The Physiology of Weight Change




All professionals dealing with obese people, especially obesity surgeons who deal with extremely obese people, have observed the discrepancy existing between what many fat persons say and what are the common beliefs for both lay people and medical doctors. One example indicates what I mean. Practically all of the about 10,000 obese patients who came to my observation during the last 25 years told me a story of progressive lifelong weight gain, whereas no one of them ever told me of a progressive lifelong increase in food intake. The easier explanation for this apparently inexplicable phenomenon is that obese people lie. But, why should they lie to the bariatric surgeon, and, above all, is it possible that all of them lie?

Other common statements of the obese persons (especially women) which may be difficult to explain are: 1) that there are many other persons (especially men) of the same age and height who eat much more than them and still are much thinner than they are; 2) that they do not lose weight even staying on a diet; 3) that, after losing weight on a diet, when they go back to the previous food intake they stabilize at a higher weight than the previous one. Common sense teaches that when many people tell the same story they are probably telling the truth. So, the conclusion is that there must be something we have not understood.

The aim of this paper is to clarify the relationship between energy balance and body weight, in order to avoid grossly mistaken evaluations on which are based many false assumptions, beliefs and expectations that are today frequently accepted also, if not especially, among professionals dealing with obesity.




Like any other natural entity, human body must obey physical laws, in this case represented by the first principle of thermodynamics, according to which energy cannot be created nor destroyed, but only transformed. Consequently, all excess energy introduced must necessarily modify the internal energy of the system, being retransformed in potential chemical energy. And, since our main energy store is fat, an energy intake greater than energy expenditure unavoidably causes an increase of adipose tissue, which is always accompanied by an increase of lean body mass, and thus an increase of body weight.

Starting from a condition of weight stability, a permanent increase of energy intake causes an increase of body weight. Since body energy expenditure is proportional to body weight, an increase of the latter also causes an increase of energy expenditure. The body weight will then stabilize when the energy expenditure of the weight gain will equal the increase of energy intake, the balance condition being re-established. Exactly the opposite happens if the energy intake is permanently reduced. It may then be concluded that, under the same conditions (and then in each individual), to each definite energy intake corresponds a definite body weight, which is the one that produces an energy expenditure equal to that energy intake, and to each definite permanent change of energy intake corresponds a definite change of body weight, which is the one that produces a change of energy expenditure equal to that change of energy intake. For the same permanent change of energy intake, the size of weight change evidently depends on the energy expenditure of the weight that is gained or lost, which in turn depends on its composition and on the unit energy expenditure of its components.

The weight gained or lost by a person with normal body weight is composed of two-thirds adipose tissue and one-third lean body mass, which consume about 5 and 40 Cal/kg/day, respectively. One kilogram of such weight consumes then about 17 Cal/day. Therefore, if a normal weight person increases or decreases permanently his/her energy intake by 100 Cal/day, his her body weight will eventually increase or decrease by about 6 kilograms, which produce an energy expenditure equal to the variation of energy intake. Considering that 100 Calories represent more or less the energy content of a cappuccino, the corresponding weight change appears decidedly greater than what is commonly believed.

How long does it take to gain or lose the above 6 kilograms? It evidently depends on how many Calories are cumulatively necessary to build a new kilogram of body weight (energy cost of weight gain = energy content of the tissue + energy needed to build it) and, vice versa, on how many Calories are provided by the demolition of one kilogram of existing body weight (energy yield of weight loss = energy content of the tissue – energy needed to demolish it). The energy cost of adipose tissue and lean body mass correspond to about 10,000 and 2,000 Cal/kg, and the energy yield to about 7,500 and 800 Cal/kg, respectively. Therefore, energy cost and yield of one kilogram of weight with the above-mentioned composition (two-thirds adipose tissue and one-third lean body mass) are equivalent to about 7,000 and 5,400 Calories, respectively. Since 6 kilograms are to be gained or lost, the overall cost or yield would amount to 42,000 and 32,000, and, as the positive or negative energy price paid per day is 100 Calories, one would be tempted to conclude that total gain or loss would be completed in 420 and 320 days, respectively. However, energy balance is positive or negative by 100 Cal/day only initially, then it gradually reduces because the body energy expenditure changes according to the consumption of the weight gained or lost. Times for gain or loss are consequently lengthened, with an asymptotic curve such as 99% of the total weight is gained or lost in about 4 and 3 years, respectively. However, due to this type of curve, 75% of the total weight is gained or lost in little more and little less than one year, respectively.

