Why are there people who gain weight even if they eat little?

By Juan Ignacio Pérez Iglesias, University of the Basque Country / Euskal Herriko Unibertsitatea

Gaining or losing weight is, simplifying things a bit, the result of a balance between acquisition and energy expenditure. The organism acquires the one it assimilates from the ingested food. And due to the activities it develops it also spends energy, it loses it in the form of dissipated heat.

The activities are very varied: some are necessary for the normal functioning of the organic systems (cardiac, renal, nervous activities), including the maintenance of body temperature. Others are linked to the renewal and repair of cells and tissues. They are, on the other hand, responsible for the synthesis of new structures. Finally, also the muscles involved in physical exercise. When the acquisition of energy exceeds spending, the body gains mass. If the expense is greater than the assimilation of energy, the opposite occurs.

Put that way things seem very simple. Although in these matters nothing is as simple as it may seem, the acquisition of energy is a relatively simple term. There are two main factors to consider: the amount of food and its composition, which affects its energy content and the ease or difficulty of digesting and assimilating it. Energy expenditure has more complications because several factors influence.

If we consider what are the determinants of the elements of acquisition and energy expenditure, nobody is surprised that overweight and obesity have reached a high prevalence in today's societies. There is a lot of food and, in addition, in general we have a predilection for succulent foods, with high energy content and easy absorption. Regarding spending, we have a notable tendency towards sedentary lifestyle and physical inactivity. We also live in environments of great thermal comfort: we are hardly in conditions that force the body to spend a lot of energy to keep the body temperature stable.

Things seem very simple. And yet, they are not so much.

We all know people who, although thin, are not deprived at mealtime. Those who gain weight easily contemplate them with envy, aware of how hard it is to limit themselves to a good table, not snack between hours or suffer running through the streets, or pedaling in an unbridled way on an exercise bike. The opposite is also known to us, people who seem not to eat much and who, however, have excessive weight.

Why eating similar amounts of food and maintaining similar levels of physical activity some are fatter and others thinner? What do those apparent obey inconsistencies metabolic? Some may be due to factors not taken into consideration. But another important element to consider is the genetic factor, especially because of its incidence on metabolism and, therefore, on energy expenditure.

Obesity is inherited

A study published this year compared the genome of 1600 very thin people, 2000 very obese and 10 400 of normal weight. The results confirmed that obesity is an inheritable condition to a degree not very high but important. To be precise, they estimated their heritability at 32%.

Thinness is also inheritable, although to a slightly lesser extent, 28%. These percentages indicate the proportion of the variability of the trait studied – obesity or thinness – in the population that is due to hereditary factors.

They have also identified a set of genetic variants linked to a condition, some of which were already known, and others linked to the opposite. Therefore, obesity and thinness, in part at least, are also inherited. Although this, to some, does not help us much comfort.


A version of this article was originally published in the Notebook of Scientific Culture, a publication of the Chair of Scientific Culture of the UPV / EHU.


Juan Ignacio Pérez Iglesias, Professor of Physiology, University of the Basque Country / Euskal Herriko Unibertsitatea

This article was originally published in The Conversation. Read the original.

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