For decades, a myth dominated the health industry: people who are lean have a “naturally fast metabolism” and people with obesity have a “naturally slow metabolism”.

Since metabolism is your body’s process for converting food to energy, this idea seemed logical and fuelled metabolism-boosting trends.

People did everything from taking stimulant drugs to loading their diet with sardines. (Yes, it’s athing.) These “quick fixes” promised to trick our bodies into burning calories at a faster rate.

But newer research has shown that while metabolism does impact weight, that alone doesn’t fully explain the rise in obesity.

In fact, a recent Duke Universitystudy published in Proceedings of the National Academy of Sciences found that metabolic rates remain pretty stable across populations, which suggests that other factors of modern life are more plausible drivers.

Obesity is more prevalent in developed countries than in less developed nations. Historically, health experts attributed the difference to industrialised populations that are less active and eat more calories.

But the Duke study revealed that physical activity may play less of a role than we thought.

Researchers evaluated the daily calorie burn, body fat percentage and BMI for thousands of people across 34 different countries. The biggest finding was that although there was a small decline in body-size-adjusted total calorie burn in developed countries, it wasn’t enough to explain the rise in body fat.

“When we look at different populations with very, very different lifestyles – for example, pastoralists, who are moving with herds of sheep and camels – they have similar body-size adjusted total energy burn as someone who is living in the U.S. sitting at their desk typing on a computer all day,” said Amanda McGrosky, an evolutionary anthropologist at Elon University and co-author of the study.

The finding aligns with previousresearch that showed our bodies adjust based on the ways we expend energy to keep our calorie burn at a stable level.

“We do see that people will increase their calorie burn when they start picking up a new exercise regime, and they might lose some weight initially. But over the long term, they tend to plateau,” McGrosky said. This happens because your metabolism becomes more efficient, learning to burn fewer calories to complete the same activities.

Exercise is still critical for overall health. It improves cardiovascular, cognitive and mental health and reduces your risk of chronic diseases. It can also help you maintain your weight and build muscle.

But looking at the obesity crisis that spans across developed nations, lack of exercise doesn’t seem to be a main cause.

“On the broad scale, across individuals in different populations, habitual energy expenditure and human metabolism seem to be fairly constrained within this narrow range that we’re all kind of stuck in,” McGrosky said.

So if energy expenditure and a “lack of metabolism” isn’t the answer, what’s really impacting us? Here are two major factors society tends to dismiss when talking about obesity (spoiler alert: the cards are stacked against us):

Processed food

“The current food environment makes it easy to gain weight,........

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