Physical Activity, Relative Body Weight, and Risk of Death among Women
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《新英格兰医药杂志》
In 1949, the late Ancel Keys warned of a possible epidemic of obesity.1 Keys wrote that under the right economic and social circumstances, obesity from overeating would be a dominant nutritional problem. He further recognized the role of energy expenditure in weight control: "While our calorie intake goes up our output goes down. The wonderful advances of technology not merely free us from back-breaking toil; they make it almost impossible to get a decent amount of calorie-using exercise."2
Long before the observation by Lee et al.3 that fit men with a high body-mass index could be healthy, Keys focused clearly on the inadequacy of gross weight as a measure of "overfatness." He felt that gross body weight and the Metropolitan Life Insurance Company guidelines for relative weight are only crude approximations of the "ideal" body composition, since the size of a person's frame can affect energy needs, and gross measures ignore body composition. Furthermore, Keys observed that the difficult clinical and public health problem of fatness and body composition does not reside in the very fat, who are easily identified by clinical acumen and the assessment of disease states.4 Rather, the important issue exists for those in the middle range of relative weight, where the mix of muscle and fat is highly varied among persons.4 In current terms, such a middle range might include people with a body-mass index (the weight in kilograms divided by the square of the height in meters) of 23 to 27.
With the use of data from 24 years of follow-up in a study of more than 100,000 nurses, Hu et al.5 present elegant analyses in this issue of the Journal that address both old1,2,4 and recent3 concerns in this area. The authors come to a basic and helpful conclusion in terms of public health: be fit and lean if you can be. Hu and colleagues contribute to an understanding of the person who is fit but has a high body-mass index by substituting physical activity for fitness in their analyses and concluding that increased levels of physical activity and a low body-mass index are independently associated with reduced mortality rates among the women involved in the Nurses' Health Study. Neither component alone sufficed in the Hu study,5 in contrast to the Lee study,3 which found that a high body-mass index conveyed no extra risk in the most fit people. Thus, neither being active and fat nor being sedentary and thin was associated with as good an outcome as being active and thin. Hu et al. acknowledge that there may be muscular women with a high body-mass index who are quite fit and active and whose risk of death is similar to that for women who have a low body-mass index and are fit and active; however, considering that this subgroup must be quite small, the findings carry less public health significance.
Several comments are relevant to the interpretation of this article. First, imprecision in the measurement of physical activity may have caused some misinterpretation of the data. A group of fit women with a high body-mass index would be harder to identify with the use of a self-reported physical-activity questionnaire than with a treadmill test; in addition, men are physiologically more inclined toward muscularity than are women, and younger women may find it culturally acceptable to train to a high degree of muscularity, so such characteristics might be more common among men and younger people than among older women. A more difficult issue is the possibility that overweight subjects understated their weights and overstated their level of physical activity. These biases would tend to cause an underestimation of the influence of physical activity on the reduction of both fatness and the risk of death. Also, physical activity of light-to-moderate intensity, which constitutes the bulk of the energy expenditure in most people's lives, was not included in the analysis by Hu et al. This omission might also result in the understatement of the true influence of physical activity on the risk of death. That is, with the levels of moderate and vigorous physical activity held constant, one would expect to see a broad range in the amount of energy expended during low-intensity activity and in time spent being inactive (e.g., watching television) — a hypothesis supported by studies in a variety of populations.6,7
Second, weight loss was associated with an increased risk of death; the nature of the weight loss was not specified, and, presumably, the result is confounded by the combination of unintentional weight loss due to disease and intentional weight loss (which was probably beneficial). Furthermore, the survey about physical activity was conducted after the period in which the weight had changed. Perhaps the responses about physical activity were differentially misstated because of wishful thinking by individual subjects that they were exercising enough to combat weight gain.
