Energy Imbalance and Body Weight

Positive energy balance leads to body weight gain and negative energy leads to body weight loss. There is no fixed relationship between these two variables so that relatively small energy retention can be accompanied by large body weight gain and vice versa. The confounding factor is the associated water storage.

Long-term fluctuations in fat stores will be reflected in body weight. There is a difference in the energy value of fat mass and fat-free mass, the latter including the glycogen-water pool and the protein-water pool (Table 5).

Energy density of the tissue stored (or the substrate pool stored) represents an indicator of the composition of tissue stored or mobilized. It is defined as the total calorie per gram of substance. It is about 8 kcal g-1 for adipose tissue compared to the fat value (triglyceride) of 9 kcal g-1. This lower former value is due to the fact that fat is diluted out by the small amount of water (5-10%) and proteins the adipose tissue contains. As explained previously, the energy density of the glycogen-water pool is low, about 1kcalg-1, since glycogen (4.2 kcal g-1) is associated with approximately 3 times its weight of water.

Let us take an energy imbalance of say 1000 kcal. The body weight change will be approximately 8 times lower (i.e., «125 g) if fat is stored in adipose tissue, as compared to glycogen stored (under the form of glycogen-water pool) in liver and muscles («1000 g). In other words, rapid weight gain (or weight loss) means little fat storage despite what the layman thinks. Day-to-day energy imbalance is generally accommodated by water retention due to changes in carbohydrate storage and sodium intake.

In real life, it is more reasonable to consider that the reserve is composed of a mixture in different proportions of fat and glycogen. If about half of the energy imbalance is accounted for by fat and half by glycogen storage, the energy density will be 4-5 kcal g-1. With the imbalance value described above, it will generate a body weight change of 400-500g.

Table 5 Body stores of energy as different macronutrients

Substrate Form of Pool size Tissues storage

The energy balance varies from day to day. The changes in daily energy intake and expenditure are not necessarily synchronized. Positive energy balance on one day may not be spontaneously compensated by negative energy balance on the subsequent day, so that it is important to consider the overall energy balance regulation over a prolonged period of time. Short-term day-to-day energy imbalance is mostly accommodated by rapid changes in carbohydrate balance, whereas over a prolonged period of time, positive energy balance is mostly expressed as fat storage since carbohydrate stores are small (Figure 6).

To what extent do alterations in energy output contribute to the regulation of energy balance and stability of body weight? To understand the regulation of a system, it must be subjected to perturbation. Excess food intake during overfeeding or deficit in food intake during underfeeding disrupts the balance system.

Overfeeding Studies (Figure 8)

In a perfectly regulated system, any increase in energy intake should be offset by a change in energy expenditure of the same magnitude and direction. However, a 100% efficient adaptive process would obviously be counter productive, since this would signify that an increase in energy storage (required during nutritional rehabilitation) or an increase in energy mobilization (required for decreasing body weight) would be very limited. Adaptation to energy imbalance only occurs at the cost of increasing (or decreasing) body weight. In fact, excess energy intakes result in an increased metabolic turnover and energy flux through the mechanism of adaptive thermogenesis. The efficiency of energy storage is not constant and depends upon several factors including the magnitude of energy imbalance and the composition of the surfeit energy fed, as well as endogenous factors. As shown in Figure 8, the energy expenditure increases during acute overfeeding, an evidence of the 'flexibility' of the metabolism.

% Water in Daily Energy density Postprandial tissues imbalance (kcal/g^1) thermogenesis

70-80 Large '4 Average

5-10 Small '8 Low

70-75 Small '4 High

Carbohydrate Glycogen Small (limited) Liver + muscles

Fat Triglycerides Moderate-large Adipose tissue

(unlimited)

Protein Protein Moderate Lean tissue

(limited) (muscle)

Total E expenditure

E equilibrium

ADAPTATION

Figure 8 Energy balance in underfeeding (below maintenance) and overfeeding (above maintenance) conditions. E, energy.

Total E expenditure

E equilibrium

ADAPTATION

Figure 8 Energy balance in underfeeding (below maintenance) and overfeeding (above maintenance) conditions. E, energy.

Underfeeding Studies (Figure 8)

Analysis of underfeeding experiments shows that the decrease in energy expenditure has three components. First, if energy intake is decreased the thermic effect of feeding (about 10% of energy intake) is similarly decreased. Second, there is an adaptive decrease in metabolic rate during the first week, related in part to a decrease in sympathetic activity. The magnitude of this decrease is significantly related to the initial metabolic rate, and is usually about 5-8%. Third, there is a decrease in metabolic rate related to the weight lost: most investigators find a decrease of 10-12 kcal per day per kg weight loss. The effect of all three processes is that a person who lost weight from, say, 100 kg to 70 kg (a 30% reduction in weight) would experience about a 15% reduction in energy requirements for weight maintenance. Thus, a decrease in energy intake causes a reduction in body weight but, provided the decrease is not too great, a new equilibrium will be reached at which the reduced requirement will be satisfied by the reduced intake, and body weight will stabilize. Taken together we can conclude that the efficiency of energy utilization is lower in overfeeding than in underfeeding conditions because, substrate storage in tissues is energetically costly (ATP needs), whereas the process of energy mobilization requires little energy. In the former situation excess energy must be dissipated.

Adaptive changes in thermogenesis do attenuate the impact on energy balance of excessive or insufficient food consumption (as compared to requirement). The magnitude of adaptive thermogeneis varies as a function of the nature of excess substrates fed (protein is higher than carbohydrate and fat).

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