Consumption of food results in a significant increase in metabolic activity. This activity relates the energy required to absorb, metabolise, store and excrete food components. Such thermogenesis is called the thermic effects of food (TEF), and it can contribute significantly to the total energy expenditure in humans. In fact in a typical 2500 kcal diet, the thermic effect of food may account for around 250 kcals of energy per day. It is known that the obese show a blunted thermic effect to food, and this has been shown to be caused by a significant reduction in circulating catecholamines (mainly adrenaline) following food consumption, when compared to healthy normal weight individuals. Natural cycles, called circadian rhythms are known to occur in many of the hormone systems in the body and increasingly it is being shown that variations exist in the circadian rhythms associated with thermogenesis. Studies show that the thermic effect of food shows significant variations throughout the day.
For example, in one study1, researchers gave 20 healthy subjects a standard meal either in the morning, or in the evening. Following the meal the researchers analysed the change in the energy expenditure of the subjects. The fasting metabolic rate, taken before the meal, was not different between the morning and the evening for the subjects. However, the resting metabolic rate was significantly elevated after the morning meal compared to the evening meal. In addition, the glucose and free fatty acid concentrations were significantly lower following the morning compared to the evening meal. Therefore the same meal can have significantly different effects depending on the time the meal is taken. Eating later in the day may reduce the energy required for the absorption, metabolism and storage of ingested food and may alter the plasma levels of circulating macronutrients, particularly glucose and free fatty acids. This illustrates the importance of meal timing in any nutritional strategies aimed at altering health outcomes.
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