The idea of an “optimal” weight has shifted around several times over the past few decades and numbers out of Denmark may be moving the post once again—into a range currently considered to be overweight.
From past research, it has been known that body mass index (BMI) has a U-shaped association with mortality. In other words, being underweight or overweight is known to raise death rate but the closer someone is to an ideal midpoint, the longer they live. Where that midpoint actually falls has shifted over the decades as studies are done and refined. This current bit of research may move the bar again, though the reason is still unknown.
Denmark BMI Study: Summary
- The study design was relatively simple: get about 100,000 people from the same part of Denmark (Copenhagen) as the studies from the 1970’s and 90’s, track them for ten years, and see which average BMI dies and when
- The BMI associated with the lowest death from cardiovascular causes in the 70’s was 23.2, in the 90’s it was 24, and in the recent study it was 26.4
- The BMI associated with the lowest death for all other causes of death in the 70’s was 24.1 in the 70’s, 26.8 in the 90’s, and 27.8 in the recent study
- Cancer was excluded from the study since patients typically lose significant weight during the course of the disease and it is capable of skewing findings
BMI is calculated by weight divided by height squared, but there are many handy calculators available that can be used. Going by World Health Organization guidelines, a BMI of 27.8 is in the middle of the ‘overweight’ category (25-29) and would translate to a 5’10” individual who weighs about 190 pounds. How much your BMI changes per-pound depends on a person’s height, but it generally works out to around a 1.5 BMI change for every 10 pounds gained or lost.
It is worth keeping in mind that a high BMI does not automatically mean someone is overweight or obese, since muscle is denser than fat. However, while BMI is not the best measure to tell if a single individual is overweight or obese, it can be generally relied on for population-level assessments.
What This Means
The correlation between a the higher BMI and living longer likely exists since these findings are also in line with other research that shows a certain protective effect to being slightly overweight, especially after middle-age. However, this does not mean the benefit is coming from the weight itself. It could be that living longer at these BMI levels is because being slightly overweight is connected to certain income levels or lifestyles that are the main contributor. Or this BMI range has some interaction with Denmark’s healthcare system that helps. The point to keep in mind is that there is a correlation but causation can’t be identified without more specific research.
It’s also important to keep in mind that these results may not be generalized well. As the researchers themselves note:
- The participants were all located in Copenhagen. People who lived in different countries, more rural or industrial areas, or climates could end up seeing alternate results
- The participants were also all white, which could affect how generalized the findings can be to other ethnicities. Asian and Indian individuals, for example, tend to show negative effects from weight gain at lower weights than other groups
- Being slightly overweight, going by this study and past research, may lead to a slightly longer lifespan
- The BMI level best suited for this, as well as who can actually benefit, is still unclear
- Also unclear is why this link exists. It could be the actual reason is associated with higher BMI but does not require higher BMI
Afzai, S., et. al., “Change in Body Mass Index Associated With Lowest Mortality in Denmark, 1976-2013,” JAMA, 2016; 10.1001/jama.2016.4666.