This group of people is often overlooked by society. Nowadays, the world’s attention is primarily focused on the billion-strong population suffering from . However, on the other end of the spectrum are those who are extremely thin and do not wish to be. Researchers estimate that about 1.9 percent of the global population is thin, so to speak, “by nature.” In the U.S. alone, there are 6.5 million such individuals, according to Live Science.
Constitutionally thin people (those with such a physiological constitution) typically eat as much as their peers. Yet, their body mass index (BMI) is below 18.5, and sometimes even as low as 14, which corresponds to 32.6 kg for a height of 1.5 meters.
Extreme thinness has not been studied extensively. There are fewer than 50 clinical studies dedicated to constitutionally thin individuals worldwide. Meanwhile, the issue of excess weight has been the subject of thousands of scientific papers.
Recently, however, scientists have begun to delve deeper into the characteristics of naturally thin people, particularly their metabolism. This knowledge may eventually help them gain weight. On the flip side, it could also assist those struggling with excess weight in shedding pounds. Since by nature may serve as a “mirror model” of obesity, believes Melina Bai, co-author of a recent study from the metabolism research lab at the University of Clermont Auvergne (France).
The Programmed Set Point
Research has shown that naturally thin individuals largely “resist” weight gain, especially when consuming calorie-dense foods. The pounds they gain from overeating quickly disappear once they return to a normal .
Many experts believe that our bodies have a predetermined norm, a “set point” weight to which they strive to return. This is one reason why few dieters manage to maintain their weight over the long term. Their metabolism slows down, burning fewer calories and making it easier to gain weight, especially after they stop restricting their calorie intake.
As a group, thin individuals are likely just as heterogeneous as those with excess weight. Some may remain thin due to a reduced appetite or quick satiety, while others consume as many calories as heavier individuals.
One study found that constitutionally thin people consume about 300 calories more daily than their metabolism requires. “They have a positive energy balance, yet still resist weight gain,” noted Bai.
A 2021 meta-analysis surprised researchers. When Bai and her colleagues compiled data on the body composition of thin individuals, they discovered something unexpected. Constitutionally thin people exhibited nearly normal levels of body fat. “It’s truly unusual to have such a low body mass combined with a completely normal fat mass,” said Bai.
Modest Muscle and Bone Mass
Clearly, naturally thin individuals exhibit a deficiency in muscle mass. Recent studies have confirmed that extremely thin people also have lower bone mass, and their muscle fibers are, on average, 20 percent thinner than those of individuals with normal weight.
This could negatively impact their health. Bai suggested that, with age, particularly thin women may face an increased risk of developing osteoporosis. Additionally, a decrease in muscle mass could complicate everyday tasks, such as opening jars or carrying groceries.
Supergenes
Scientists speculate that individuals with constitutional thinness may possess unique patterns of activity in related to energy production.
One such gene that has caught researchers’ attention is ALK (anaplastic lymphoma kinase). When scientists removed this gene from mice, the animals became resistant to weight gain when fed a high-fat diet. The brain, influenced by the ALK gene, likely sends signals that affect the rate at which fat cells burn energy.
Understanding these genetic mechanisms could pave the way for new treatments for individuals with abnormal weight.