Third, we need to heed the concept put forth by Keys1,2,4 that body-mass index is a gross measure that does not tell the whole story of fatness. Indeed, we believe that researchers in the field are only now scratching the surface in regard to the physiology of fat and muscle as it relates to human health and disease. It is clear that, if the body-mass index is held constant, the amount of muscle, fat, and fat distribution around the body will vary considerably among individual persons.8,9 Although excessive visceral fat in the intraabdominal region has direct effects on liver metabolism, which lead to fatty liver, insulin resistance, and elevated risks of cardiovascular disease and type 2 diabetes,9,10 the role of subcutaneous fat depots is less clear. In fact, there is accumulating evidence that certain subcutaneous fat depots may confer protection against diabetes and cardiovascular disease through a variety of plausible mechanisms, such as the provision of a "sink" for the clearance of circulating fatty acids or differences in the secretion of inflammatory mediators between subcutaneous and visceral fat stores.8,11,12 As new research accumulates along these lines, the scientific community should surely revisit guidelines and definitions of anthropometric characteristics as they relate to the promotion of health. In addition, we should note that, for cultural reasons, what is considered obese in one population may not be considered as such in another, and for genetic or other physiological reasons, the body-mass index may not have the same association with fatness in all ethnic groups.
Finally, we should keep in mind that body weight and fat regulation are the products of an extremely complicated system that includes a large number of complex interactions of genes and the environment. Physical activity clearly resides among the environmental factors, and objectively assessed physical fitness involves some combination of genetics and behavior. Therein may lie a major source of the discrepancy between the results of the study by Hu et al. on physical activity in women and those of the previous study by Lee et al.3 on physical fitness in men. We should also keep in mind that the overall findings by Hu and colleagues need to be considered in the context of this very complex system that governs the regulation of body weight. That is, one would not expect the level of physical activity, particularly that which is subjectively recalled with at most a moderate level of validity in the best hands, to explain more than a moderate amount of the effect of body-mass index on the risk of death.
There remains much to learn about the associations among physical activity, fitness, and fatness in regard to the risk of chronic disease. Nevertheless, from a public health perspective, the real challenge we face in our current "obesogenic" environment is the promotion of a healthy lifestyle, including ample physical activity, a prudent, balanced diet, moderate or no alcohol consumption, and the avoidance of drugs and tobacco.
Dr. Jacobs reports having received grant support from General Mills, and Dr. Pereira consulting fees from the Breakfast Advisory Board, the Florida Citrus Council, and the Dairy Weight Management Board.
Source Information
From the Division of Epidemiology and Community Health, School of Public Health, University of Minnesota, Minneapolis (D.R.J., M.A.P.); and the Department of Nutrition, University of Oslo, Oslo (D.R.J.).
References
Keys A. The caloric requirement of adult man. Nutr Abstr Rev 1949;19:1-10.
Keys A. The management of obesity. Minn Med 1965;48:1329-1331.
Lee CD, Blair SN, Jackson AS. Cardiorespiratory fitness, body composition, and all-cause and cardiovascular disease mortality in men. Am J Clin Nutr 1999;69:373-380.
Keys A. Energy requirements of adults. JAMA 1950;142:333-339.
Hu FB, Willett WC, Li T, Stampfer MJ, Colditz GA, Manson JE. Adiposity as compared with physical activity in predicting mortality among women. N Engl J Med 2004;351:2694-2703.
Fitzgerald SJ, Kriska AM, Pereira MA, de Courten MP. Associations among physical activity, television watching, and obesity in adult Pima Indians. Med Sci Sports Exerc 1997;29:910-915.
Kronenberg F, Pereira MA, Schmitz MKH, et al. Influence of leisure time physical activity and television watching on atherosclerosis risk factors in the NHLBI Family Heart Study. Atherosclerosis 2000;153:433-443.
Thamer C, Machann J, Haap M, et al. Intrahepatic lipids are predicted by visceral adipose tissue mass in healthy subjects. Diabetes Care 2004;27:2726-2729.
Snijder MB, Zimmet PZ, Visser M, Dekker JM, Seidell JC, Shaw JE. Independent association of hip circumference with metabolic profile in different ethnic groups. Obes Res 2004;12:1370-1374.
Cnop M, Landchild MJ, Vidal J, et al. The concurrent accumulation of intra-abdominal and subcutaneous fat explains the association between insulin resistance and plasma leptin concentrations: distinct metabolic effects of two fat compartments. Diabetes 2002;51:1005-1015.
Van Pelt RE, Evans EM, Schechtman KB, Ehsani AA, Kohrt WM. Contributions of total and regional fat mass to risk for cardiovascular disease in older women. Am J Physiol Endocrinol Metab 2002;282:E1023-E1028.
