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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