Bee Thao

ANS 419

19 APR 2005

Desirable Body Weight: Body Weight and Mortality

Americans have become more obese in the past century (1,2). Countering this increase in

size and weight has been the weight consciousness of recent years, which has led to a focus on

various weight reduction methods. The focus on weight may largely be attributed to the media

and a result of changing aesthetic values; however, scientists have begun to ascertain the effects

of body mass on human life expectancy and morbidity. Scientists have been researching the

effects of body weight on human life expectancy since the 1970s. However, research has not yet

found clear indications of a desirable body weight beyond the knowledge that a very high body

mass index may shorten life expectancy and increase morbidity (1,2,3).

Research has shown that body weight and composition have a substantial effect on

quality of life as well. Though the quantification of quality of life is quite difficult, many studies

utilize the risk of morbidity and affected health rates to determine quality. Most researchers also

utilize body mass index (BMI), where the BMI is body weight in kilograms divided by the

square of height in meters. Within many societies, average desirable BMI ranges from 19-24

depending upon gender and ethnicity. For instance, the National Institutes of Health sets the

desirable, healthy BMI in the United States between 18.5 and 24.9 (2). Within children, any

deviation from this range leads to progressively slower growth and maturation in lower ranges.

With adolescents and adults, there is an increased risk for cancer as well as various obesity

related disorders in the higher ranges (2,4,5,6). Du et al clarified the effects of low body weight

within Chinese ethnicity with a study of the bone health and pubertal maturation in a sample of

1,214 Chinese girls 12-14 years of age (5). Their findings indicated that, when compared to

average Chinese girls of BMI 18-21, Chinese girls with BMI below 18 had less bone mass,

decreased sexual development, and stunted growth. The effects of greater than average BMI

have also been studied. Utilizing a cohort of 155, 989 adults, Heo et al found a J-shaped

association between BMI and health-related quality of life, defined as the detrimental loss of

quality due to time spent recovering from illness or disease (6). The J-shaped curve was such

that the relative risk of morbidity in Americans begins to increase at BMI greater than 25 with

similar risk found below 18.5; morbidity was highly associated with BMI at levels higher than

35. Decreased health related quality of life in higher BMI individuals was found to be caused by

obesity and the related health problems such as joint pain, diabetes, cardiovascular disease and

their associated increases in risk of mortality. Heo et al also suggested that impaired quality of

life at lower BMI is due to the effects of subclinical diseases and deleterious behaviors such as

smoking, dieting and excessive repetitive motions from some physical activities and sports.

While these studies do not specifically mention mortality, the risk of death increases with

increasing occurrence of disease, especially cardiovascular disease (2).

The Caerphilly Prospective Study illustrates the effect of BMI on morbidity, and thus

mortality (7). In this study, 2,335 men from South Wales were surveyed between 1979 and

1983. Using the recollected BMI of the men at 18 years of age, their BMI at the time of the

survey, and their BMI at a follow up after 14 years, Yarnell et al found that at 18 years of age,

BMI of 30 or higher increased the relative risk by 29% for mortality and 73% for major ischemic

heart disease by the age of 45. Yarnell et al also concluded that substantial weight gain in men

from 18 years of age to middle age increases the risk of cardiovascular disease in middle age; but

men with moderate weight gain, approximately 16 kg, had the lowest rate of mortality.

Lev-Ran, however, found that even though the world population is accumulating more

fat, and while obesity carries increased risk for mortality, the average lifespan has increased from

50 years of age in 1900 to 77 years of age in the present day (2). Even though obesity has only

become an issue in the past ten years, Lev-Ran continues to argue that after 50 years of age,

increased BMI may have little or no effect on mortality of any cause. This statement is echoed

in other research, which suggests that increased relative risk may only be associated with obesity

in younger subjects (7,8). In a study utilizing mortality data over 12 years from the American

Cancer Society’s Cancer Prevention Study I, from 1960 through 1972, Stevens et al found a

positive association between BMI and mortality up to 75 years of age, such that the elderly, aged

65 and older, with BMI between 29.0 to 31.9 had among the lowest relative risk of mortality (8).

They concluded that “the relative risk associated with greater body mass index declined with

age”.

It appears that in the elderly, age becomes the most important factor (2). Though clear

explanations could not be given, questions are raised as to the effect of obesity related illness.