Tanko LB, Bagger YZ, Alexandersen P, Larsen PJ, Christiansen C. Peripheral adiposity exhibits an independent dominant antiatherogenic effect in elderly women. Circulation 2003;107:1626-1631.(David R. Jacobs, Jr., Ph.)
Long before the observation by Lee et al.3 that fit men with a high body-mass index could be healthy, Keys focused clearly on the inadequacy of gross weight as a measure of "overfatness." He felt that gross body weight and the Metropolitan Life Insurance Company guidelines for relative weight are only crude approximations of the "ideal" body composition, since the size of a person's frame can affect energy needs, and gross measures ignore body composition. Furthermore, Keys observed that the difficult clinical and public health problem of fatness and body composition does not reside in the very fat, who are easily identified by clinical acumen and the assessment of disease states.4 Rather, the important issue exists for those in the middle range of relative weight, where the mix of muscle and fat is highly varied among persons.4 In current terms, such a middle range might include people with a body-mass index (the weight in kilograms divided by the square of the height in meters) of 23 to 27.
With the use of data from 24 years of follow-up in a study of more than 100,000 nurses, Hu et al.5 present elegant analyses in this issue of the Journal that address both old1,2,4 and recent3 concerns in this area. The authors come to a basic and helpful conclusion in terms of public health: be fit and lean if you can be. Hu and colleagues contribute to an understanding of the person who is fit but has a high body-mass index by substituting physical activity for fitness in their analyses and concluding that increased levels of physical activity and a low body-mass index are independently associated with reduced mortality rates among the women involved in the Nurses' Health Study. Neither component alone sufficed in the Hu study,5 in contrast to the Lee study,3 which found that a high body-mass index conveyed no extra risk in the most fit people. Thus, neither being active and fat nor being sedentary and thin was associated with as good an outcome as being active and thin. Hu et al. acknowledge that there may be muscular women with a high body-mass index who are quite fit and active and whose risk of death is similar to that for women who have a low body-mass index and are fit and active; however, considering that this subgroup must be quite small, the findings carry less public health significance.
Several comments are relevant to the interpretation of this article. First, imprecision in the measurement of physical activity may have caused some misinterpretation of the data. A group of fit women with a high body-mass index would be harder to identify with the use of a self-reported physical-activity questionnaire than with a treadmill test; in addition, men are physiologically more inclined toward muscularity than are women, and younger women may find it culturally acceptable to train to a high degree of muscularity, so such characteristics might be more common among men and younger people than among older women. A more difficult issue is the possibility that overweight subjects understated their weights and overstated their level of physical activity. These biases would tend to cause an underestimation of the influence of physical activity on the reduction of both fatness and the risk of death. Also, physical activity of light-to-moderate intensity, which constitutes the bulk of the energy expenditure in most people's lives, was not included in the analysis by Hu et al. This omission might also result in the understatement of the true influence of physical activity on the risk of death. That is, with the levels of moderate and vigorous physical activity held constant, one would expect to see a broad range in the amount of energy expended during low-intensity activity and in time spent being inactive (e.g., watching television) — a hypothesis supported by studies in a variety of populations.6,7
Second, weight loss was associated with an increased risk of death; the nature of the weight loss was not specified, and, presumably, the result is confounded by the combination of unintentional weight loss due to disease and intentional weight loss (which was probably beneficial). Furthermore, the survey about physical activity was conducted after the period in which the weight had changed. Perhaps the responses about physical activity were differentially misstated because of wishful thinking by individual subjects that they were exercising enough to combat weight gain.
Third, we need to heed the concept put forth by Keys1,2,4 that body-mass index is a gross measure that does not tell the whole story of fatness. Indeed, we believe that researchers in the field are only now scratching the surface in regard to the physiology of fat and muscle as it relates to human health and disease. It is clear that, if the body-mass index is held constant, the amount of muscle, fat, and fat distribution around the body will vary considerably among individual persons.8,9 Although excessive visceral fat in the intraabdominal region has direct effects on liver metabolism, which lead to fatty liver, insulin resistance, and elevated risks of cardiovascular disease and type 2 diabetes,9,10 the role of subcutaneous fat depots is less clear. In fact, there is accumulating evidence that certain subcutaneous fat depots may confer protection against diabetes and cardiovascular disease through a variety of plausible mechanisms, such as the provision of a "sink" for the clearance of circulating fatty acids or differences in the secretion of inflammatory mediators between subcutaneous and visceral fat stores.8,11,12 As new research accumulates along these lines, the scientific community should surely revisit guidelines and definitions of anthropometric characteristics as they relate to the promotion of health. In addition, we should note that, for cultural reasons, what is considered obese in one population may not be considered as such in another, and for genetic or other physiological reasons, the body-mass index may not have the same association with fatness in all ethnic groups.