Interestingly, if the permanent change of energy intake, and thus the weight change, was smaller or greater (e.g., 200 Cal/day for 12 kilograms), the composition and consequently the cost and yield of the weight gained or lost being the same, the time for the total weight change would remain exactly the same. In fact, the quicker variation of body weight due to the greater energy imbalance would be exactly counterbalanced by the quicker variation of energy expenditure that accompanies the weight variation. Instead, if the composition of weight change was different, for the same variation of energy intake times for gain or loss would be the longer or shorter the greater or smaller the percentage of adipose tissue (and thus the cost or the yield) in the weight gained or lost.

Whereas there is no theoretical upper limit to body weight in humans, there is probably a limit to lean body mass size, as there is for stature, that may be indicated as 100 kg for men and 70 kg for women. The more the weight increases and the lean body mass approaches that limit the smaller is its increase in proportion to body weight increase, and thus the composition of weight gain or loss varies according to the starting body weight. Consequently, the weight gained or lost by a heavier individual, containing less lean body mass, consumes less and costs or yields more in comparison with the same weight gained or lost by a lighter individual. Therefore, a heavier person, for the same energy intake variation, gains or loses more weight and in a longer time than a lighter person. To give an example, one kilogram of weight gained or lost by a very lean person, composed of 50% lean body mass, consumes about 22.5 Cal/day and costs or yields about 6,000 or 4,000 Calories, whereas one kilogram gained or lost by a person so heavy that the percentage of lean body mass in his/her weight variation can be considered negligible consumes about 5 Cal/day and costs or yields about 10,000 or 7,500 Calories, respectively. A permanent change in energy intake of 100 Cal/day would cause the first person to gain or lose 4.5 kilograms in about 2.5 or 1.8 years, while the second person would gain or lose as much as 20 kilograms, even if in about 19 or 14 years, respectively.

Contrarily to what is generally believed, the heavier one is, the easier it is to gain further weight, and this occurs for smaller and smaller increases of energy intake, so small that it becomes practically impossible to perceive them; in addition, the long time necessary to reach the stabilization makes it very difficult to notice the weight variation.

This phenomenon explains the common clinical observation of the patient who reports a history of progressive lifelong weight gain with no progressive lifelong increase in food intake. Moreover, it is quite possible that the energy intake has actually remained the same, the progressive weight gain being caused by decreased energy expenditure due either to the progressive reduction of physical activity that generally accompanies weight gain or to the progressive reduction of lean body mass which inevitably occurs with aging.

In reality, the fact that weight gain, since the ratio fat/lean in what is gained is inverse to that of what exists, alters body composition at the advantage of adipose tissue, thus causing a progressive decreasing energy expenditure per unit of body weight, has been known for a long time. What is interesting to note is the extent that this phenomenon can reach considering that the composition of the weight gain is not constant, but containing a percentage of lean body mass which progressively decreases with the increase of body weight. As a consequence, with progression of weight gain, any further gain corresponds to smaller and smaller increases of energy expenditure, with a progressively greater discrepancy between the increase of body weight and the increase of energy expenditure. The obvious result is that the differences in energy expenditure/intake among subjects with relatively great differences in body weight is relatively small. And this explains another common clinical observation (especially in women) which is ignored or denied by many bariatric professionals.

The obese patient eats on the average more than the lean person, but not much more.

In our population, the difference in total energy expenditure, and thus of energy intake, between a group of 34 lean subjects with mean stable body weight of 65 kg (range 55-83 kg; mean TEE: 1,989 Cal/day, range 1,167-3,176) and a group of 86 obese patients with mean stable body weight of 124 kg (range 90-203 kg; mean TEE: 2,246, range 1,322-5,064) was little more than 250 Cal/day. Furthermore, due to the enormous interindividual variability, caused by many variables other than composition of weight change, the overlap between the two groups was such that it was well possible for a subject much heavier than another one to consume and eat much less than the latter.




