Origin and Nutrient Determinants

Since the time of Lavoisier, it has been known that the ingestion of foods by animals and humans produces an increase in oxygen consumption. This increase in metabolic rate, originally called 'specific dynamic action' (SDA) is now widely referred to as the 'thermic effect' (TE) of food or 'diet-induced thermogenesis' (DIT) [1]. This effect starts generally 1 h after ingestion, reaches a maximum after 3 h later, and continues at this level for several hours [2]. The DIT is a component of the total energy expenditure, which includes energy expenditure required for performance of cellular and organ functions (basal metabolism [BM]), physical activity, and thermoregulation of body temperature. Supplementary energy is required for metabolic processes taking place during growth, pregnancy, and lactation [3]. In quantitative terms DIT represents about 10% of total energy expenditure (15%

together with cold-induced thermogenesis). BM accounts for about 60-75% of total energy expenditure, with the remaining due to physical activity (10-20% of total energy expenditure) (Fig. 1) [4].

There are two energetic aspects of the food intake effect: the first, and major one, is the obligatory expenditure in order to digest, absorb, distribute, and store the nutrients ingested; the second is the facultative expenditure inducing additional heat production by activation of brown adipose tissue (BAT) [5]. The amount of energy required for handling incoming food is related to the type and the quantity of carbohydrates, fats, and proteins ingested. Fat is the least 'expensive' in terms of DIT, since it requires relatively little hydrolysis and has a fairly direct pathway to storage tissue (3-4% of ingested calories). Protein is the most 'expensive' for DIT, requiring expenditures up to 30% of the inherent energy for processing, which includes removal of nitrogen, synthesis of urea, and gluconeogenesis (on average,

Basal metabolism

thermogenesis

Fig. 1. Categories of energy expenditure

15-20% of ingested calories). Carbohydrate is intermediate with respect to DIT; it requires considerable metabolism when converted and stored as triglyceride and less when converted to glycogen (10-15% of ingested calories). Carbohydrate (and also fat) can also elicit increased heat production not related to the use of energy for nutrient digestion, transport, and storage.

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