Thermogenic responses

Thermogenesis describes increases in the MR above RMR in response to things such as food intake, exposure to cold or heat and the body’s physiological responses to fear or stress.

Thermic effect of food. The thermic effect of food (TEF), also called diet-induced thermo-genesis, represents increases in MR above RMR resulting from the energy costs of digestion, absorption, transport, metabolism and storage of food consumed. Consumption of food produces a transient increase in MR until it is assimilated. When these processes are complete, MR returns to the pre-meal state. The TEF is the major form of thermogenesis, accounting for approximately 10-15 per cent of daily energy expenditure. Many studies have been done to examine if obesity is related to a decrease in TEF. In a recent review of all these studies it was discovered that several found a difference between the lean and obese and several found no difference. It is important to remember that even in those studies which found a difference, it was only of the order of about 1 per cent of energy expenditure.

The major factors which influence the TEF are the meal size and composition. A higher TEF is seen with a higher calorie load and a higher proportion of protein. The energy cost of processing protein is very high (~ 20% of its calories) compared to fat and carbohydrate which only cost a few per cent of its calories. Other factors which affect TEF are palatability of the food and time of the meal, as well as a person’s age, genetic factors, physical fitness and some specific foods.

Caffeine: Several studies have shown an increase in MR of up to 10-30 per cent for 1-3 hours after administering oral or intravenous caffeine. It appears that the optimal amount is the equivalent of about 2—3 cups of instant coffee per day. Caffeine use in athletes has also been shown to increase performance by increasing lipolysis and, therefore, sparing carbohydrate during endurance events. However, recent work also suggests that the effect of caffeine is not as great in the obese as in normal weight individuals.

Spicy foods: Spicy foods such as chillies and curries, which contain an ingredient known as capsaicin, have been shown to elicit small increases in TEF. The use of spices not only adds flavour to low-fat dishes therefore, but also may have a small, transient effect on increasing MR.

The importance of TEF. Most researchers do not regard TEF as a major cause of obesity.

According to Ravussin and Swinburn:

. . . one can safely state that individual differences in the thermic effect of food can only account for small differences in daily energy expenditure. This implies that a minimal weight gain will increase energy expenditure (mostly in relationship to resting metabolic rate and the energy cost of physical activity) and will therefore be sufficient to offset any impairment in the thermic effect of food.

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