We have dealt above with the variations of body weight consequent to a permanent variation of the energy intake. They also entail a variation of energy expenditure which is secondary to the variation of body weight and stops when the new energy expenditure reaches the balance with the new energy intake, with the ensuing stop of the body weight variation. Let us see now what happens in the opposite case, i.e. when the variation of energy expenditure is primitive, and, the energy intake remaining unchanged, it causes the variation of body weight.

Total energy expenditure (TEE) is composed about 70% of “resting energy expenditure” (REE), about 20% of “activity energy expenditure” (AEE), and about 10% of “diet-induced thermogenesis” (DIT), which represents the energy expenditure necessary for mastication, transit, digestion, absorption, and metabolism of food. What varies with body weight is essentially REE. Let us refer initially to the most common case, that is the variation of AEE due to decrease or increase of physical exercise. A permanent change of AEE, the energy intake remaining unchanged, obviously causes an energy imbalance, with consequent increase or decrease of body weight until the ensuing change of REE brings TEE back to its starting level, corresponding to the unchanged energy intake. Exactly the same occurs following a possible change of DIT in percent of energy intake which, the latter being unchanged, evidently results in a change of the DIT absolute value. Another very common case is the disappearance of the thermogenesis induced by cigarette smoking, which inevitably causes weight gain if the smoke cessation is not accompanied by a reduction of energy intake. In all these instances, where the energy intake is unchanged, the variation of one of the TEE components is counterbalanced by the variation of opposite sign of another component, namely REE, so the energy expenditure goes back to its starting value thanks to a variation of body weight.

Only apparently more complex is the case of energy expenditure change due to increase or decrease of REE per unit of body weight, i.e. a variation of REE not caused by a variation of body weight, and then of the consuming mass, but the weight being unchanged, by the fact that the same mass consumes (in resting conditions) more or less than before. This simply occurs because of change of the ratios among the components of body mass, which, as we know, have very different unit resting consumption. The most common case is the variation of the fat/lean ratio, that, as said above, inevitably follows any body weight change, but can occur, body weight being unchanged, for instance after weight loss and regain for a drastic diet, or simply due to aging. However, let us remember that the two main components of lean body mass also have very different REE, that of viscera (about 360 Cal/kg/day) much greater than that of muscles (about 18 Cal/kg/day). Therefore, if for any reason, with body weight remaining unchanged, within lean body mass muscle/viscera ratio changes, REE of lean body mass increases or decreases because the proportions of tissues with very different REE have changed. And this also obviously results in a variation of REE per unit of body weight.

In case of variation of REE per unit of body weight, as a rule the energy balance is not re-established by variation of opposite sign of another component of TEE (even if this is a possible eventuality), but by variation of REE itself which, by increasing or decreasing as absolute value with the variation of body weight, compensates the relative variation that had determined the energy imbalance, thus reestablishing the equilibrium. In other words, the energy intake being unchanged, if each kilogram of body weight consumes more or less than before, the number of kilograms must decrease or increase to produce an energy expenditure equal to the energy intake. This phenomenon is better understood if we think that the change of body composition that causes it does not occur contextually (even if generally the long time necessary to completion of the overall variation could make it believable), but first the loss or gain of a weight with a certain composition and thus a certain unit consumption, and subsequently the gain or loss of another weight with a different composition and unit consumption may be identified, the overall energy expenditure of the second being necessarily equal to that of the first.

The physiological loss of lean body mass that occurs with aging, if the energy intake remains the same, compulsorily causes the gain of a body weight which, as we know, is composed of lean and fat, and thus consumes less and must be gained in a greater amount to produce the same consumption of what was lost. To give another example, drastic diets cause a weight loss containing a percentage of lean body mass which may exceed 50%, whereas the composition of the weight that is subsequently regained, as we saw, depends on the starting weight and contains anyway less lean in comparison with the weight lost, thus consuming less than the latter. In both cases the result is a person who, eating as much as before, necessarily weighs more.

