A recent study published in JAMA determined that overweight and obesity was responsible for 25,814 deaths in the United States in 2000. The study, conducted by a team of researchers from the Centers for Disease Control and Prevention (CDC) and the National Institutes of Health (NIH), stands in stark contrast to the CDC’s overblown and mathematically flawed estimate of 400,000 deaths per year.
This new study finds that putting on a few extra pounds is not a lethal mistake. The researchers found no link between being overweight (a Body-Mass Index or BMI between 25 and 29.9) and an increased risk of death. In fact, it indicates that being overweight may actually be safer than being a “normal weight.”
The authors found that 111,909 people die from obesity, as opposed to overweight. But when they add their obesity and overweight deaths, they conclude: “…for overweight and obesity combined, our estimate was 25,814 excess deaths.” So according to the JAMA article, overweight in America saves 86,094 lives a year.
The vast majority of deaths associated with obesity (82,066 of 111,909) come from individuals with a BMI of 35 and above. According to The New York Times’ Gina Kolata, just 8 percent of the population has this BMI level. Meanwhile, the government continues to warn that 65 percent of Americans weigh too much — those with a BMI of 25 and above.
An internal review committee at the CDC that looked into the 400,000-deaths estimate concluded: “While there was at least one error in the calculations and both the presentation of the paper and limitations of the approach could have been expressed more clearly, the fundamental scientific problem centers around the limitations in both the data and the methodology in this area.” This new JAMA study is substantially superior to the CDC’s previous estimate, both with respect to data and methodology.
- Better Data — Despite having access to data as recent as 2002, the CDC based its 400,000-deaths conclusion on a number of studies that began as long ago as 1948. The average start date was 1963. Since then, we’ve seen substantial improvements in medical technology that reduces the risk of obesity, as well as sustained increases in life expectancy. As the authors of the recent JAMA study note:
The impact of obesity on mortality may have decreased over time, perhaps because of improvements in public health and medical care. These findings are consistent with the increases in life expectancy in the United States and the declining mortality rates from ischemic heart disease.
To further complicate matters, the original CDC report of 400,000 deaths used data from a number of studies that were not representative of the entire U.S. population. The authors of the recent JAMA study write:
Previous estimates used data from a variety of studies to estimate relative risks, but the studies had some limitations. Four of the 6 included only older data (2 studies ended follow-ups in the 1970s and 2 in the 1980s), 3 had only self reported weight and height, 3 had data only from small geographic areas, and 1 study included only women. Only one dataset, the National Health and Nutrition Examination Survey (NHANES) I, was nationally representative.
To address these substantial problems, the JAMA authors relied on the CDC’s own National Health And Nutrition Examination Surveys (NHANES). Unlike the previous estimate’s data, which was partially drawn from the NHANES I follow-up study (1971-1992), the recent study includes data from the entire set of NHANES surveys, including data from as recently as 2002. Not surprisingly, the newer data show mortality risks declining significantly. In fact, the most recent NHANES III data show no increased risk among those with a BMI under 35:
Excess deaths associated with obesity (BMI >30) were calculated as 298,808 according to the NHANES I relative risks, 26,917 according to the NHANES II relative risks, or 43,640 according to the NHANES III relative risks. In all 3 cases, however, the majority of deaths associated with obesity were associated BMI 35 and above 186,498, 21,777, or 57,515 deaths respectively. (NHANES III relative risks produced a negative estimate for BMI 30 to < 35) For overweight (BMI 25 to <30), the data consistently suggest no excess deaths overall ... the largest difference [in deaths] is due to the inclusion of the mortality data from NHANES II and NHANES III, which decreased estimates by 63% of more relative to NHANES I mortality data alone ... Relative to NHANES I, the more recent data from NHANES II and NHANES III suggest the possibility that improvements in medical care, particularly for cardiovascular disease, the leading cause of death among the obese, and its risk factors may have led to a decreased association of obesity with total mortality. Cardiovascular risk factors have declined at all BMI levels in the US population, but, except for diabetes, the decline appears to be greater at higher BMI levels.
- Better Methodology — Before it was even published, the CDC’s method for counting obesity-related deaths came under fire both inside and outside the agency. These criticisms related in part to the failure of the CDC’s original estimate to considers age as an important factor. Since older people have a lower risk of death from obesity, failing to account for this would lead to an overestimation of deaths. Months before the CDC’s 400,000-deaths announcement was made, three of the authors of the recent JAMA study had submitted for publication papers that both pointed to this problem and offered a solution. Writing in the American Journal of Epidemiology, they argued:
Existing estimates of the number of deaths attributed to overweight and obesity were calculated by using a method likely to produce biased estimates, when the effects of obesity vary by age or other characteristics. Estimates of deaths attributable to overweight and obesity arrived at by using this approach may be biased and should be viewed cautiously.
Another problem with the methodology of the CDC’s 400,000-deaths study is that it failed to report a level of uncertainty in their conclusion. As the authors of the updated study point out, because the risk of obesity is low but the number of obese people is high, minor fluctuations in the risk can result in huge differences in the mortality estimate:
Obesity is associated with a modestly increased relative risk of mortality, often in the range of 1 to 2. In this range, estimates of attributable fractions, and thus numbers of deaths, are very sensitive to minor changes in relative risk estimates. Thus, results are affected by the precision and bias in relative risk estimates.
An editorial accompanying the recent JAMA study reinforces the point:
An important and possibly overlooked contribution by the study by Flegal et al is the formal calculation of confidence intervals around the estimate of obesity-related deaths. When relative risk estimates are only modestly elevated, as in the case of obesity, very small changes in the relative risk translate to large differences in the population-attributable fraction. Thus, it should come as no surprise that the 95% confidence interval around the estimate of 112,000 deaths ranges from 54,000 to 170,000, greater than a 3-fold difference reflected within the range. Although the other studies that previously estimated obesity-attributable death did not include confidence intervals, the estimates from those studies should be assumed to have underlying uncertainty at least as great.
The quote above refers to confidence intervals, which are a way to measure statistical significance. If the range of the confidence interval includes the result of no increased risk, then there is no statistically significant increased risk. Keeping this in mind, there are a few very interesting results:
- The conclusion that being overweight is safer than being a normal weight is statistically significant. The 86,000 lives saved from overweight have confidence intervals that go from 161,000 lives saved to 11,000 lives saved.
- As noted above, there is no risk of BMI 30-35 in the most recent data set.
- The final conclusion that overweight and obesity combined causes excess deaths is not statistically significant. The number of 25,814 deaths from overweight and obesity combined has a range from 86,284 lives saved to 137,913 lives lost.