Roughly 40 percent of the world's adult population was overweight in 2014, the last time the World Health Organization checked. Thirteen percent was obese — at least 20 percent above a healthy body weight — and therefore at increased risk of heart disease, diabetes, stroke and cancer, among other serious health problems.
Common knowledge says most of these people ate too much and moved too little. A recent international study suggests movement may have had little to do with it.
In one of the latest efforts to explore the link between sedentary lifestyle and weight gain, researchers couldn't find one.
The study, published in the journal PeerJ on Jan. 19, followed 1,944 people from five countries for two years, tracking weight gain against baseline activity levels, and found no correlation at all. Loyola University announced the counterintuitive findings in early February, noting they fly in the face of the popular theory that inactivity, "especially in the workplace, has been a key contributor to the obesity epidemic."
While genetics, pharmaceuticals and diseases such as hypothyroidism and Cushing's syndrome play into the obesity picture, an overabundance of energy is the main factor. When people ingest more food energy (calories) than they spend through basic metabolic processes (organ functions), digestion and physical activity, the extra energy is stored as fat.
Lead author Dr. Lara R. Dugas, assistant professor of public health sciences at Loyola Chicago's Stritch School of Medicine, says the study results were surprising.
"In-line with popular rhetoric, we had expected that more physically active individuals at baseline would be more protected from future weight gain," Dugas writes in an email.
At the start of the study, each subject wore an accelerometer, a device that detects motion and velocity, around his or her waist for one week, establishing a baseline physical-activity level — what Dugas describes as "daily, habitual" movement, or a subject's typical energy expenditure in a day. The study then tracked subjects' weights for two years.
Final data revealed that U.S. men and Jamaican women experienced the least yearly weight change, South African men and Ghanaian women experienced the most, subjects who began the study overweight gained less than those who began at a healthy weight, and none of it correlated with baseline activity levels.
"We also examined our data by whether people met the US Surgeon General Guidelines for PA [physical activity]" — at least 150 minutes of moderate-intensity movement per week for adults (and one hour a day for children) — "and again found no relationship for weight change between those who met the guidelines compared to those who didn't," Dugas writes.
Age, baseline weight and sex were the only factors predicting whether a subject gained or lost weight over the study period.
A Complex Process
The findings add to a growing body of evidence suggesting obesity isn't what it seems. Over the last decade, studies have linked childhood obesity to maternal Caesarean section and gestational diabetes. A 2016 study found that rats exposed to Beijing's air pollution gained weight without any increase in food intake. In 2012, scientists discovered a virus that appears to make the carrier simultaneously more likely to become obese and, oddly, less likely to become diabetic.
Research has identified possible roles for inflammation, depression and insomnia, too.
Not everyone is onboard with the new perspective, writes California-based physician Valerie Ulene, on the L.A. Times website. Many medical professionals "believe that obesity is caused almost exclusively by an unhealthful diet and a lack of exercise. In their minds, it's a problem that people inflict upon themselves that could be solved if patients were motivated enough."
Twenty-two percent of tomorrow's doctors feel similarly, according to a 2015 survey of thousands of fourth-year medical students at 50 U.S. universities.
A Controversial Body of Research
There's an argument to be made for inactivity driving weight gain. Research has uncovered strong links between obesity and sedentary lifestyle. It's possible, though, that weight gain promotes inactivity, rather than the other way around, by triggering changes in brain chemistry that reduce the desire to move.
Complicating factors aside, Dugas agrees it does come down to diet and exercise. When it comes to weight gain, though, she thinks a lot of people are looking at the wrong side of the energy equation. The level of physical activity required to significantly impact weight just isn't possible for most people.
"The number of minutes you will need to exercise to burn the required caloric deficit required for weight loss is approximately 1 hr/day ... Few people find the time to exercise for 30 mins/day, let alone 1 hr every day," Dugas writes.
Plus, "when people begin exercise training they may experience changes in their appetite, to a point where they are working against these physiological cues," she adds.
Other studies — though notably not the ones funded by Coca-Cola — have come to similar conclusions: It's more about what people are eating.
"The significant increase in sugar sweetened beverage consumption since the 1980s may be a factor in the current obesity epidemic...," writes Dugas. She thinks a widespread soda tax could actually make a difference in the obesity rate. But the legislation often fails under the weight of beverage-industry lobbying, and some prominent health nonprofits pulled their support for the tax after receiving grants from Coke or Pepsi.
Instead, writes Dugas, "people are just being told to exercise more."