Before weight coach Bella Barnes consults with new clients, she already knows what they’ll say. The women struggle with their weight, naturally. But they don’t want to lose pounds. They want to gain them.
Her clients find themselves too thin, and they’re suffering. “Last week, I signed up a client who wears leggings that have bum pads in them,” says Barnes, who lives in Great Britain. “I’ve had another client recently that, in summer, wears three pairs of leggings just to try and make herself look a bit bigger.”
These women belong to a demographic group that has been widely overlooked. As the world focuses on its billion-plus obese citizens, there remain people at the other end of the spectrum who are skinny, often painfully so, but don’t want to be. Researchers estimate that around 1.9 percent of the population are “constitutionally thin,” with 6.5 million of these people in the United States alone.
Constitutionally thin individuals often eat as much as their peers and don’t exercise hard. Yet their body mass index is below 18.5 — and sometimes as low as 14, which translates to 72 pounds on a five-foot frame — and they don’t easily 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 is also understudied: Fewer than 50 clinical studies have looked at constitutionally thin people, compared with thousands on unwanted weight gain.
Recently, researchers have started to investigate how naturally thin bodies are different. The scientists hope to unlock metabolic insights that will help constitutionally thin people gain weight. The work may also help overweight people lose pounds, since constitutional thinness appears to be “a mirror model” of obesity, says Mélina Bailly, a coauthor of the recent review and a physiological researcher at AME2P, a metabolism research lab at the University Clermont Auvergne in France.
Individuals who eat heartily but remain inexplicably skinny were first reported in the scientific literature in 1933. Decades later, a landmark 1990 experiment demonstrated how profoundly people differ in regulating their weight.
Twelve pairs of identical twins were fed 1,000 surplus calories for six days a week. After three months of such overfeeding — equivalent to an extra Big Mac and medium fries daily — the young men had gained an average of…
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