Being physically inactive is associated with twice the risk of dying prematurely than being obese, according to a study published last week in the American Journal of Clinical Nutrition by a multinational team of researchers.

Furthermore, even a small increase in physical activity among individuals who are currently sedentary — the equivalent of, say, a 20-minute brisk walk daily — may help lower their risk of premature death, whether they are obese or not, the study suggests.

A quick caveat: These findings shouldn’t be interpreted as meaning that obesity can be healthy as long as you exercise regularly. As the “Behind the Headlines” website of Great Britain’s National Health Service (NHS) points out in its discussion of the new study, “In practice it’s hard to disentangle the two [physical activity and weight], since exercise, along with diet, helps maintain a healthy weight. Also, obesity is an established risk factor for diseases such as type 2 diabetes, which is best tackled with a combination of diet and exercise.”

Study details

Plenty of previous studies have linked physical inactivity to an increased risk for various diseases and premature death, and other studies have done the same for obesity. The authors of this new study wanted to measure something more complex: the interaction between body weight and level of physical activity on early mortality.

For the study, the researchers analyzed data collected from more than 334,000 European adults aged 25 to 70 for an average of 12 years. All were part of a large research project known as the European Prospective Investigation into Cancer and Nutrition (EPIC) study. The data included the participants’ waist circumference and body mass index (two determinants of obesity), as well as self-reports of their physical activity levels, both at home and at work.

The analysis revealed that physical inactivity increased the risk of premature death by 7 percent, while being obese increased the risk by 4 percent.

The results held even after adjusting for sex, education, smoking and alcohol intake.

The greatest difference in risk was found when comparing the two groups with the lowest levels of activity — groups the researchers categorized as “inactive” and “moderately inactive.”

If inactive people would add just a modest amount of exercise to their daily routine — an activity (like a brisk 20-minute walk) that would burn between 90 and 100 calories — they would reduce their risk of premature death between 16 and 30 percent, the researchers estimated. The impact of that increased activity would be greatest on people of normal weight (BMI of 18.5-24.9), but even those with higher BMIs would see a benefit, they added.

Other findings

The researchers also calculated, using statistical modeling, that of the 9.2 million deaths that occurred in Europe in 2008, about 337,000 could be attributed to obesity and 676,000 to sedentariness.

Interestingly, the study also found that that having an “obese” waist circumference (40-plus inches in men and 35-plus inches in women) was more strongly associated with premature death than having an “obese” BMI (one equal to or greater than 30).

That finding is not all that surprising, however. Many experts have long argued that waist circumference is a much more accurate indicator of unhealthy levels of body fat than BMI. 

Strengths and weaknesses

As the NHS website points out, this observational study has several strengths, especially its large size and relatively long follow-up period. But it also has several limitations, including the fact that the participants’ BMI and physical activity were measured only once, at the start of the study.

And an observational study can show only a correlation between two things. It can’t prove that one thing caused another.

“It is quite possible that people’s BMI changed over time, and that this would have had an effect on mortality rates,” the NHS reviewers write. “For example, if physical activity helped reduce obesity over time, it is not possible to say that physical activity reduced the risk of mortality, independent of people’s weight.”

“It would be a bad idea to ignore the risks of obesity, whatever your levels of physical activity,” they warn. “Obesity is an established risk factor for a range of conditions such as diabetes and cardiovascular disease and it is best tackled by both diet and exercise.”

“But no one would argue with the notion that everyone should be encouraged to increase levels of physical activity, whatever their size,” they add.

You can read the study in full on the American Journal of Clinical Nutrition website.