More exercise, fewer calories? Weight loss is more complicated than that.

For years people have been told that weight gain is a simple mathematical problem. Consume more calories than you burn and you'll gain weight, do the opposite and you'll lose.

But now a growing consensus of experts in nutrition say it's not that simple.

Instead, they say, not all calories are created equal, and weight gain is a complicated process involving food quality, metabolism, genetics, medication and the bugs that live in people's guts.

This is more than an academic argument. With obesity levels and their associated health problems on the rise, this delicate dance between food, exercise and weight gain has huge implications for American's health and longevity.

"We are in an out-of-control epidemic that is going to continue to drive down life expectancy unless we make some major changes," said Dr. Walter Willett, a professor of epidemiology and nutrition at the Harvard T.H. Chan School of Public Health. "It's more serious than most people realize."

Obesity has been rising in America for the past half century and picked up steam around 1990, when low-fat diets were all the rage. The most likely explanation is people have been eating and drinking more.

"We've been telling people to eat less and move more – but it doesn't seem to be working. We need new ways of thinking," said Dr. David Ludwig, an endocrinologist at Boston Children's Hospital.

Monday, Ludwig, Willett and other Harvard-affiliated researchers published a paper in the "American Journal of Clinical Nutrition," laying out their concerns and the approach they believe will work best to counter obesity.

One problem with the simple calories-in-calories-out model, Ludwig said, is it may implicitly blame people for aspects of their metabolism beyond their conscious control. No one blames someone who is feverish for not reducing their own body temperature, he said, so we shouldn't expect people whose bodies send them powerful hunger signals to simply eat less.

"We've got to make weight loss easier," Ludwig said.

The question is how. Unfortunately, while academics have a good handle on what causes obesity, they haven't yet figured out a solution.

There is general consensus that it would be best for everyone to cut out the "crappy carbs," as one scientist put it – the sugar-sweetened drinks and highly processed foods including breakfast cereals, white bread, chips and most of what's available in vending machines, fast food places and the middle aisles of grocery stores.

Replace them instead with fruits, vegetables and whole grains, everyone agrees.

Beyond that, the academics dissolve into infighting.

Some, including the authors of the new paper, insist that reducing all carbs and replacing them with fatty foods like olive oil, fatty fish, avocado and nuts is the best way to lose weight and stay healthy.

Potatoes might not be "crappy," but they're not the best approach for weight control or diabetes prevention, Ludwig says. People who are already diabetic may need to cut even more carbs to restore metabolic health.

Others argue that good-quality carbs, like beans, fruit, legumes and whole grains, are just fine, and the crux is cutting calories.

"If you cut carbs out of the diet you would fix obesity? I don't see any evidence for that looking around the world," said Herman Pontzer an evolutionary anthropologist at Duke University.

Pontzer studies the Hadza, an indigenous group of hunter-gatherers in Tanzania who get 10% to 20% of their calories from honey. "They eat more sugar and carbs than Americans do as a percentage of their diet," said Pontzer, author of the 2021 book "Burn: New Research Blows the Lid Off How We Really Burn Calories, Lose Weight, and Stay Healthy." But they don't have America's obesity problem.

He says overconsumption has made people fat, rather than specific food categories.

Individual variability is also a factor in weight gain and loss, said Penny Gordon-Larsen, a distinguished professor of nutrition at UNC Gillings School of Public Health. Low-carb diets may not be right for everyone, and the most important factor for weight loss is whether someone can stick to whatever diet they're on.

But she still praised the new Harvard study as "a thought-provoking piece that really illustrates the complexity of all of these relationships and a pretty strong argument against the energy-balance model" of simply calories in-calories out.

Both camps, and those in between, seem to agree that people of all sizes should cut down on those "crappy carbs" that make up 40% of the typical American diet, said Christopher Gardner, a nutrition scientist at Stanford University's Prevention Research Center.

"America has a weight issue," he said. "What would you go after first? I'd go after the low-quality carbs. I've never found anybody who disagreed with that."

The hormone insulin causes cells to store energy as fat, said Daniel Lieberman, a Harvard paleoanthropologist and co-author of the paper. It makes sense that eating a lot of food that causes spikes in insulin – a lot of processed carbohydrates, for instance – would cause the body to store more energy as fat. "If I eat a high-fat diet, I will not elevate insulin the way I will if I eat a high-carb diet," he said.

Exercise plays a role in helping prevent weight gain, but diet matters more when trying to lose pounds, said Lieberman, author of the 2021 book "Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding."

The body also digests highly processed foods differently from whole foods, said Dariush Mozaffarian, a cardiologist and dean of the Tufts Friedman School of Nutrition Science and Policy.

A food that has already been highly processed is digested mostly in the stomach and upper gut, leaving little left over for the microbes that live lower in the intestines, he said. Consume 100 calories and "if it's minimally processed, you share those calories with your microbiome," Mozaffarian said. With processed foods, "when it says 100 calories, your cells get all 100 calories."

Advances in food production in the 1950s and 1960s led to massive production of starchy crops – rice, wheat and corn – that led to the industrialization of food. That was great for the world's population, Mozaffarian said, dramatically reducing hunger and allowing the global population to explode from under 3 billion to nearly 8 billion in a century.

But that same food-system industrialization is also a primary driver of the global obesity epidemic, he said.

He doesn't blame the food industry for this unintended consequence. But "they are responsible now for not moving in a new direction" to help combat obesity, he said.

What exactly the food industry should do, though, depends on what scientists say is the best approach – and that consensus is still missing.

Weight loss is made more difficult because of the economic incentive food companies have to create an environment that encourages overeating of highly processed foods, Gardner said.

"Until we fix that," he said, "we're just chewing around the edges."

Contact Karen Weintraub at kweintraub@usatoday.com.

Health and patient safety coverage at USA TODAY is made possible in part by a grant from the Masimo Foundation for Ethics, Innovation and Competition in Healthcare. The Masimo Foundation does not provide editorial input.

This article originally appeared on USA TODAY: Fewer calories, more exercise for weight loss? It's more complicated