obesity to health problems, including risk of cardiovascular disease, type 2 diabetes, some cancers, and all-cause mortality.(1) These associations are dose-related, temporally consistent, and biologically plausible, which support a causal hypothesis. Physical inactivity also has a dose-related, temporally consistent, and biologically plausible relationship to the same health outcomes as those as for obesity,(2) and both obesity and inactivity have similar patterns of association with clinical risk indicators such as blood pressure, fasting plasma glucose, and inflammatory markers.(3-5) Furthermore, declines in average daily energy expenditure are a likely underlying cause of the obesity epidemic.
3) However, the majority of studies examining obesity and health have not adequately accounted for physical activity. When physical activity has been considered, investigators have often relied on simple self-report questionnaires in which inaccuracy can increase proportionally with the respondent's weight. Failure to adequately quantify physical activity when examining the risks of obesity is similar to exploring risk factors for cancer and misclassifying tobacco use. Physical activity and weight are closely linked and each must be measured accurately and considered carefully when examining the other.
4) Obese individuals with at least moderate cardiorespiratory fitness (CRF) have lower rates of cardiovascular disease (CVD) or all-cause mortality than their normal-weight but unfit peers. In fact, death rates in the former group are about one half those of the latter. These results are similar whether BMI, percent body fat, body fat mass, or waist circumference is used as the body habitus measure and are consistent for women and men and for men with type 2 diabetes. These results also pertain to nonsmokers and after exclusion of individuals with chronic disease or deaths within the first 5 years of follow-up. Moreover, 150 minutes a week of moderate intensity physical activity is sufficient to avoid the low-fitness category. These results are consistent with those reported in 24 studies identified in a systematic review on physical activity, CRF, obesity, and health.
References (abridged):
1. National Institutes of Health, National Heart, Lung, and Blood Institute. Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults: The Evidence Report. Rockville, Md: National Institutes of Health, National Heart, Lung, and Blood Institute; 1998:1-228
2. Physical Activity and Health: A Report of the Surgeon General. Atlanta, Ga: US Dept of Health and Human Services, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion; 1996
3. Church TS, Barlow CE, Earnest CP, et al. Associations between cardiorespiratory fitness and C-reactive protein in men. Arterioscler Thromb Vasc Biol. 2002;22:1869-1876
4. Fagard RH. Physical activity in the prevention and treatment of hypertension in the obese. Med Sci Sports Exerc. 1999;31:S624-S630.
5. Wei M, Gibbons LW, Mitchell TL, et al. The association between cardiorespiratory fitness and impaired fasting glucose and type 2 diabetes mellitus in men. Ann Intern Med. 1999;130:89-96
J. Am. Med. Assoc. http://www.jama.com
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Related Material:
ON CHILDHOOD OBESITY
The following points are made by Albert P. Rocchini (New Engl. J. Med. 2002 346:854):
1) Childhood obesity has reached epidemic proportions. Worldwide, approximately 22 million children under 5 years of age are overweight, and during the past 3 decades, the number of overweight children in the US has more than doubled. In 1983, 18.6 percent of preschool children in the US were defined as overweight, and 8.5 percent were defined as obese. By the year 2000, 22 percent of preschool children were overweight and 10 percent were obese. Data indicate that the prevalence of overweight has increased by 21.5 among non-Hispanic black children, 21.8 percent among Hispanic children, and 12.3 percent among non-Hispanic white children. Similar increases in the prevalence of obesity have been observed worldwide, and childhood obesity is the most serious and prevalent nutritional disorder in the US.
2) Obesity has a substantial effect on cardiovascular risk. Childhood obesity is directly linked to abnormalities in blood pressure, lipid, lipoprotein, and insulin levels in adults, as well as to the risk of both coronary artery disease and diabetes. It has been documented that 80 percent of obese adolescents have elevated systolic blood pressure, diastolic blood pressure, or both. Furthermore, 97 percent of such adolescents have 4 or more of the following cardiovascular risk factors: elevated serum triglyceride levels (more than 100 milligrams per deciliter), low levels of high-density lipoprotein cholesterol, increased total cholesterol levels (more than 200 milligrams per deciliter), elevated systolic blood pressure, diastolic blood pressure, or both, diminished maximal oxygen consumption, and a strong history in the immediate family of coronary heart disease, myocardial infarction, angina pectoris, or high blood pressure.
New Engl. J. Med. http://www.nejm.org
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TELEVISION WATCHING AND OTHER SEDENTARY BEHAVIORS IN RELATION TO RISK OF OBESITY AND TYPE 2 DIABETES MELLITUS IN WOMEN
The following points are made by F.B. Hu et al (J. Am. Med. Assoc. 2003 289:1785):
1) Current public health campaigns to reduce obesity and type 2 diabetes have largely focused on increasing exercise levels, but have paid little attention to the reduction of sedentary behaviors. Television (TV) watching is a major sedentary behavior in the US. In a survey conducted in 1997, an adult male spent approximately 29 hours per week watching TV, and an adult female spent 34 hours per week.(1) In recent decades, in parallel with increasing obesity, there has been a steady increase in the number of homes with multiple TV sets, videocassette recorders (VCRs), cable TV, and remote controls, as well as the number of hours spent watching TV.(1)
2) Compared with other sedentary activities such as sewing, playing board games, reading, writing, and driving a car, TV watching results in a lower metabolic rate.(2) Constant exposure to food advertising leads to increased food and calorie intake and unhealthy eating patterns.(3-5) It is well established that prolonged TV watching is associated with obesity in children. However, the role of TV watching compared with other sedentary behaviors, such as sitting at work or reading, in the development of obesity and type 2 diabetes among adults has not been well studied, especially among women.
3) The authors examined prospectively the relationship between several common sedentary behaviors and incidence of obesity and type 2 diabetes in a large cohort of women. A prospective cohort study was conducted from 1992 to 1998 among women from 11 states in the Nurses' Health Study. The obesity analysis included 50,277 women who had a body mass index (BMI) of less than 30 and were free from diagnosed cardiovascular disease, diabetes, or cancer and completed questions on physical activity and sedentary behaviors at baseline. The diabetes analysis included 68,497 women who at baseline were free from diagnosed diabetes mellitus, cardiovascular disease, or cancer.
4) The authors conclude: Independent of exercise levels, sedentary behaviors, especially TV watching, were associated with significantly elevated risk of obesity and type 2 diabetes, whereas even light to moderate activity was associated with substantially lower risk. The authors suggest this study emphasizes the importance of reducing prolonged TV watching and other sedentary behaviors for preventing obesity and diabetes.
References (abridged):
1. Nielsen Report on Television. New York, NY: Nielsen Media Research; 1998
2. Ainsworth BE, Haksell WL, Leon AS, et al. Compendium of physical activities. Med Sci Sports Exerc. 1993;25:71-80
3. Lank NH, Vickery CE, Cotugna N, Shade DD. Food commercials during television soap operas. J Community Health. 1992;17:377-384
4. Dietz WH, Gortmaker SL Jr. Do we fatten our children at the television set? Pediatrics. 1985;75:807-812
5. Hu FB, Leitzmann MF, Stampfer MJ, Colditz GA, Willett WC, Rimm EB. Physical activity and television watching in relation to risk for type 2 diabetes mellitus in men. Arch Intern Med. 2001;161:1542-1548
J. Am. Med. Assoc. http://www.jama.com
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