Eating increases the body’s metabolic rate, a phenomenon referred to as diet-induced thermogenesis (also known as the “thermic effect” of food). Diet-induced thermogenesis begins about an hour after eating, peaks about two hours later, and then maintains a steady level for several more hours. Approximately 5 to 15 percent of a person’s daily energy expenditure– an estimate of how many calories a person burns per day – is due to diet-induced thermogenesis.
A few factors influence the degree of diet-induced thermogenesis, including meal size, macronutrient content (protein versus fat, for example) and environmental temperature. Age and physical activity may also play roles in diet-induced thermogenesis. Findings from a new study suggest that circadian variations in energy expenditure influence diet-induced thermogenesis.
The randomized, cross-over, laboratory study involved 16 healthy, normal-weight men. Each of the men ate three meals per day in the laboratory for three days and maintained a regular sleep pattern. The authors of the study conducted indirect calorimetry tests to determine the participants' energy expenditure and collected participants' blood samples before and after meals to gauge glucose tolerance. The participants rated their feelings of hunger on a Likert scale.
Meals consisted of a high-calorie breakfast and low-calorie dinner or the converse – a low-calorie breakfast and a high-calorie dinner. The high-calorie meals provided 69 percent of the participants' calorie needs and the low-calorie meals provided 11 percent. All lunches were identical and provided 20 percent of the participants' calorie needs.
The indirect calorimetry tests revealed that the participants' diet-induced thermogenesis after breakfast was generally 2.5 times higher than after dinner. The participants' glucose levels were, on average, lower after breakfast than after dinner. Glucose levels were 17 percent higher after eating the low-calorie dinner compared with levels after eating the low-calorie breakfast. The participants reported having greater feelings of hunger, especially for sweet foods, on days when they ate the low-calorie breakfast.
These findings highlight the role of circadian variation in metabolism and underscore the need for modifying food intake to exploit this variation.
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