While many experts have offered that rising obesity rates are due to declining physical activity as societies become more industrialized, the findings show that people in wealthier countries expend just as much -- or even more -- energy daily. In an article recently published in PNAS, Duke researchers point instead to higher caloric intake as the primary driver, suggesting that diet rather than idleness plays the bigger role in the global obesity crisis.

"Despite decades of trying to understand the root causes of the obesity crisis in economically developed countries, public health guidance remains stuck with uncertainty as to the relative importance of diet and physical activity. This large, international, collaborative effort allows us to test these competing ideas. It's clear that changes in diet, not reduced activity, are the main cause of obesity in the U.S. and other developed countries," says Herman Pontzer, principal investigator with the Pontzer Lab and professor in the Department of Evolutionary Anthropology.

The researchers analyzed thousands of measurements of daily energy expenditure, body fat percentage and body mass index (BMI) from adults aged 18 to 60 across 34 populations spanning six continents. The more than 4,200 adults included in the study came from a wide range of lifestyles and economies, including hunter-gatherer, pastoralist, farming and industrialized populations. To further categorize the level of industrialization, they also integrated data from the United Nations Human Development Index (HDI) to incorporate measures of lifespan, prosperity and education.

"While we saw a marginal decrease in size-adjusted total energy expenditure with economic development, differences in total energy expenditure explained only a fraction of the increase in body fat that accompanied development. This suggests that other factors, such as dietary changes, are driving the increases in body fat that we see with increasing economic development," says Amanda McGrosky, a Duke postdoctoral alumna and lead investigator for the study who is now an assistant professor of biology at Elon University.

The researchers hope the study helps clarify public health messaging and strategies to tackle the obesity crisis and explain that the findings do not mean that efforts to promote physical activity should be minimized. Instead, the data support an emerging consensus that both diet and exercise should be prioritized. "Diet and physical activity should be viewed as essential and complementary, rather than interchangeable," the study notes. They will next work to identify which aspects of diet in developed countries are most responsible for the rise in obesity.

But new University of Colorado Boulder research shows the popular sugar substitute and specialty food additive comes with serious downsides, impacting brain cells in numerous ways that can boost risk of stroke.

The study was published in the Journal of Applied Physiology.

"Our study adds to the evidence suggesting that non-nutritive sweeteners that have generally been purported to be safe, may not come without negative health consequences," said senior author Christopher DeSouza, professor of integrative physiology and director of the Integrative Vascular Biology Lab.

First approved by the Food and Drug Administration in 2001, erythritol is a sugar alcohol, often produced by fermenting corn and found in hundreds of products. It has almost no calories, is about 80% as sweet as table sugar, and has negligible impact on insulin levels, making it a favorite for people trying to lose weight, keep their blood sugar in check or avoid carbohydrates.

But recent research has begun to shed light on its risks.

One recent study involving 4,000 people in the U.S. and Europe found that men and women with higher circulating levels of erythritol were significantly more likely to have a heart attack or stroke within the next three years.

DeSouza and first author Auburn Berry, a graduate student in his lab, set out to understand what might be driving that increased risk.

Researchers in the lab treated human cells that line blood vessels in the brain for three hours with about the same amount of erythritol contained in a typical sugar-free beverage.

They observed that the treated cells were altered in numerous ways: They expressed significantly less nitric oxide, a molecule that relaxes and widens blood vessels, and more endothelin-1, a protein that constricts blood vessels. Meanwhile, when challenged with a clot-forming compound called thrombin, cellular production of the natural clot-busting compound t-PA was "markedly blunted." The erythritol-treated cells also produced more reactive oxygen species (ROS), a.k.a. "free radicals," metabolic byproducts which can age and damage cells and inflame tissue.

"Big picture, if your vessels are more constricted and your ability to break down blood clots is lowered, your risk of stroke goes up," said Berry. "Our research demonstrates not only that, but how erythritol has the potential to increase stroke risk."

DeSouza notes that their study used only a serving-size worth of the sugar substitute. For those who consume multiple servings per day, the impact, presumably, could be worse.

The authors caution that their study was a laboratory study, conducted on cells, and larger studies in people are needed.

That said, De Souza encourages consumers to read labels, looking for erythritol or "sugar alcohol" on the label.

"Given the epidemiological study that inspired our work, and now our cellular findings, we believe it would be prudent for people to monitor their consumption of non-nutrient-sweeteners such as this one," he said.