Effect of Meal Size on Energy Expenditure

Total energy expenditure (EE) can generally be divided into three major components: basal metabolic rate (BMR), thermogenesis, and physical activity (Table 1). In order for meal size to have an effect on the EE side of the energy balance equation, it must have an effect on one or more of these components. There is no evidence that meal size has an effect on BMR, which refers to the energy expended to maintain the body on a day-to-day basis. Thermogenesis broadly refers to the body's production of heat, which is divided into three categories: dietary, thermoregu-latory, and adaptive. It is the dietary category, commonly known as dietary-induced thermogenesis (DIT), that is of greatest relevance to the discussion of the effect of meal size on energy expenditure. It refers to the heat lost by the body as a result of the absorption and metabolism of a recently ingested meal. DIT represents approximately 10% of energy intake, and therefore the energy expended on DIT increases and decreases in relation to the size of the

Table 1 Major components of energy expenditure

Component Total energy Represents expenditure (%)

BMR 60-75 Day-to-day running costs of an individual (e.g., circulation) Thermogenesis 10-20 Heat produced by the body through dietary, adaptive, and thermoregulatory processes

Physical 100 - (BMR + The sum of work carried activity thermogenesis) out by an individual

BMR, basal metabolic rate.

meal and, more important, the energy value of the meal consumed. The larger the meal, the more energy will be expended to absorb, transport, and metabolize the nutrients consumed during that meal. For example, in the case of a meal containing 2000 kJ (478 kcal) of energy, approximately 200 kJ (48 kcal) will be expended on DIT alone. It is in the physical activity component of energy expenditure that the greatest variation between individuals is observed because physical activity levels (and therefore the energy expended on activity) are contingent on lifestyle choices such as employment and leisure time activities. The effect that meal size may have on physical activity is somewhat difficult to quantify. Meal size is perhaps more important to elite athletes, whose energy expenditure is two or three times greater than that of untrained weight-matched athletes with up to 40% of their energy expenditure being the cost of training.

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