For example, obesity can lead to diabetes in middle aged adults, but there is not much

information on the effects of obesity in the elderly. Regardless, age is considered a confounder

in some studies and should be discounted when considering the effect of BMI itself on mortality

(4,6,9). Even if age is not corrected for, its effects are ambiguous. For example, Ajani et al

specifically challenge the findings of Stevens et al regarding an association between increased

age, BMI and mortality (9). Utilizing 85,078 men aged 40 to 84 from the Physician’s Health

Study cohort with 5 years of follow-up, Ajani et al observed increased relative risk of mortality

in the elderly with greater BMI, concluding that BMI directly affects mortality, regardless of age.

However, Ajani et al utilize absolute mortality rates. Considering the current life expectancy of

most Americans is between 75 and 85 years of age, it is not unexpected for these subjects to die

during a study, thus the study needs to be corrected.

With the associations corrected for age, gender, ethnicity, and other confounding factors,

research points to a J-shaped curve (1,2,6,9) or U-shaped curve for BMI versus all-cause

mortality (1,2,8,9). These curves suggest those lean and obese individuals, who have BMI less

than 18 and greater than 30 respectively, are at increased risk for mortality of all causes. Other

research finds that BMI is linearly and positively associated with mortality, even at low BMI

(1,4,7,9). These findings suggest an ambiguity in risk at lower BMI levels, most likely as a

result of confounders such as smoking and physical activity levels. In regards to smoking, the

Caerphilly Prospective Study found that the cessation of smoking caused weight gain; thus

smoking can be attributed to decreased BMI and the increased relative risk of death from

smoking related illness must also be discounted from BMI mortality studies (7). Deleterious

effects are also attributed to damage caused on the musculoskeletal structure from physical

activity and metabolic stress from yo-yo dieting (2).

In any case, current research suggests that a desirable weight is that weight which will

provide a BMI within the range of 19-24, supplemented with a healthy, active lifestyle (10).

Unfortunately, further research must be undertaken to provide the optimal BMI for maximum

life expectancy. Future research must also consider confounding factors, such as smoking, age,

gender, and ethnicity. These should be corrected for through their elimination from ideal studies.

Research may find that there is essentially an optimal BMI for an individual that must account

for the individual’s physical activity, smoking and eating habits, age, body composition, and

ethnic background. Even within a specific ethnic background, optimal levels may vary.

References

1. Samaras, TT, Storms, LH, Elrick, H. 2002. Longevity, mortality and body weight. Aging

Research Reviews. 1: 673-691.

2. Lev-Ran, A. 2001. Human obesity: an evolutionary approach to understanding our bulging

waistline. Diabetes Metab Res Rev. 17: 347-362.

3. Fang, J, Wylie-Rosett, J, Cohen, HW, Kaplan, RC, Alderman, MH. 2003. Exercise, body

mass index, caloric intake, and cardiovascular mortality. Am J Prev Med. 25(4): 283-289.

4. Jeffreys, M, McCarron, P, Gunnell, D, McEwen, J, Smith GD. 2003. Body mass index in

early and mid-adulthood, and subsequent mortality: a historical cohort study. International

Journal of Obesity. 27: 1391-1397.

5. Du, X, Greenfield, H, Fraser, DR, Ge, K, Zheng W, Huang, L, Liu, Z. 2003. Low body

weight and its association with bone health and pubertal maturation in Chinese girls. European

Journal of Clinical Nutrition. 57: 693-700.

6. Heo, M, Allison, DB, Faith, MS, Zhu, S, Fontaine, KR. 2003. Obesity and Quality of Life:

Mediating Effects of Pain and Comorbidities. Obesity Research. 11(2): 209-216.

7. Yarnell, JWG, Patterson, CC, Thomas, HF, Sweetnam, PM. 2000. Comparisons of weight in

middle age, weight at 18 years, and weight change between, in prediction subsequent 14 year

mortality and coronary events: Caerphilly Prospective Study. Epidemiol Community Health.

54: 344-348.

8. Stevens, J, Cai J, Pamuk, ER, Wialliamson, DF, Thun, MJ, Wood, JL. 1998. The effect of

age on the association between body mass index and mortality. New England Journal of

Medicine. 338(1): 1-7.

9. Ajani, UA, Lotufo, PA, Gaziano, JM, Lee, I, Spelsberg, A, Buring, JE, Willet, WC, Manson,

JE. 2004. Body mass index and mortality among US male physicians. Ann Epidemiol. 14(10):

731-739.

10. Jacobs, PR Jr, Pereira, MA. 2004. Physical activity, relative body weight, and risk of death

among women. New England Journal of Medice. 351(26): 2694-703.