According to the criteria of the WHO (World Health Organization), we speak of thinness when the body mass index (BMI, calculated by dividing the weight by the height squared) is less than 18.5. It can follow a significant weight loss due to an illness (Cancer, depression), emotional shock, hospitalization after an accident, anorexia nervosa. But it can also be natural, it is “constitutional thinness”. “Like height, weight is genetically programmed”, explains Professor Bruno Estour. This constitutional thinness is hereditary, with the random nature implied by the genetic lottery. Certain families thus count in their ranks of the thin ones, whatever they eat, and since birth. “Between 0 and 20 years, their weight curve is always 2 to 3 standard deviations below the normal curve,” confirms the endocrinologist.
The microbiota, the key to weight?
“These people who do not gain weight despite significant caloric intake lead us to rethink the equation of metabolism usually established,” says Professor Estour. Namely that we gain weight during energy intake greater than expenditure, and that we lose weight in the opposite case. The Saint-Étienne CHU team thus carried out two studies on people with constitutional thinness (BMI between 14.5 and 16.5). Results : despite an energy intake of 2200 Cal and an expenditure of 2000 Cal, they do not gain weight by one gram. Hence this question: where did the unspent 200 cal go? The answer may well lie in the microbiota. “He could indeed intervene in this loss of calories, and explain why the contribution / expenditure equation is not valid for these people, ”suggests the specialist. This line of research could make it possible to better understand and, ultimately, help those who suffer from their constitutional thinness, but also those at the other extreme: obese. “Everything we can learn about constitutional weight loss can help us better understand healthy obesity (obesity without a metabolic disorder). They are like two mirror images, ”continues the endocrinologist. These millions of bacteria which make up our intestinal flora, this “second brain” which is of increasing interest to researchers, could thus hold the key to this resistance to be taken or taken. losing weight.
Variable health consequences
“However, scans show that in constitutionally thin people, all masses are reduced: muscle mass, lean mass but also bone mass, which leads to weakening of the bone. From the age of 20, these people can suffer fromosteoporosis », Underlines Professor Estour. Apart from this osteoporotic risk, thinness does not constitute a danger to health: the organs are functioning properly, fertilityis not impacted (which is generally not the case with anorexia or weight loss due to illness or emotional shock). “But we should not underestimate the psychiatric complications,” continues the specialist. These people are often embarrassed in their social relations. Envied for their slender silhouette while they eat without depriving themselves, the skinny do not always live well the eyes of others. “Men would like more muscle; women, more feminine curves. To this aesthetic penalty is added that of suspicion. Since it is established that when we eat we gain weight, we do not believe them when they claim that they are eating. They are therefore suspected of lying, or even of hiding a anorexia », Analyzes our endocrinologist.
How to get fat when nature has made us thin?
Teams from Toulouse University Hospital have experimented with a weight gain protocol with an additional daily caloric intake of 700 Cal in the form of fat for 15 days. Results : while the control group gained an average of 1.3 kg, the constitutionally skinny group only displayed an additional 700 g on the scale, naturally lost one week after stopping the protocol, while the control group put on 3 months to eliminate them. Same result with another protocol. “In the event of constitutional thinness, there is a real resistance of the body to gain weight. This resistance is difficult to counter, and even more so in the long term, ”analyzes Professor Estour. If high calorie intake can take a few hundred grams, and muscle building a little muscle, the body naturally loses these weights and forms obtained with a lot of calories and dumbbells. “All the more so since these are dietary intakes that are difficult to maintain over the long term,” adds our expert. It remains to rely on what the endocrinologist calls “the natural history of weight”: “Studies have shown that between 20 and 50 years, we gain on average 10 kg. In the event of constitutional thinness, this weight gain is less important: between 3 and 4 kg only. In the meantime, the right solution is to accept your weight, to live with it as well as possible. “For that, mentalities must change, that people understand that constitutional thinness exists,” said Professor Estour.
What is the equilibrium weight?
The balance weight is that for which our body is programmed. It is the weight to which we return naturally after a gap, and which we maintain year after year, without making any effort. Nothing to do with the BMI (body mass index), which is “only a statistical standard valid at 20 years, estimates Professor Estour. BMI allows statistical comparisons to be made, but does not say anything about our genetic weight. »Problem: aesthetically, this weight is not always suitable for the people concerned, hence a sometimes perpetual struggle. “Or, on the contrary, we can accept what nature has given us, and live in harmony with this weight”, concludes the specialist.
Our expert:
Prof. Bruno Estour, Department of Endocrinology, Nutrition and Eating Behavior Disorders at the CHU de Saint-Étienne
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