Wednesday January 2 2013
Being overweight, but not obese, lengthened life expectancy
“Being overweight can extend life rather than shorten it,” is the headline in The Independent.
This and related headlines come from a large review of previous research that found that those categorised as overweight were around 6% less likely to have died by the end of a study than those of a healthy weight.
Having a body mass index (BMI) of between 30 and 35 (medically termed ‘obese’) causes more deaths, but people whose BMI was higher than 35 were 29% more likely to die by the end of the study than their normal-weight counterparts.
It is widely known that BMI is an imperfect measure of body fatness (or ‘plumpness’ as one headline puts it) and a predictor of death or disease. BMI does not account for many important weight-related measures linked to death and disease risk such as differing fat levels, fat distribution, muscularity, nutritional balance and others.
Hence, this finding is not as startling as it first appears and only tells us half the story of how fatness and risk of death are related.
The bottom line from this study was that being obese (all categories combined) increased the chance of dying compared to those with a normal BMI, although this was not the case for overweight individuals (BMI of between 25 and 29) or the lowest category of obesity (grade 1) on its own.
However, a slight increase in lifespan doesn’t necessarily equate to an increased quality of life. Even being ‘just’ overweight can increase the chance of developing long-term health conditions, which while may not be fatal, can make life a lot less enjoyable.
Where did the story come from?
The study was carried out by a collaboration of researchers from US universities and the US government-funded National Centers for Disease Control and Prevention. There was no additional external funding for this research.
The study was published in the peer-reviewed The Journal of the American Medical Association.
The media coverage was generally accurate, with useful discussion of the potential explanations of the results. However, the important limitations of using BMI to estimate fatness were not given due prominence.
The headlines also did not make clear that the increase in life expectancy in people who were overweight or ‘mildly’ obese was modest – they were only 6% less likely to have died by the end of the study period than those of a healthy weight
What kind of research was this?
This was a systematic review aiming to pool and summarise previous research looking at the risk of dying in relation to the weight of an individual categorised using BMI.
BMI is a formula that uses a person’s height and weight to assess whether they are ‘normal weight’. A common misconception is that it directly measures fat levels, which it does not.
What did the research involve?
The researchers searched electronic databases for articles that reported the hazard ratio (HR) of dying (all-cause mortality) across standard BMI categories from prospective studies of adults. The BMI categories used were as follows:
- Underweight: BMI <18.5
- Normal weight: BMI ≥18.5 and <25
- Overweight: BMI ≥25 and <30
- Obese (grade 1): BMI ≥30 and <34
- Obese (grade 2): BMI ≥35 and <40
- Obese (grade 3) :BMI ≥40 (often referred to as being morbidly obese)
Studies eligible for inclusion were selected by consensus among multiple reviewers. Data from identified studies were extracted by one reviewer, then checked by three others.
Studies in adolescents, or studies specifically conducted in people with medical conditions or undergoing medical procedures, were excluded. This was because these groups may not be representative of the population at large.
The statistical methods were appropriate and included a meta-analysis. Sub-analysis was performed for different age groups and for whether height and weight were measured or whether it was self reported. Researchers also factored in whether they deemed results to have adequately taken into account other risk factors including smoking, age and gender.
What were the basic results?
In total, 97 studies were analysed comprising data on more than 2.88 million people and more than 270,000 deaths.
Relative to normal weight, both obesity (all grades combined) and obesity grades 2 and 3 (BMI ≥35) were associated with a significantly higher death rate. All obesity grades combined increased death risk by 18%, whereas the more severe grades (2 and 3 combined) had a 29% increase in risk. Grade 1 obesity (BMI ≥30 and <34) was not associated with a significantly higher death rate.
Interestingly, the overweight group (BMI ≥25 and <30) had significantly lower death rates than the normal group (HR 0.94, 95% confidence interval (CI) 0.91 to 0.96). This translated into a 6% lower risk of dying compared to the normal group.
All the BMI categories were compared to the normal weight category: the main relative risk results were:
- Overweight: HR 0.94, 95% CI 0.91 to 0.96
- Obese (grade 1): HR 0.95, 95% CI 0.88 to 1.01
- Obese (grade 2 and 3 combined): HR 1.29, 95% CI 1.18 to 1.41
- Obese combined (grades 1-3 combined): HR 1.18, 95% CI 1.12 to 1.25
The results were similar when results were restricted to self-reported BMI versus measured BMI respectively. The same pattern was also seen in the subgroup of results deemed to be adequately adjusted for age, gender and smoking status.
How did the researchers interpret the results?
The researchers concluded that, relative to normal weight: “both obesity (all grades) and grades 2 and 3 obesity were associated with significantly higher all-cause mortality. Grade 1 obesity overall was not associated with higher mortality, and overweight was associated with significantly lower all-cause mortality”.
This systematic review provides high-quality evidence that obesity grades 2 and 3 are associated with higher death rates from any cause compared to normal weight individuals (around 30% increased risk). However, it also shows that lower grades of obesity (grade 1) do not increase the risk of death relative to normal-weight individuals and, in fact, overweight people had a small but significant reduction in their risk of death in the region of 6%.
The strengths of this review include the large number of studies it included and its standardised approach to searching and extracting data from the literature. Hence, we can be reasonably confident these results reflect reality.
However, a limitation of the study is that it only assessed the risk of dying from any cause (‘all-cause’ mortality), rather than death from specific diseases such as cancer, heart disease or diabetes. The association between weight and risk of death for different disease categories may vary. Disability and living with long-term diseases are also important to people and some conditions such as diabetes may show stronger links with weight at lower thresholds of BMI.
The review also crucially chose BMI as the measure of weight, which has been mistakenly taken by the media to mean an accurate measure of unhealthy ‘fatness’. BMI is a pragmatic choice to assess weight but it does not account for other known disease and death risk factors such as differing fat levels, fat distribution, muscularity, nutritional balance and others. BMI is also an imperfect measure of fatness as it only measures weight and height. Hence, those who are overweight are not all necessarily overweight because they are carrying excess fat. For instance, someone who is very muscular can have a high BMI and therefore be categorised as overweight.
Other measures such as waist circumference offer alternative ways of quickly estimating a person’s body fat levels and whether they are of a healthy weight. In practice, BMI is not the only measure used when establishing risk of disease or death. Doctors will be checking for a host of risk factors like high blood pressure, high cholesterol and high blood glucose levels in addition to BMI and/or waist circumference. Hence, the BMI category is only one of many measures used by a doctor to assess health, and by no means the best.
The finding that overweight individuals were at slightly less risk of dying than their normal-weight counterparts had been reported in research before (this is often referred to as the obesity paradox).
Possible explanations why a bit of extra weight may extend life include:
- People with more fat reserves to rely on may survive better if they lose weight due to ill health as they get older.
- Problems associated with being overweight (high blood pressure and diabetes) are picked up and treated earlier in overweight people compared with normal weight, as doctors are more vigilant of risk factors in overweight people. This treatment improves their health overall.
However, it is important to note that these are largely unsubstantiated theories and have not been further explored or proven.
The bottom line is that being obese (all categories combined) increased the chance of dying compared to those with a normal BMI. This was not the case for overweight individuals or the lowest category of obesity (grade 1) on its own.
It is important to consider that individual risk factors for developing disease and death will vary person to person and BMI is only one of many measures used to assess the risk of developing disease in the future.
Even if you choose to ignore the limitations of this research, it would be unwise to interpret its findings as proof that being overweight is ‘healthy’ – rather it may be slightly less unhealthy than perceived.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.