Knowledge for better food systems

Largest meta-analysis on BMI and mortality shows clear relationship between weight and risk of death

Photo: Steve Baker, Obesity, Flickr, creative commons licence 2.0

The Global BMI Mortality Collaboration presents results in the Lancet from the largest pooled dataset on the relationship between weight and mortality rates. The analysis shows that both overweight and obesity in otherwise healthy participants were associated with increased mortality from all causes.

The research includes individual-participant data for 10.6 million adults in 239 prospective cohort studies in 32 countries, mainly in Asia, Australia and New Zealand, Europe, and North America. About 4 million of these were (at the start of the study) never-smokers without reported chronic diseases (e.g. cardiovascular disease, cancer, or chronic respiratory disease).

The study uses standardised methods to extract so-called hazard ratios (HRs) for mortality across the studies (a hazard ratio of 2, for example, means that someone in this population is twice as likely to die than someone in the control population).

Overweight and obesity at varying levels were associated with increased all-cause mortality with hazard ratios between 1.07 for mildly overweight to 2.76 for grade 3 obesity, those whose BMI is over 40. The relationship of BMI (when above the upper threshold of normal at 25) to mortality was strong and positive in every global region studied. The effect was much greater at younger rather than older ages and more apparent in men than in women. The results challenge some recent suggestions that overweight and moderate obesity may not be associated with higher mortality. Interestingly, there was also an association found between underweight and increased mortality (BMI below 18.5) as well as for those who are normally considered healthy and “lean” with a BMI between 18.5 and 20.

The authors point out that their study is unique in that they use data from across four continents with participants from a range of ages, which has never before been done on this topic, and that this therefore provides information relevant to international strategies for overweight and obesity. However, this approach can also be seen as a weakness of the work: pooling data across diverse racial and ethnic groups, across decades of research and across countries with very different healthcare systems and management of chronic disease management, means that these conclusions cannot be drawn on to design public health guidance.

Note that a recent much smaller, country- and age specific study found contrasting results. This study, using data from a well-defined Dutch population with long-term follow-up, calculated life expectancy for individuals (>55 years) who were normal weight, overweight, and obese and the difference in years lived with and without diabetes. Obesity was associated with an increased risk of developing diabetes but not associated with mortality in men and women with or without diabetes. The study was conducted recently, in a Western European country with universal access to high quality medical care in a European ethnic population. Over the years, there have been major improvements in the prevention and treatment of cardiometabolic risk factors, so in this population overweight and obesity may no longer affect life expectancy, as the Lancet study above suggests. Note however that that diabetes is a very expensive illness to manage, and places enormous burdens on health care services as well as on the individuals affected.

Abstract

Background

Overweight and obesity are increasing worldwide. To help assess their relevance to mortality in different populations we conducted individual-participant data meta-analyses of prospective studies of body-mass index (BMI), limiting confounding and reverse causality by restricting analyses to never-smokers and excluding pre-existing disease and the first 5 years of follow-up.

Methods

Of 10,625,411 participants in Asia, Australia and New Zealand, Europe, and North America from 239 prospective studies (median follow-up 13·7 years, IQR 11·4–14·7), 3,951,455 people in 189 studies were never-smokers without chronic diseases at recruitment who survived 5 years, of whom 385,879 died. The primary analyses are of these deaths, and study, age, and sex adjusted hazard ratios (HRs), relative to BMI 22·5–<25·0 kg/m2.

Findings

All-cause mortality was minimal at 20·0–25·0 kg/m2 (HR 1·00, 95% CI 0·98–1·02 for BMI 20·0–<22·5 kg/m2; 1·00, 0·99–1·01 for BMI 22·5–<25·0 kg/m2), and increased significantly both just below this range (1·13, 1·09–1·17 for BMI 18·5–<20·0 kg/m2; 1·51, 1·43–1·59 for BMI 15·0–<18·5) and throughout the overweight range (1·07, 1·07–1·08 for BMI 25·0–<27·5 kg/m2; 1·20, 1·18–1·22 for BMI 27·5–<30·0 kg/m2). The HR for obesity grade 1 (BMI 30·0–<35·0 kg/m2) was 1·45, 95% CI 1·41–1·48; the HR for obesity grade 2 (35·0–<40·0 kg/m2) was 1·94, 1·87–2·01; and the HR for obesity grade 3 (40·0–<60·0 kg/m2) was 2·76, 2·60–2·92. For BMI over 25·0 kg/m2, mortality increased approximately log-linearly with BMI; the HR per 5 kg/m2 units higher BMI was 1·39 (1·34–1·43) in Europe, 1·29 (1·26–1·32) in North America, 1·39 (1·34–1·44) in east Asia, and 1·31 (1·27–1·35) in Australia and New Zealand. This HR per 5 kg/m2 units higher BMI (for BMI over 25 kg/m2) was greater in younger than older people (1·52, 95% CI 1·47–1·56, for BMI measured at 35–49 years vs 1·21, 1·17–1·25, for BMI measured at 70–89 years; pheterogeneity<0·0001), greater in men than women (1·51, 1·46–1·56, vs 1·30, 1·26–1·33; pheterogeneity<0·0001), but similar in studies with self-reported and measured BMI.

Interpretation

The associations of both overweight and obesity with higher all-cause mortality were broadly consistent in four continents. This finding supports strategies to combat the entire spectrum of excess adiposity in many populations.

Citation

Global BMI Mortality Collaboration. (2016). Body-mass index and all-cause mortality: individual-participant-data meta-analysis of 239 prospective studies in four continents. The Lancet.

You can find the Lancet study here (open access) and a commentary in the same journal here.

You can read related research by browsing the following categories of our research library: Health issues, and through the keyword categories Obesity/overweight; Health concerns; Diabetes; Global Health.

A paper in the research library which discusses the link of dairy and meat to diabetes 2 can be found here and a link to an entire series on obesity by the Lancet here. For more information about obesity and other forms of malnutrition see Chapter 7 of Foodsource.

You can read related research by browsing the following categories of our research library:
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