Multiple causes of obesity

Obesity is a complex health issue resulting from a combination of causes, and the factors involved in its development are not all identified.

Dietary changes and the increase of physical inactivity play an indisputable role in the recent emergence of obesity. Bigger portions, greater energy density, excessive industrial food, food availability, and changing food prices are all factors that promote excessive caloric intake. A sedentary lifestyle and leisure activities such as television or video games, the use of the car and of public transport for daily commuting all contribute to a decrease in physical activity and energy expenditure.

But these influencing factors on the energy balance alone, and therefore the nutritional status, do not explain the increased rate of obesity, nor the “inequality” of individuals with regard to weight gain: some people gain more weight than others, even though they have a similar lifestyle.

A genetic predisposition to weight gain may account for these differences in individual susceptibility to obesity. People are two to eight times more likely to be obese if their own family members are. Several French teams, from Inserm and CNRS, have identified many genes involved in weight gain, severe obesity and / or complications of obesity. Note that while each gene taken individually has only a small role in body mass and body composition, the contribution of these genes becomes significant when they interact with external factors such as energy imbalance. There are also monogenic obesities linked to an anomaly in a single gene, as is the case in rare forms of childhood obesity, which develop very early and are very severe. The identification of gene mutations in the leptin-melanocortin pathway has helped advance their diagnosis and treatment.

The role of the environment, beyond diet and physical activity, seems to be equally important. The biological clock is singled out. But stress, certain drugs, viruses, the composition of the intestinal microbiota, exposure to pollutants are seemingly also factors to blame. Early exposures and events in life are also important, including those that occur before birth or even before gestation. Six prenatal risk factors for obesity have been identified: maternal smoking, maternal diabetes or overweight, excessive weight gain during pregnancy, fetal growth deficit or excess, unfavorable socioeconomic background. The influence of mother’s diet on the onset of obesity is analyzed in particular as part of the Elfe study.

  • various studies have shown an epidemiological association between short sleep duration and a high body mass index linked to obesity.
    When sleep duration is less than 5 hours per night, the risk of obesity increases by 60%, and every hour increase in sleep duration is associated with a 9% reduction in risk. Obesity: sleeping less than 6 hours increases the risk of obesity by 4 compared to sleeping more than 7 hours. This impact is therefore much greater than that of food intake or a lack of physical activity.

This phenomenon is due to a reduction in leptin and an increase in ghrelin, a hormone which is secreted by the stomach and stimulates appetite.

In addition, obesity can be associated with bulimia and binge eating disorders, of which the causes are also multifactorial.

Too often people who are overweight are subjected to the disapproving gaze of society on their extra pounds. However, being overweight cannot be explained by poor eating habits alone.

Did you know?

One tablespoon of ketchup contains 4 grams of hidden sugar. A single can of sugary soda can contain up to 40 grams of sugar, or ten teaspoons.

“You just have to eat less”, “You just have to move and play sports” … These are the kinds of complaints that overweight people hear or can read in other peoples eyes. But being overweight is not just the result of an imbalance between the calorie intake and expenditure. Many other factors come into play. We are not all equal when it comes to weight gain.

First of all, we must keep in mind that we live in an obesogenic environment. In our westernized societies, food is in unlimited access. Fast food restaurants are open almost all the time. The food industry is constantly innovating and offering new processed meals with excess sugar, salt and fat. Much of the sugars consumed today are thus “hidden” in foods that are not considered sweet.

Society also plays a big role in the progression of obesity. Socio-economic status, in particular, leads to significant inequalities. The risk of being overweight or obese is at least 50% higher in people with less education and low income. Men with a level of education corresponding to compulsory schooling are thus almost twice as likely to be obese as those who have completed tertiary education (19% versus 9%). In women, this risk triples (19% versus 5%). Among the groups at risk, people of foreign nationality are more affected than the Swiss and people living in the countryside are more affected than city dwellers.

Genes and biology are also responsible

On an individual level, recent research shows that certain genetic factors can cause changes in appetite and fat metabolism that lead to obesity. But, while a person’s genetic predisposition may contribute to obesity (70% of obese people have at least one parent in the same situation), it is not the primary cause. Only about 2% of obesity is secondary to a medical condition. The diseases that can lead to obesity are diseases of the endocrine glands (or endocrine diseases). The hormonal origins of obesity can be hypothyroidism or a disease of the adrenal gland.

Aging is also an important factor as body composition changes. The maximum lean mass (muscle) is reached at age 20, then it decreases due to a sedentary lifestyle. Fat mass, on the other hand, tends to accumulate in the abdominal area. With age, hormonal changes also influence weight. During menopause, the decline in estrogen and progesterone secretion results in impaired lipid metabolism characterized by increased storage of visceral fat.

Substances involved

Certain medications can also promote weight gain or slow down weight loss. This is the case with certain antidepressants and antiparkinson drugs, neuroleptics and corticosteroids.

But other substances, known as obesogenic, could also be problematic. Research in recent years points to certain chemicals (see box) found in our environment as responsible being for obesity. Called endocrine disruptors, they act like hormones and disrupt the glands that release hormones into the blood by fooling them. They therefore interact with our eating behavior and physical activity habits. We’re exposed to many of them daily through food ingestion, dust and water, or by inhaling gases and particles in the air. Each one of them has harmful effects on the body but, for the moment, the “cocktail” effect linked to their interaction is unknown. Time will tell.

Known endocrine disruptors

Known chemical endocrine disruptors are only “the tip of the iceberg”. We find, in the context of food, dioxin, and in that of care products, parabens, Triclosan, silicones, 4-MBC and 3-BC used in sunscreens, phthalates, bisphenols A and S. Many other studies involving a number of chemicals have yet to be carried out.