Obesity has been associated with physical disabilities at least since the days of Shakespeare when he penned the line "Falstaff sweats to death, and lards the lean earth." Yet, it was not until the 1950s that life insurance companies began to recognize that obesity was an independent risk factor for death and diminished life expectancy and began to price policies accordingly. Since the 1950s, a multitude of epidemiologic studies have shown that being underweight, overweight, obese, or morbidly obese causes significant health problems and affects human longevity. These associations are complex, and whenever possible, the effects of age, gender, race, smoking status, and preexisting disease must be factored in to understand of the role of body mass index (BMI) on mortality. Understanding the impact of obesity on health and mortality is of interest for both medical science and public health.

Older studies found little or no association with being overweight and the increased risk of death. However, recent studies have shown that the risk of death from all causes is increased at or above the BMI of 28 (overweight BMI ranges from 26-30). This is true for men and women in all age groups. The risk of death increases as the BMI rises.

However, these studies did not address smoking and chronic disease as independent causes of mortality in obese populations. It was thought that the connection between overweight individuals and death risk was lessened partially due to improved medical management of chronic disease such as coronary heart disease, cardiovascular disease, and Type 2 diabetes mellitus. When these confounding variables are accounted for in the analysis, the higher risk of death for patients with a BMI over 28 still remains. However, it is important to realize that eliminating preexisting disease from a study ignores the fact that some chronic disease is the direct result of the health-diminishing effects caused by increased adiposity.

Because smoking and preexisting disease are predictive of low BMI and poor survival, selecting out individuals with smoking history and/or preexisting disease at the onset of a long-term study strengthens the association between being overweight and death risk. he risk of death is increased by 20 to 40 percent in the overweight category when compared to similar ideal weight patients when smoking and preexisting diseases are accounted for. When analyzed in this way, excess weight accounts for 18.1 percent of premature deaths in men and 18.7 percent of premature deaths in women. If individuals with any smoking status and with any amount of preexisting or chronic disease are included, the impact of excess weight is seemingly diminished. Analysis in this way shows that excess weight accounts for 7.7 percent of premature deaths in men and 11.7 percent of premature deaths in women. Further clarification of these associations by eliminating confounding variables will help establish more accurate guidelines for BMI that are tailored to the individual and not to humans in general.

It is estimated that 280,000 excess deaths in the United States annually can be attributed to obesity and morbid obesity when compared to cohorts who have normal BMI. he obese category (BMI 30-35) alone is estimated to contribute to nearly 112,000 of those deaths. It is thought that the impact of obesity on mortality may actually have decreased over time with improvements in public health and medical care. Even so, as the prevalence of obesity in the United States continues to increase, so does the concern for the understanding of the association between obesity and mortality.

Obesity is associated with an increased risk of death for individuals of any age, but the relative risk actually lessens as one gets older. For example, obese men with a BMI ranging from 30-31.9 have relative risks of death of 1.62 (30-64 years old), 1.42 (65-74 years old), and 1.16 (75 years and older). For women of the same BMI range, the relative risks of death are 1.51 (30-64 years old), 1.32 (65-74 years old), and 1.25 (75 years and older). he lowest risk of death for any age is generally within the normal range of BMI.

Another way to relate age and obesity to mortality is to follow the effects of elevated BMI at the age of 50. Excess weight at midlife has a strong influence on decreased life expectancy. Individuals who were overweight at the age of 50 have a risk of death that is 20 to 40 percent higher than those who are normal weight. Those who were obese or morbidly obese at the age of 50 have two to three times and three to four times the risk of early death, respectively.

Gender also influences the risk of death due excess weight. The BMI ranges associated with the lowest overall risk of mortality are higher for men. The optimal BMI range for men is 23.5-24.9 and 22.0-23.4 for women. Despite this, men tend to have a higher risk of mortality than women for above-normal ranges of BMI regardless of race or ethnicity.

Race and ethnicity play an important role in the outcomes of excess weight on mortality. The associations between obesity and mortality risk are slightly stronger among Hispanic, Asian, Pacific Islander, and Native-American men and women than white or black men and women. This is true whether or not smoking and preexisting disease are eliminated from a study population. Whether these issues are culturally related or genetically determined has yet to be proven.

