Extreme Thinness: Scientists Explore Why Some People Find It Hard to Gain Weight

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Before weight coach Bella Barnes starts consulting with new clients, she already knows what they'll say. Sure, women struggle with weight. But they don't want to lose weight. They want to gain it.

Her clients feel they are too thin and suffer. “I had a client come in last week who wears leggings with butt pads,” says Barnes, who lives in the UK. “I recently had another client who wears three pairs of leggings in the summer to look a little bigger.”

These women belong to an often-ignored demographic. While the world focuses on the billion-plus people who are obese, at the other end of the spectrum are people who are thin, often painfully thin, but don’t want to be that way. Researchers estimate that about 1.9 percent of the population is “naturally thin,” and that there are 6.5 million of them in the U.S. alone.

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Constitutionally thin people often eat as much as their peers and don’t exercise intensely. But their body mass index is below 18.5, sometimes as low as 14 — equivalent to 72 pounds at 5 feet tall — and they don’t tend to gain weight. The condition is “a real enigma,” write the authors of a recent paper in the Annual Review of Nutrition. Constitutional thinness, they say, challenges “basic dogmatic knowledge about energy balance and metabolism.” It’s also understudied: Fewer than 50 clinical studies have looked at people with constitutional thinness, compared with thousands of studies looking at people with unwanted weight gain.

Recently, researchers have begun to study the characteristics of naturally thin bodies. The scientists hope to gain metabolic insights that will help naturally thin people gain weight. The study could also help overweight people lose weight, since naturally thin bodies appear to be a “mirror model” of obesity, says Melina Bailly, a co-author of the recent review and a research physiologist at AME2P, a metabolism lab at the University of Clermont-Auvergne in France.

Genetic and metabolic factors

The first reports of people eating a lot but remaining inexplicably thin were reported in the scientific literature in 1933. Decades later, a landmark 1990 experiment demonstrated how widely people vary in how they regulate their weight.

Twelve pairs of identical twins were given 1,000 extra calories six days a week. After three months of this overeating—the equivalent of an extra Big Mac and an average number of French fries a day—the boys had gained an average of almost 18 pounds, mostly from fat, but with a wide range: One gained almost 28 pounds, while the other gained less than 10 pounds. The latter somehow dissipated about 60 percent of the extra energy.

The study also found that the difference in weight gain between pairs of twins was three times greater than within pairs, suggesting a genetic influence on the tendency to gain weight when overeating.

Other studies have confirmed that naturally thin people are highly resistant to weight gain, especially when eating fatty foods. Any pounds they gain from overeating quickly disappear once they return to a normal diet.

After periods of binge eating, the body typically loses weight. But as this graph shows, there is variability in both the response to binge eating and the return to “normal” body weight. (“Ad libitum” refers to the period of the experiment when participants eat whatever they want.)

This is somewhat consistent with modern thinking. Many researchers believe that our bodies have a preprogrammed weight “set point” or “set range” to which they want to return. This is one reason why few dieters maintain their weight over the long term. Their metabolism slows down, burning fewer calories and making it easier to gain weight, especially after the person stops restricting calories. (The system exhibits some flexibility, which explains why many of us gain a few inches in our bellies as we age.)

“Condemnation of the thin”

As a group, overweight people are probably as heterogeneous as overweight people. Some may stay thin because they have a decreased appetite or feel full quickly. Others consume as many calories as heavier people. One study found that constitutionally thin people consume 300 or more calories a day more than their metabolism requires. “They have a positive energy balance, and they still resist weight gain,” says Bailly, co-author of the NUTRILEAN project on constitutional thinness at the University of Clermont-Auvergne in France.

Like obese people, naturally thin people face social stigma. Thin men may feel too skinny to fit the masculine ideal. Thin women often complain about their lack of curves. Others may suspect that they are hiding an eating disorder. They receive “comments from random people on the street,” says Jens Lund, a postdoc in metabolic research at the Novo Nordisk Foundation Centre for Basic Metabolic Research at the University of Copenhagen. “These people feel like they can’t go to the toilet after a family dinner … because they’re afraid that they’ll be looked at as if they’re going to puke, as if they’re bulimic.”

