Recent research confirms that obesity is mainly linked to an excess of the calorie contribution and not to a lack of physical activity.
Obesity has become one of the main causes of mortality, being directly responsible for more than 4 million deaths each year worldwide.
The economic development of societies seems to represent one of the main engines of this modern obesity crisis: for example, while obesity was a very rare phenomenon in 19e century (and still remains today in traditional communities of farmers and pickers), it has become very common in the past 50 years in most industrialized populations of the globe.
Energy imbalance
Basically, weight gain is the result of an imbalance caused by a consumption of calories which exceeds the energy needs of the body.
Two major changes brought by the modern lifestyle can promote this imbalance: 1) The greatest availability of food that encourages the overconsumption of calories and 2) the decrease in physical activity associated with economic development (industrialized populations are much less physically active than traditional communities of farmers and pickers).
The exact contribution of these two aspects of the modern lifestyle to the current obesity crisis, however, remains poorly understood: have we become too big because we eat too much or because we do not spend enough calories?
Constant energy expenditure
To answer this question, a team of researchers compared the energy expenses of 4,213 men and women living in dozens of countries with very varied socio-economic conditions (industrialized countries, hunter-gatherers, farmers) (1).
To precisely measure these energy expenses, an elegant reference method is to use heavy water, where hydrogen is replaced by its isotope, Deuterium (therefore D2O instead of H2O).
When we burn calories, certain oxygen atoms present in water are used to produce carbon dioxide (Co2) that we expire. By measuring the excess of heavy hydrogen in the urine of a person a few days after drinking marked water, scientists can estimate the amount of oxygen transformed into carbon dioxide, and therefore very precisely the quantity of burnt metabolic energy.
The results obtained by this method are very surprising: the inhabitants of economically developed countries, although less physically active, spend about the same number of calories every day than those living in less industrialized countries which are more active.
It seems that the body has control mechanisms that seek to maintain our total energy expenditure at a constant level: for example, if we do a lot of exercise, the brain will decrease in return the basic metabolism to compensate. The molecular mechanisms in question remain to be better understood, but it nevertheless seems clear that sedentary lifestyle is not the main cause of the strong impact of obesity in rich countries.
Suspect number 1: ultra -formed foods
In another component of the study, the researchers noted a strong correlation between a percentage of higher body fat and the consumption of ultra -formed food. The contribution of these industrial foods to the obesity epidemic is also supported by a very large number of studies which have noted an association between the consumption of these products and the impact of obesity (2).
The obesogenic effect of these foods would be due to a combination of factors, in particular their particular orosensory properties (soft texture, absence of fibers) which encourage the overconsumption of calories, their very high energy density, as well as their content in various additives (emulsifiers, in particular) which disrupt metabolism.
In short, these results indicate that a modification of eating habits, in particular with regard to ultra -formed industrial foods, represents the absolute priority to fight against obesity. This does not mean that physical activity is not important: on the contrary, exercise is possibly the aspect of lifestyle that has the most positive influence on health, both physical and mental.
In terms of body weight control, however, it is first and foremost that we eat daily that remains the most important aspect. The same reasoning applies for those who wish to lose weight: it is first and foremost a decrease in the calorie contribution which must be prioritized to get there. To paraphrase the old adage, there is no point in running, you have to eat in time.
References
(1) McGrosky A et coll. Energy expenditure and obesity across the economic spectrum, Proc. Natl Acad. Sci. U.S.A 2025; 122: e2420902122.
(2) Juul F et coll. The role of ultra-processed food in obesity. Nat. Rev. Endocrinol.published on July 14, 2025.