The size of the change in body weight which, with the energy intake remaining constant, is caused by a change of body composition depends on the different unit resting consumption of the tissues that were exchanged. And, since the unit resting consumption of the different body constituents are very different among themselves, small changes of body composition may cause great changes of body weight.

In conclusion, we first learned that each variation of energy intake must be counterbalanced by an equal variation of energy expenditure, which occurs through a variation of body weight, so that for each individual to a definite energy intake must correspond the body weight that produces the energy expenditure equal to that intake. However, we also learned that the same energy expenditure may be produced by very different body weights, as well as the same body weight may produce very different energy expenditures, depending on different body compositions. This explains how individuals with equal body weight may have different energy intakes, and individuals with the same energy intake may have different body weights, and how a very heavy individual may have an energy intake much smaller than that of another much lighter individual, and vice versa. In summary, certainly the energy intake determines body weight and its variations, but the size of body weight and of its variations is determined by body composition and by its variations.




There are many biological phenomena, which occur as a consequence of variations of body weight, or energy intake, or energy expenditure, or energy balance, that in turn influence body weight. Some of these phenomena produce a negative feed-back system, opposing any body weight variation in any direction, and then tending to maintain body weight at any level it is. Other phenomena, again at any level of body weight, act only opposing its reduction, and other only favouring its increase. All these phenomena are to be considered “physiological”, because they occur necessarily in all individuals, and, even if they favour the onset of obesity, they do it equally in all subjects. Finally, there are phenomena favouring weight increase, congenital or acquired, that can be considered frankly “pathological”, because they occur only in some individuals, who, independently of the fact that they become obese or not, are in a condition of greater vulnerability towards weight gain as compared to other people.




The simplest among the phenomena that create a negative feed-back mechanism tending to limit the weight change is the above described variation of energy expenditure that accompanies the weight change itself and stops it when the energy balance has been reached. Other phenomena that act by limiting the energy imbalance, causing increase of energy expenditure in the case of positive energy balance and vice versa, are the variation of DIT which accompanies increase or decrease of energy intake and the variation of AEE which accompanies increase or decrease of body weight according to the corresponding variation of the mass to be moved. The increased appetite consequent to the increased energy cost of physical activity acts in the same direction.

The most important and also the most debated among these phenomena is represented by the variation of resting energy expenditure per unit of fat-free mass (REE/FFM) consequent to a consistent increase or a drastic reduction of energy intake. This phenomenon, particularly investigated in the case of reduction of energy intake, is generally admitted and considered totally reversible with the resumption of previous energy intake, whereas its permanence in the case of energy intake permanently reduced is questioned. In six subjects who had kept for three to ten years a body weight steadily reduced by a drastic diet from a mean of 103 kg to a mean of 69 kg we found a mean REE and energy intake smaller by 30% in comparison with a group of controls with equal mean body weight, while the lean body mass was smaller only by 15%. The variation of REE/FFM consequent to the great variations of energy intake could be caused, rather than by an actual variation of resting consumption of the tissue unit (very difficult to be explained), by an alteration of the viscera/muscles ratio for increase or decrease of the first sector consequent to greater or smaller trophic stimulation of the GI tract by food. This explanation, referring to a change of body composition, would also account for the persistence of the phenomenon as long as its cause persists, which in this case would not be reduction of food intake but reduced food intake.




