Study reveals ideal weight for staying healthy
International researchers have collected data from a staggering 30 million people to find out what weight you should keep to stay healthy.
BMI—or body mass index—almost everyone has seen or heard about it in the news, but what does it really indicate about our health and health risks?
Dagfinn Aune, a PhD student at NTNU, set out to find some answers. Together with a team of Norwegian, British and Indian researchers, he has carried out the largest meta-analysis of research on BMI and mortality to date.
Aune and his colleagues summarized the data from 230 studies that involved a total of 30 million participants.
“We went through all the studies published in the field, and gathered them into a giant meta-analysis. That gives us much clearer answers than if we just looked at single studies,” Aune tells forskning.no.
Smile-shaped curve
We know that obesity can have unfortunate consequences.
A BMI score approaching 30 increases the risk significantly for many diseases, such as diabetes and cardiovascular disease. A BMI of 30 corresponds to someone who is 170 centimetres tall and weighs 87 kilograms, for example.
But it's not healthy to be too thin, either. Having a BMI below 18.5 means you’re underweight and also at risk.
Almost all studies of BMI and premature death risk show the same smile-shaped curve: the risk of death increases for the way too thin and the way too fat, while those in the middle do best.
But just where is the bottom of that smile – the weight range with the lowest risk of mortality?
Obesity paradox
At this point, scientists can't seem to agree.
Many studies show that the BMI with the lowest risk is approximately halfway through the normal weight range – which extends from BMI 18.5 to 25. At 170 centimetres, that puts your weight somewhere between 55 and 70 kilograms.
But paradoxically, some large studies are showing that this normal BMI is actually not the best weight to prevent premature death.
In 2013, the JAMA published a huge study with nearly three million participants. It showed that it was safest to be overweight with a BMI between 25 and 30.
Does extra weight actually have a protective effect?
Not everyone is convinced.
The 2013 study had some weaknesses. For example, the researchers excluded a great many participants who potentially could have been in the study. They have also been criticized for not taking sufficient account of factors that could interfere with the results.
On 10 May 2016, JAMA announced another study, in which Danish researchers concluded that the most favourable BMI quite simply seems to have increased over the last 30 years.
Studies that began in the 1970s show an optimal BMI of 24, while studies from the 2000s show that the safest option to be a BMI around 27 – well into the obesity range.
The authors behind the JAMA study ask whether we should perhaps change our definitions for overweight and normal weight.
But Aune and his colleagues believe there is little reason to doubt the prevailing categories.
Meta-analysis suggests that normal weight is best
Their study shows that the safest weight is still within the normal weight range. People who had never smoked had the lowest risk of mortality at BMI 23 to 24.
This aligns well with a previous large meta-analysis published in the Lancet in 2009.
BMI and the risk of mortality in people who never smoked. (Illustration: Aune et al/BMJ.)
Professor Jøran Hjelmesæth at the University of Oslo and the Morbid Obesity Centre at Vestfold Hospital has not been involved in the new study. He believes Aune’s results are solid.
“This is a top-notch study,” he tells forskning.no. “It strengthens the existing knowledge that the risk of mortality grows with increasing BMI.”
Findings primarily linked to obesity
However, it is worth noting that the smile curve is quite flat at the bottom. Although the safest BMI is 23 to 24, the risk does not increase much just below and above that level.
Aune says a slight increase in risk exists for overweight individuals, but the correlation with obesity is stronger.
Hjelmesæth believes experience and other studies confirm this, especially BMIs closer to 35 and 40.
“Healthy eating is the best thing you can do if you’re overweight, along with preventing further weight gain and being physically active. That can reduce the increased risk from overweight to almost nothing,” he says.
Cause and effect still not fully clear
Hjelmesæth points out that a lot of uncertainty still exists in this kind of research.
“These studies can’t tell us anything about cause and effect. The fact that increased BMI is associated with higher mortality, doesn’t mean that one is the cause of the other,” he says.
That said, there is still reason to believe that heavy weight may increase the risk of mortality.
Aune says that many other studies have examined the underlying mechanisms.
“Overweight and obesity are linked to high cholesterol, triglycerides, blood pressure, low-grade inflammation, insulin resistance and hormonal changes that may explain the increased risk of heart disease, stroke and some cancers,” he adds.
Interfering factors
Another issue is that the results of observational studies – such as Aune’s – can be skewed by other factors.
So much can affect both weight and the risk of dying, like diet, physical activity, education and social status.
Simply put: If you’ve eaten too much junk food and become obese, both the poor diet and the obesity lead to a higher mortality risk. It can be difficult to know how much is due to one or the other.
And what happens to smokers?
Smoking increases the risk of illness and death significantly, it but also affects weight. Smokers often weigh less than non-smokers but are more often sick.
Smokers and non-smokers separated
Since smoking is so closely linked to mortality and weight, Aune and his colleagues separated smokers from non-smokers to conduct their analyses. They believe that by limiting the variables in this way, they could gain a more accurate picture of the impact of overweight and obesity.
This also reduces a strange effect that recurs in some BMI and mortality risk data, where the slimmest normal weight individuals appear to have at least as high a mortality risk as people with obesity. Having a BMI of 18.5 seems to be as bad as having a BMI of 35.
Is it dangerous to be slim?
When all participants are included, it may seem as if the mortality rate rises sharply in the lower range of normal weight – when approaching a BMI of 18.5. A BMI of 20 would thus imply a higher mortality risk than a BMI over 30. (Illustration: Aune et al / BMJ)
Scientists can’t fully explain why the data shows up this way. But smoking plays a role. Smokers are often leaner than non-smokers, but may have a much higher risk of mortality due to disease.
Heart failure and certain cancers, for example, may cause both weight loss and earlier death. Some neurological and lung diseases and diseases can cause weight loss long before the patient has a clear diagnosis.
Some participants might have already had one of these diseases when they joined the study, without knowing it. Some of the analyses in the survey indicate that such a disease may partially explain the apparent upturn in mortality for the very slim.
Neither Aune nor Hjelmesæth think the data suggests that low normal weight in itself causes a higher mortality risk.
“You can find thin people who have unhealthy diets, aren’t physically active and smoke a lot. But then it's not the BMI that causes an early death, but the other factors,” says Hjelmesæth.
“If you eat healthy, stay active and maintain a BMI of 19 or 20, you don’t have to worry,” he says.
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Read the Norwegian version of this article at forskning.no
