The proportion of people suffering from overweight or obesity is greater in adults (18 – 64 years) than in children and adolescents (from 3 to 17 years). Overweight and obesity are indeed linked to age:

• body composition develops with age (from 20-30 years, the lean mass, like the skin, bones, muscles and organs, decreases while the fat mass increases)
• basal metabolism (expenditure of energy at rest), the thermic effect of foodstuffs (expenditure of energy in order to digest) and physical activity decrease with age
• hormonal changes (after menopause the production of oestrogen and progesterone decreases and leads to an increase in the laying down of visceral fat in women).

The percentage of obesity (based on the BMI [1]) tends to increase with lower education. People with a high level of education seem better informed about obesity risks [2] and the means of preventing [3] weight gain, particularly with regard to adopting good dietary habits. On the other hand, people with a lower level of education are less likely to have physically active leisure-time activities (Anthropometry, Food consumption survey, 2014-2015, WIV-ISP, the predecessor of Sciensano, only in French [4] and Dutch [5]).
 
The level of education also influences the level of income and reinforces social disparities. People with a higher level of education have greater financial means for the purchase of healthy foodstuffs, such as fruit and vegetables. Conversely, people with a lower level of income will be more inclined to consume industrial, processed products and fast food, which cost less and often contain a lot of saturated fats, quick-release sugars and salt.

Children and adolescents are a more vulnerable group of the population with regard to the problem of overweight and obesity.

While childhood obesity is partly determined genetically, environmental factors are promoting sedentary life style and the consumption of unhealthy foods by children and adolescents:

• availability of processed industrial food
• technology and omnipresence of screens (computers, mobile phones, television etc.)
• advertising and marketing campaigns for unhealthy products targeting children
• quality of food in school canteens.

If childhood is a critical period for the development of obesity, it is also the ideal period to prevent it or take care of it, in so far as food consumption and activity patterns are developing during this period.

It must be said, however, that the progression of obesity among children and adolescents has tended to become stabilised in recent years (Anthropometry, Food consumption survey, 2014-2015, WIV-ISP, the predecessor of Sciensano, only in French [4] and Dutch [5]).