Underweight: definition, causes, when to worry?

2023-06-20 09:00:00

Underweight (or thin): definition

According to the WHO, 462 million adults worldwide are underweight. In France, approximately 2.5% of the population would be affected. In adults, we speak ofunderweight (or underweight) when the body mass index (IMC) is less than 18.5. The WHO (World Health Organization) defines it as a low ratio between weight and height (source 1). “It’s a statistical norm, there are people who are 55 kg for 1.70m, who are still that weight and who will be fine, and others who have lost weight, who support well or badly… There are many nuances, it’s individual”, specifies the doctor nutritionist Jacques Fricker, author of “ Take your heart to heart » et « Lose weight fast and well » published by Odile Jacob.

When do we speak of being underweight in children or at birth?

According to Public Health France, about 13% of children aged 6 to 17 are considered thin. The prevalence of underweight reaches even close to 20% for girls aged 11-14 (source 2).

Although the formula for calculating BMI in children is the same (weight, in kg, divided by height, in meters, squared), the interpretation of the result is different. It is necessary to refer to reference curves available in the health recordwhich take into account the age and sex of the child.

Low birth weight is defined by the WHO as a birth weight less than 2.5 kg.

What are the possible causes of thinness or weight loss?

Underweight can be constitutional: the person will then always have a low weight, it is his morphology. But unconstitutional thinness or significant weight loss can be the sign of a disorder or diseaseas :

Being underweight can also be linked to substance abuse (alcohol, drugs, medication, etc.).

While the causes do not vary between children and adults, particular attention should be paid to eating disorders as teenage anorexia.

Older people more at risk of being underweight

“The impact of unintended weight loss increases with aging, often reaching 50% in residents of care facilities,” the MSD manual states (source 3). According to a study by Public Health France, approximately 15% of people aged 75 and over at home are thin, particularly women (source 4).

What causes underweight in seniors? In the elderly, thinness and/or weight loss “may be linked to loss of appetite, chewing problemssalivation, of transitesophagus, gastric emptying…”, indicates the nutritionist doctor.

Being thin, is it serious in adults? What can be the consequences?

“The consequences will vary from one individual to another. If the underweight is constitutional, the person is still the same weight, there is no risk. By nature, these people generally have thin muscles, little fat and an appetite suited to their needs,” explains Dr. Jacques Fricker.

On the other hand, “a unwanted weight loss (involuntary, or voluntary as in anorexia) exceeding four/five kilograms or 5% of body weight over a period of a few months is cause for concern,” the MSD Manual states. A balance sheet is then necessary in the event of unexplained weight loss – unless there is an emotional shock preceding the weight loss, “because a loss of a few kilos, often transient, is frequent in the event of bereavement, divorce…”, specifies the specialist.

Dr. Jacques Fricker: It is the thinness linked to an illness or that one imposes on oneself that is worrying.

The risks can then be:

a fatigue ; disturbances of balance or attention; qualitative degradation of his muscles; malnutrition (lack of vitamins, minerals, proteins, etc.). “People suffering from undernutrition, and children in particular, are much more likely to fall ill and die”, specifies the WHO; increased sensitivity to infections ; delayed healing.

What are the risks for children?

In children, weight monitoring is particularly important. When should you worry and consult? ” When the child’s weight chart falls below desirable ranges or breaks. In other cases, do not be alarmed: a child between the ages of 3 and 10 generally appears thin by nature, according to Dr. Fricker. Children who are a little rounder, that’s not normal. A normal adult BMI in children is a sign of future obesity”.

Conversely, according to the WHO, “a truly underweight child may present with growth retardation and/or suffer from emaciation”. If in doubt, it is better to talk to your doctor or pediatrician.

What to do ? How to fix underweight?

Constitutional underweight does not represent a risk to health, so it is not necessary – and difficult – to remedy it. When it is acquired and undesirable on the other hand, the first thing is to treat the underlying cause of the thinness. Then, you have to “feed the person again if they are tired, malnourished, lacking in vitamins…”, says Dr. Fricker. For this, protein or vitamin supplements can be prescribed. It is also sometimes necessary “to review the diet, to see the foods that pass better”, specifies the doctor nutritionist. “Making several small meals is also a good solution, it promotes weight gain,” he explains.