Mortality seems to be less influenced by elevated BMI in black men (relative risk of death is 1.35) than for white men (relative risk of death is 1.66) for BMI ranges from 32 to 34.9. This fact is true regardless of smoking history or disease status.

With a BMI of 35 or higher, black women have a 20 to 30 percent increased risk of death, while the same BMI group of white women have a 75 to 100 percent increase risk of death. It is thought that this is due to the tendency of black women to have a more peripheral distribution of fat than white women. This distribution has been shown to be less associated with atherogenic risk factors such as increased levels of cholesterol, triglycerides, and degree of peripheral insulin resistance.

Another example of this is seen when comparing Asian and Western populations. Asians generally have higher percentages of body fat for the same BMI levels when compared to Western populations. On average, an Asian's BMI is lower by 1.9-3.2 for the same percentage of body fat. This is usually attributed to Asians having a more slender build and less muscle mass and connective tissue. It is because of these differences that the World Health Organization has recommended that cutoff values for the overweight and obese categories of BMI be lower for Asian populations than for Western populations.

The relationship between excess body fat, early death, and risk of developing comorbidities has been supported by numerous studies. The medical risks of overweight, obese, and morbidly obese persons include cardiovascular disease, coronary heart disease, pulmonary disease, hypertension, Type 2 diabetes, and cancer. The mechanisms for disease that link excess body fat and the risk of death include lipid abnormalities, hormonal alterations, insulin resistance, glucose intolerance, and chronic inflammation.

A high BMI is most predictive of death from cardiovascular disease. This is a great concern when considering that cardiovascular disease and coronary heart disease are leading causes of death within most industrialized countries. This is especially true for men whose risk is nearly three times that of men within the normal category of BMI. Even a BMI as low as 26.5 for men and 25.0 for women puts an individual at increased risk of mortality from cardiovascular causes.

The risk of cancer-related death increases 40 to 80 percent for morbidly obese men and women. Current estimates attribute 14 percent of all U.S. deaths from cancer in men and 20 percent of those in women to elevations in BMI above normal. Eliminating those who have a history of smoking strengthens the relationship between obesity and cancer death in both men and women. There is no relationship between low BMI and increased risk of cancer death, highlighting the fact that obesity is the major cause of numerous cancer deaths in the United States.

In men, positive increases in death rates are associated with BMI for all cancers (relative risk: 1.52 for BMI at least 40), as well as liver cancer (4.52 for at least 35), pancreatic cancer (2.61 for at least 35), stomach cancer (1.94 for at least 35), esophageal cancer (1.91 for at least 30), colorectal cancer (1.84 for at least 35), gallbladder cancer (1.76 for at least 30), multiple myeloma (1.71 for at least 35), kidney cancer (1.70 for at least 35), non-Hodgkin's lymphoma (1.49 for at least 35), prostate cancer (1.34 for at least 35), and all other cancers.

In women, positive increases in death rates are associated with BMI for all cancers (relative risk: 1.88 for BMI at least 40), uterine cancer (6.25 for at least 40), kidney cancer (4.75 for at least 40), cervical cancer (3.20 for at least 35), pancreatic cancer (2.76 for at least 40), esophageal cancer (2.64 for at least 30), gallbladder cancer (2.13 for at least 30), breast cancer (2.12 for at least 40), non-Hodgkin's lymphoma (1.95 for at least 35), liver cancer (1.68 for at least 35), ovarian cancer (1.51 for at least 35), colorectal cancer (1.46 for at least 40), multiple myeloma (1.44 for at least 35), and all other cancers (1.88 for at least 40).

Being overweight (BMI at least 28) or obese elevates mortality risk and decreases longevity. As BMI increases, this risk can worsen. While all ethnicities are affected, for some such as African-American and Hispanic, the implications of obesity can be worse. In order to combat these trends, it is important that individuals take steps to maintain healthy weights, and treat current obesities and comorbid conditions that arise.

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