Most of the developing world is getting fatter, with a noteworthy uptick in BMI, the scale that researchers use to measure obesity, in 1980.
But despite the clear evidence that people are gaining weight, researchers still can’t say exactly why.
A popular hypothesis is that changes in our lifestyle, with more time in front of computers and watching television, have led us to exercise less.
In many ways, this idea makes sense. If we burn less energy because we’re lounging on the couch instead of out jogging, then all the food that might have been burned up by our activity should get turned into fat, right?
Some studies conclude that the overall decline in exercise may explain weight gain in the developed world over the past 30 years.
But a number of studies also present a classic chicken-and-egg problem: are people heavier because they are less active, or are they less active because they are fat?
Several large studies have not found any clear relationship between activity levels and being overweight or obese. Some research even raises doubts about something that we take for granted, which is that more physically active you are, the more calories you burn.
“I think a decline in physical activity is an important factor” in the obesity epidemic, says Sidsel Graff-Iversen, a senior researcher at the Norwegian Institute of Public Health.
Graff-Iversen cites the results of a recently published study in support of her opinion.
“The idea is that the body’s regulation of appetite and satiety stops working properly when our activity levels fall below a certain point. Maybe we need to be moderately to intensely active at least one to two hours every day to avoid becoming obese,” she said.
But Ulf Ekelund, a professor at the Norwegian School of Sport Sciences, disagrees.
“There is only a very weak correlation between physical activity and change in weight in the population,” he said. “Several factors come into play, but how much we eat is probably more significant than physical activity.”
So what does Norwegian research have to say about the relationship between weight and physical activity?
Fifteen years ago, Graff-Iversen and her colleagues studied information on weight and activity among older people in Norway, collected between 1974 and 1994.
The figures showed that the number of people with sedentary jobs increased during the same period that the average weight of the population increased. You might think this suggests that activity has a big impact on your waistline.
But the data they collected didn’t actually support this assumption.
Women with jobs that were sedentary or involved light physical work actually had lower BMIs than those who were more active throughout the period. The researchers found a similar pattern for men, but the trend was less clear.
Another Norwegian study from 2006 looked at information about people's physical activity at work and in their free time from 1972 to 2002.
“We found that a small group of women who were very active, both at work and in their free time, seemed to be protected from the weight gain that affected the general population during this period,” says Sigmund Anderssen of the Norwegian School of Sports Sciences, one of the researchers behind the study.
Women who are extremely active are equally thin today, while virtually all other groups are heavier now.
The figures also showed that those who were least active had higher average BMI both in 1972 and in 2002. This might suggest that exercise can prevent people from becoming overweight and obese.
But even here there were some confusing findings.
Instead of only inactive people gaining weight during this period, BMIs increased significantly in almost all groups, regardless of activity level. This was especially evident in men.
Both moderate and highly active men in 2002 had actually higher BMIs than the most sedentary men in 1974. So this means that even the couch potatoes of the 1970s were slimmer than active people today.
If we disregard the small group of very active women, it seems that physical activity has not protected people from gaining weight over time.
In short, the study makes it difficult to argue that lower amounts of physical activity are the main reason behind the obesity epidemic.
Fat construction workers, thin teachers
International studies of obesity in various professions also offer similar, confusing findings, where those with active professions, such as construction workers, were overweight, whereas those with more sedentary jobs, such as teachers, were not.
One American study published in 2013 with nearly four million subjects showed that even when people participated in an activity programme designed to prevent weight gain, they gained weight anyway.
Consequently, Ekelund from the Norwegian School of Sport Sciences believes there is little to suggest that decreased physical activity has been decisive in the increases in obesity over the last 30 years.
“The large and rigorous studies that have been done find little relationship between physical activity and weight gain over time,” he says.
Several researchers who have studied the obesity epidemic argue that food intake plays a much bigger role in the problem than how active we are.
There is a small problem with this assertion, however: little indicates that we have increased our food intake over the last three decades.
Unfortunately, there is a great deal of uncertainty in the research on people’s habits when it comes to food and exercise.
But if it should turn out that we do not actually eat more or less than before, and that physical activity does not play a very important role in weight regulation, why are we still getting steadily fatter?
Could there be an x-factor that is causing the obesity epidemic? Could something other than calories and activity be making us fatter, although we are not lazier or eating more than before?
The next issue of this series examines this exact question.