As concern about the health implications of the obesity epidemic has increased one frequently mentioned claim is that the current generation will be the first to live shorter lives than their parents. The assumption is that the effects of obesity on the risk for chronic disease are so significant that life expectancy will inevitably fall. But new evidence suggests that this assumption may be wrong.

New meta-analysis

A new meta-analysis of the effects of overweight and obesity on all-cause mortality was published this month in the Journal of the American Medical Association. The analysis included data from 97 studies from around the world, providing a combined sample size of more than 2.88 million subjects and more than 270,000 deaths.

The risk of death of all obese subjects (BMI>30) was substantially and significantly higher (18%) than that of subjects of normal weight (BMI 18.5-25) – the sort of finding that we have come to expect. However, the results told a different and perplexing story when narrower weight categories were considered. For example, those who were overweight (BMI 25-30) experienced 6 per cent lower risk of mortality than subjects of normal weight. Subjects with grade 1 obesity (BMI 30-35) had the same mortality risk as subjects of normal weight. Consequently, the mortality risk associated with obesity appeared to be due entirely to higher grades of obesity – BMI greater than 35, which was associated with a 29% increase in risk.

How can it be?

Previously, a J-shaped relationship between body weight and mortality has been observed i.e. very low body weight was linked with higher risk; normal body weight was associated with the lowest risk; and mortality risk climbed again as body weight increased above the normal range. However, the latest study suggests that we may be looking at a long, drawn out J shape with low mortality risk associated with a broad range of body weights, including normal weight, overweight and grade 1 obesity.

But how come? Isn’t obesity supposed to be associated with increased risk for type 2 diabetes, coronary heart disease, stroke and some cancers? Maybe relying on BMI alone is just not accurate enough. As highlighted in the accompanying editorial, BMI is crude measure of health status. People with the same BMI can have widely differing metabolic health depending on their fitness, the amount of fat on their bodies and, importantly, the distribution of that fat. Waist circumference has been suggested as a better measure of risk than BMI as it is more likely to capture central obesity and its associated risks.

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An obesity paradox?

More challengingly, the editorial suggests that there appears to be an ‘obesity paradox’ i.e. a protective effect of overweight or grade 1 obesity in old age and in the presence of chronic conditions, such as heart disease and diabetes. They speculate that small excess amounts of adipose tissue may provide energy reserves needed during acute catabolic illnesses, have beneficial mechanical effects with some types of traumatic injuries and convey other effects that need to be investigated. The implication is that it can’t be assumed that the life expectancy of all overweight subjects, even those with chronic disease, will increase with weight loss. Another paradox – that’s just what we need!

What about morbidity?

Another recent study found no association between overweight, obesity and mortality though it was a different story when the researchers looked at the risk for disability. Being overweight was associated with a 33% increase in the risk for becoming disabled and the risk was doubled for those with obesity. Furthermore, recovery from disability was negatively associated with obesity.

So, although being overweight may not kill you, it could make you sick.