Governments and the medical profession frequently warn that being ‘overweight’ or ‘obese’ puts us in mortal danger, and encourage us to conforms to weight norms defined by a measure known as the body mass index (BMI). The BMI is calculated by dividing someone’s weight in kilograms by the square of their height in metres (kg/m2). According to conventional wisdom, a BMI of 18.5 – 24.9 is regarded as ‘normal’ or ‘healthy’, while BMIs of 25.0-29.9 are considered ‘overweight’, and those of 30.0 or more are classified as ‘obese’. While the BMI usually forms the basis of the advice health professionals give to individuals about their weight, does the advice stand up to scientific scrutiny?
When considering the relationship between BMI and health, it pays to take as wide a view as possible. The way to do this is to assess its relationship, not with the risk of individual conditions, but with overall risk of death. Ultimately, risk of death is 100 per cent, of course. In studies ‘risk of death’ (sometimes termed ‘overall mortality’ or just ‘mortality’) refers to likelihood of death over a specified period of time (usually many years).
A study just published in the Journal of the American Medical Association should cause us to consider whether making health recommendations based on the current bands makes good sense. In this study, US researchers amassed evidence from 97 studies which assessed the relationship between BMI and overall mortality. The studies taken together involved 2.88 million people and a total of 270,000 deaths. This is the biggest ever study of its kind.
Individuals with a BMI of 18.5-30) relative risk of death was 1.18 (an 18 per cent increase compared to those of ‘normal’ weight). However, the researchers also looked at those whose were ‘mildly obese’, as defined by BMIs of 30.0 –
In, short, this evidence suggests that on a population basis, the BMI band that Governments and health professionals should be urging us to be in is the one that goes from 25.0 –
Some have suggested though that these results are skewed by two basic facts:
1. smoking overall tends to shorten life and smokers tend to be lighter in weight
2. when people get sick they can lose weight before they die
The idea here is that the presence of smokers and individuals with long-term illness in these studies makes being a bit heavier look healthier than it is in reality.
The authors of this latest study address this and say this:
“The results presented herein provide little support for the suggestion that smoking and preexisting illness are important causes of bias. Most studies that addressed the issue found that adjustments or exclusions for these factors had little or no effect.”
We should not be too surprised by these results, though, as time and again, previous evidence has found pretty much the same thing. One thing that has come out of the research is that the relationship between BMI and mortality is influenced, to some degree, by age. In one study , being ‘overweight’ was associated with an enhanced risk of death in individuals under 60 years of age, but not after this. Other studies have yielded findings that support the idea that as we age, being technically ‘overweight’ is not a concern in itself, and may in fact signal superior survival.
For instance, in a study of Japanese men and women aged 65 years and older, having an overweight BMI was not associated with an enhanced risk of death, and this was even true for men technically classified as obese . In another study, lowest mortality in was found in older Norwegian men and women with BMIs of 25.0-
One theory that has been put forward to explain why larger BMIs are associated with improved survival in the elderly is that some surplus fat can be used as a store of energy which individuals can draw on in times of need – such as during a critical illness. In one study, researchers assessed the relationship between ‘fat mass’ (overall levels of fat in the body), and risk of illness and mortality in individuals aged 65 and older . Over time, compared to those with the lowest fat masses, individuals with most fat had a 70 per cent reduced risk of mortality. These findings provide support for the idea that decent fat stores can come in handy in later life.
Also, it’s a pain a simple fact that BMI tells us nothing about body composition, so that individuals of big build who are quite heavily muscled and maybe very healthy and robust are penalised if judged by weight or BMI alone.
Overall, the evidence suggests that the hazards of being overweight (and possibly even obese) have been overstated. It seems some individuals may have less need to lose weight on health grounds than conventional wisdom dictates. This applies particularly to elderly individuals, as well as those who are of ‘big build’ and relatively muscular.
Also, even if someone is carry extra fat, the distribution of that fat appears to be important in terms of its likely impact on health. ‘Visceral’ fat (fat within the abdomen and organs) appears to be particularly toxic, and a decent proxy for this is an expanded waist size.
1. Flegal KM, et al. Association of All-Cause Mortality With Overweight and Obesity Using Standard Body Mass Index CategoriesA Systematic Review and Meta-analysis JAMA 2013;309(1)
2. Berrington de Gonzalez A, et al. Body-mass index and mortality among 1.46 million white adults. N Engl J Med. 2010;363(23):2211-9
3. Tamakoshi A, et al. BMI and all-cause mortality among Japanese older adults: findings from the Japan collaborative cohort study. Obesity (Silver Spring). 2010;18(2):362-9
4. Kvamme JM, et al. Body mass index and mortality in elderly men and women: the Tromso and HUNT studies. J Epidemiol Community Health. 2011 Feb 14. [Epub ahead of print]
5. Flicker L, et al. Body Mass Index and Survival in Men and Women Aged 70 to 75. Journal of the American Geriatrics Society 2010;58(2): 234-241
6. Bouillanne O, et al. Fat mass protects hospitalized elderly persons against morbidity and mortality. Am J Clin Nutr 2009;90(3):505-10