Finally, we should keep in mind that body weight and fat regulation are the products of an extremely complicated system that includes a large number of complex interactions of genes and the environment. Physical activity clearly resides among the environmental factors, and objectively assessed physical fitness involves some combination of genetics and behavior. Therein may lie a major source of the discrepancy between the results of the study by Hu et al. on physical activity in women and those of the previous study by Lee et al.3 on physical fitness in men. We should also keep in mind that the overall findings by Hu and colleagues need to be considered in the context of this very complex system that governs the regulation of body weight. That is, one would not expect the level of physical activity, particularly that which is subjectively recalled with at most a moderate level of validity in the best hands, to explain more than a moderate amount of the effect of body-mass index on the risk of death.
There remains much to learn about the associations among physical activity, fitness, and fatness in regard to the risk of chronic disease. Nevertheless, from a public health perspective, the real challenge we face in our current "obesogenic" environment is the promotion of a healthy lifestyle, including ample physical activity, a prudent, balanced diet, moderate or no alcohol consumption, and the avoidance of drugs and tobacco.
Dr. Jacobs reports having received grant support from General Mills, and Dr. Pereira consulting fees from the Breakfast Advisory Board, the Florida Citrus Council, and the Dairy Weight Management Board.
Source Information
From the Division of Epidemiology and Community Health, School of Public Health, University of Minnesota, Minneapolis (D.R.J., M.A.P.); and the Department of Nutrition, University of Oslo, Oslo (D.R.J.).
References
Keys A. The caloric requirement of adult man. Nutr Abstr Rev 1949;19:1-10.
Keys A. The management of obesity. Minn Med 1965;48:1329-1331.
Lee CD, Blair SN, Jackson AS. Cardiorespiratory fitness, body composition, and all-cause and cardiovascular disease mortality in men. Am J Clin Nutr 1999;69:373-380.
Keys A. Energy requirements of adults. JAMA 1950;142:333-339.
Hu FB, Willett WC, Li T, Stampfer MJ, Colditz GA, Manson JE. Adiposity as compared with physical activity in predicting mortality among women. N Engl J Med 2004;351:2694-2703.
Fitzgerald SJ, Kriska AM, Pereira MA, de Courten MP. Associations among physical activity, television watching, and obesity in adult Pima Indians. Med Sci Sports Exerc 1997;29:910-915.
Kronenberg F, Pereira MA, Schmitz MKH, et al. Influence of leisure time physical activity and television watching on atherosclerosis risk factors in the NHLBI Family Heart Study. Atherosclerosis 2000;153:433-443.
Thamer C, Machann J, Haap M, et al. Intrahepatic lipids are predicted by visceral adipose tissue mass in healthy subjects. Diabetes Care 2004;27:2726-2729.
Snijder MB, Zimmet PZ, Visser M, Dekker JM, Seidell JC, Shaw JE. Independent association of hip circumference with metabolic profile in different ethnic groups. Obes Res 2004;12:1370-1374.
Cnop M, Landchild MJ, Vidal J, et al. The concurrent accumulation of intra-abdominal and subcutaneous fat explains the association between insulin resistance and plasma leptin concentrations: distinct metabolic effects of two fat compartments. Diabetes 2002;51:1005-1015.
Van Pelt RE, Evans EM, Schechtman KB, Ehsani AA, Kohrt WM. Contributions of total and regional fat mass to risk for cardiovascular disease in older women. Am J Physiol Endocrinol Metab 2002;282:E1023-E1028.
Tanko LB, Bagger YZ, Alexandersen P, Larsen PJ, Christiansen C. Peripheral adiposity exhibits an independent dominant antiatherogenic effect in elderly women. Circulation 2003;107:1626-1631.(David R. Jacobs, Jr., Ph.)