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 fueled metabolism-boosting trends. People did everything from taking stimulant drugs to loading their diet with sardines. (Yes, it’s a thing.) 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 University study 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 industrialized 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 previous research 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, and it makes it hard to lose weight,” said Diane Enos, CEO of the American Society for Metabolic and Bariatric Surgery. “Obesity is a chronic relapsing disease. It’s not a decision by a person.”
Many experts are now focusing on diet patterns and food systems that impact people starting from childhood, especially ultraprocessed foods. Kids and teens get 62% of their calories from ultraprocessed foods, and for adults, it’s 53%.
Processed foods are more affordable and easier to access for many families.
Experts are concerned about ultraprocessed foods because people tend to eat more of these manufactured products ― many times because of their availability and affordability. Your body will often end up absorbing more calories than when you eat unprocessed foods. This happens because the structure of food can actually impact digestion.
Most of your digestion happens in the stomach and small intestine, where your body quickly absorbs calories. Some foods, especially minimally processed, fiber-rich and plant-based foods, make it all the way to the large intestine, where gut bacteria take over and start absorbing calories instead.
“It’s not so much that you’re increasing your metabolism; it’s that you’re shifting who’s doing the metabolizing,” said Dariush Mozaffarian, director of the Food is Medicine Institute at Tufts University.
For example, raw fruits and vegetables can keep a portion of the calories locked in their cell structure while they pass through the stomach and small intestine. When they reach the large intestine, the gut bacteria break them down and absorb some of the calories instead of the person eating the food.
Ultra-processed foods are often broken down before they reach the gut bacteria in the large intestine, so your body absorbs nearly every calorie. Over time, this makes weight gain more likely.
Think about eating 200 calories of steel-cut oats versus 200 calories of instant oatmeal. With steel-cut oats, the natural structure is still intact, so some starches make it to the gut bacteria, which can help you absorb up to 20% less calories. With the instant oatmeal, the grains have already been broken down before you eat them, so your body absorbs more of the calories.
“Ultra-processed foods loaded with refined grains, starch and sugar also slow metabolism,” Mozaffarian said.
So when it comes to weight management, the challenge and solution likely involve the foods we eat, and while metabolism matters, it isn’t something we can easily overhaul with tricks or fad diets.
“I think people should pay attention to metabolism; try to have more muscle mass, try to eat foods that are going to benefit their metabolism and be digested by their gut microbiome … but they shouldn’t look for a quick fix, like just taking a pill or thinking, ‘I’m going to eat protein and my metabolism is going to change.’ Those are the quick fixes that I think are more harmful than beneficial,” Mozaffarian said.
Public Policy
If we know that ultraprocessed food is less beneficial and leads to overeating, why do we still have so much of it? As mentioned, for many people, it’s the most accessible option.
“In the U.S., a lot of people in lower socioeconomic groups don’t have easy access to fresh fruits and vegetables,” McGrosky said. “You can go to the store and see how expensive it is to buy fresh foods versus stopping at the corner store, where it’s much easier to grab packaged foods that are less nutrient-rich for the number of calories they provide.”
While that’s the case in developed countries, the situation for those facing poverty in other parts of the world can look very different.
Looking across societies, when there’s a shift from hunting, herding or subsistence farming into market economies, body fat percentages rise. Market economies give people access to mass-produced foods that may trigger increases in obesity.
McGrosky shared that the pastoralist families she works with in Kenya live on very little money, but they don’t eat the packaged meals that are commonly served to low-income families in the U.S. Their staples include a dense cornmeal porridge called ugali and milk from goats and sheep. Meat is rare and considered a luxury.
“I think it’s really a product of your social environment and the types of foods that are accessible to you,” McGrosky said. “That does correlate with socioeconomic status, but it varies a lot depending on the country and the population … From the policy standpoint, I think it’s really important to ensure that everyone has access to these fresh foods that are not processed, foods that actually occur in nature, because our bodies did not evolve to digest ultraprocessed foods.”
Oscar Wong via Getty Images
Your metabolism and the way you eat and move your body is vital to your health, but it’s not exactly the easy answer when it comes to why obesity rates have risen.
When Metabolism Is The Answer To Weight Gain
Weight is just one measure of health status, and it doesn’t tell the whole story of obesity as a chronic, relapsing disease.
“Even if you lose weight, you still have obesity, you still have a metabolic disorder.” Enos said.
The factors that contribute to it are complex and go beyond food environment and social conditions. A person may also be struggling with genetic factors like an inherited metabolic disorder or hormonal disorders like polycystic ovary syndrome. They may be taking medications that can cause weight gain, like antidepressants or corticosteroids. Even common conditions like sleep apnea or chronic stress can disrupt metabolism or increase cortisol production, which can contribute to changes in body composition.
Consider all of those biological factors, and then add in our current food environment and social issues like housing and food insecurity. It’s easy to see why the American Medical Association now views obesity as a disease that requires comprehensive, individualized care.
So, What’s The Solution?
About half of all Americans are projected to have obesity by 2030, signaling an urgent need for prevention and destigmatization. This means reducing bias, shifting focus to overall health and equitable care, and changing the food environment.
“The only way to help people evolve and combat metabolic syndrome and this disease of obesity is to start [from childhood] with easier healthy choices that are affordable, accessible and incentivized over ultra-processed foods,” Enos said. “You can’t put it all on the person if you’re not putting the policies in place to help protect them to begin with.”
Certain initiatives are already in the works. Funded by government grants and nonprofits, farm-to-school programs give children fresh meals, cooking classes and nutrition education. Many schools do not have access to these programs, though, highlighting the need for broader funding and implementation.
At the legislative level, federal and state bills have proposed changes to the labeling and advertising of sugar-sweetened and ultraprocessed foods and beverages. Some localities are adjusting their zoning to restrict fast-food restaurants and incentivize the creation of grocery stores. In terms of food assistance, there are proposed changes to SNAP benefits, and WIC packages were revised last year to encourage families to choose more nutrient-rich foods — but this adjustment isn’t a simple one.
“If people don’t know how to use the produce, or if they don’t have the ability to store it in a safe or healthy way, then it doesn’t do anybody any good,” Enos said. “People think, ‘[Give] them more fresh produce and people will naturally have a better outcome,’ but there’s a system-wide approach that has to be used.”
While many of these large-scale changes will take time, there are slight improvements that can be made in the short term.
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“I talk a lot about small steps, not needing people to be perfect,” said Becky Ramsing, senior program officer II at the Johns Hopkins Center for a Livable Future.
Ramsing recommends trying to make easy, low-cost switches, like avoiding sugar-sweetened beverages, buying more frozen fruit and vegetables or choosing whole grains instead of simple carbohydrates.
“People’s food decisions are complex,” Ramsing said. “You have culture, gender, community, family, your food environment, and your own health issues. We have to go deeper than just throwing healthy food in the middle of the community. We need to actually make it affordable and have the education so people can make those choices for their families.”