Weight-gain coach Barnes was never technically considered thin, but she experienced this “skinny-shaming” herself. Her family commented on her weight but dismissed her concerns. “I felt like I could never talk about it,” she says. “People would say, ‘It’s not a problem,’ or, ‘Just lose weight.’”

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Researchers are learning that there are significant individual differences in how the body uses calories. (Thermogenesis is the metabolic process in which calories are burned to produce heat.)

A 2021 meta-analysis yielded some surprises. When Bailly and his colleagues collected body composition data from lean people, they found something unexpected: Constitutionally lean people had nearly normal amounts of fat throughout their bodies. “It’s really unusual to have such a low body weight combined with a fairly normal fat mass,” Bailly says.

Apparently, there is a lack of muscle mass. Naturally thin people have less of it – studies have shown that their muscle fibers are on average about 20% thinner than those of normal-weight people. Naturally thin people may also have reduced bone mass.

These facts suggest that being thin has negative health effects. Although research is still limited, Bailly suggests that as women age, they may be at higher risk of developing osteoporosis, a dangerous weakening of the bones. Decreased muscle mass can also make it harder to perform everyday tasks, such as opening jars or carrying groceries.

“And this could mean a decrease in protein stores during illness,” says Julien Verny, a research physiologist at the University of Clermont-Auvergne's metabolic lab and co-author of the paper in the Annual Review of Nutrition.

In addition to differences in body composition, researchers suggest that naturally thin people “waste” calories. For example, some studies show that while thin people exercise less, they fidget more.

They may also excrete more calories than others. Although this has not been studied specifically in normal-weight people, it is known that some people lose up to 10% of their calories in their stool (and, to a lesser extent, urine), while others lose as little as 2%. In one study, a woman excreted 200 calories a day, the equivalent of half a liter of soda.

Additional metabolic features of constitutionally lean people may yet be discovered. “We recently found some signatures that may indicate increased metabolic activity in their fat tissue,” says Bailly. “That’s really surprising.” Other studies have already shown that naturally lean people have more “brown fat,” the tissue that burns calories and produces heat.

To find more concrete answers, Lund plans to conduct a stationary study at the University of Copenhagen. The study will use a metabolic chamber to track energy intake, expenditure, and all energy loss pathways, including feces, urine, and exhaled gases, in constitutionally lean people. Since 2020, Lund’s team has built a network of Danes who consider themselves naturally lean, which represents a unique resource for future research.

Constitutional thinness has a strong genetic component, as a 1990 study of twins showed: Research shows that 74 percent of very thin people have relatives with similar body types. As researchers identify gene variants, they are realizing that many of them—like FTO, MC4R, and FAIM2—are also involved in processes that lead to obesity. Although it’s not yet clear what exactly they are, scientists speculate that people with constitutional thinness may have unique patterns of activity in genes related to energy production.

One such gene that has caught the attention of researchers is ALK (anaplastic lymphoma kinase). When scientists deleted this gene in mice, the animals became resistant to weight gain when fed a high-fat diet — even mice that are genetically predisposed to obesity. The ALK gene appears to act in the brain, which then sends signals that affect how quickly fat cells burn energy.

Genetics plays a major role in constitutional thinness, and researchers are beginning to identify the genes responsible. In mice, deleting one of them, the Alk gene, made the animals resistant to weight gain when fed a high-fat diet.

Understanding these genetic mechanisms could lead to new treatments for both the thin and the obese, says Lund. “If we can figure out what’s protecting them from becoming overweight, whatever that mechanism is, we can try to turn it into a drug,” he says. “There are so many signaling molecules in the body that we don’t even know exist.” The dream is to make a breakthrough as revolutionary as the latest obesity drugs.

While researchers look for biological clues, Bella Barnes is navigating the complexities of weight gain on her own. After years of trial and error, she gradually gained about 40 pounds by combining strength training with smart, mindful eating. At first, if she didn’t hit her daily calorie goal, she’d grab a bag of cookies—anything to gain weight. But over time, she found balance. “Not all calories are created equal. You have to eat whole foods,” she says. And lots of them.