As we saw, if the energy intake is permanently reduced, body weight also reduces until it produces en energy expenditure equal to the intake, and the size of the reduction is inversely proportional to the consumption of the weight loss, which in turn depends on its composition that is determined by the starting weight. As already mentioned, a drastic diet, either in absolute terms (less than 1,200 Cal/day) or in relative (more than 500 Calories less than the starting TEE), causes the loss of a weight containing a percentage of lean body mass greater than what would be physiological for that definite starting weight. This phenomenon, which was proved in fertile women, while the clinical experience and the results of our studies suggest that it does not occur in postmenopausal women or in men, has two types of negative consequences. On one side, starting from the same body weight, since the weight loss consumes more, for a definite cumulative reduction of energy intake less weight is lost, even if in shorter time, than what would be if the weight loss composition was the physiological one. On the other side, with the weight attained being the same, since more lean than due was lost, body weight contains less lean and more fat, and then it consumes less. This means that to maintain any weight after a drastic diet one has to eat less than another person who has lost the same weight with the physiological composition, or a person with equal body weight who never lost weight. But the worst aspect of this phenomenon is that the alteration of body composition that it causes is permanent. In fact, in the case of weight regain after a drastic diet, the composition of the weight regain is the one physiological for the starting weight. Thus, less lean and more fat are regained in comparison with what was lost, and consequently also the maintenance of a weight equal to that prior to the diet entails an energy intake smaller than that before dieting. And, since the alternative is to go back to the previous energy intake stabilizing at a higher weight than that before dieting, it is easily understood how a series of such ups and downs (which is defined as “weight cycling”) may lead to a progressive weight gain progressively more difficult to control. Many young women with no genetic predisposition to obesity, misled by mass media and not protected by specialized surveillance, vainly pursue unrealistic beauty ideals and eventually condemn themselves to become obese or to stay on a diet all their life.




As said above, with aging the lean body mass physiologically reduces, and, if this is not counterbalanced by an adequate reduction of energy intake, the obvious result is an increase of body weight. But the most perverse of the physiological mechanisms that favor weight gain can be identified with the already described variation of the composition of the weight gain or loss according with the starting weight. As we saw, the weight gained or lost contains the leaner the smaller is the body weight, and vice versa. Now, if the weight decreases this phenomenon opposes the weight loss (and, as to this aspect, it should have been described in the preceding paragraph). Actually, since the decrease of lean body mass is progressively increasing, the reduction of energy expenditure also progressively increases, and this obviously opposes further weight reduction. Indeed, due to the increasing lean/fat ratio in the weight loss, each kilogram successively lost consumes more and more, and then requires a negative variation of energy balance greater and greater to be lost. On the contrary, when body weight increases, the same phenomenon, instead of opposing the weight gain, favours it. In fact, the increase of lean body mass progressively decreases, and the same is true for the increase of energy expenditure. The lean/fat ratio in the weight gain decreases; each kilogram successively gained consumes less and less and is gained for a smaller and smaller positivity of energy balance. As we already saw, this explains how, especially in the already obese subject, relatively great increases of body weight are accounted for by relatively small increases of energy intake. Therefore, the variations of energy expenditure consequent to the modifications of body composition which accompany the weight changes produce effects that go in the same direction in both cases of weight gain and loss.

The sum of all the above physiological regulation mechanisms causes the human organism to tend not to the maintenance of body weight, even if at any level, as it is commonly believed, but on the whole to favor weight gain and to oppose weight loss, an aptitude that was selected during many thousands of centuries in conditions of food scarcity and that was certainly in other days providential for the preservation of our species.




The word “thermogenesis” (improperly used, since all types of energy consumption eventually result in heat production) defines the increase of REE consequent to stimuli such as cold, anxiety, cigarette smoking, caffeine (as well as pharmacological agents able to reproduce their effects), but above all food. The diet-induced thermogenesis (DIT), differently than that produced by other stimuli, was found reduced in obese subjects in comparison to lean individuals, in a proportion that, according to different studies, varies from one-third to two-thirds of cases. This necessarily entails, for the same energy intake, a greater body weight. A genetically reduced lean body mass would evidently produce the same effect. However, it is practically impossible to demonstrate the primitivity of both these phenomena, because there are no studies in the pre-obese, and the results of those carried out in the post-obese are conflicting and biased by other variables.

Other pathological phenomena, similarly, only hypothetical so far, that could favour weight gain are congenital or acquired alterations of the composition of weight gain or loss, but above all of the visceral mass size. On the contrary, it is beyond doubt that any pathological reduction of AEE, of any type and due to any cause, favours weight gain.