Resting metabolic rate is used as a tool in clinical practice to determine the effects of nutrients on energy expenditure. However, these measurements can only be useful if there is a firm understanding of the changes that occur to resting metabolic rate during the course of the day and through other external factors. Little research has been performed on the day to day variability in resting metabolic rate, although it is widely used as a research tool. There are many factors that could contribute to changes in resting metabolic rate including pharmaceutical drugs, the thermic effect of food, natural variation, instrument variability, as well as errors. Because of this resting metabolic rate protocol is standardised to minimise external variables, and is usually measured after an 8 to 12 hour fast with avoidance of exercise for 12 hours.
Researchers1 have investigated the effects of diurnal variation on resting metabolic rate in 12 healthy men and 25 health women aged 21 to 57 years. Subjects had body mass indexes of between 17 and 34 kg/m2 and body fat levels of between 6 and 54%. Measurements of resting metabolic rate were taken in the morning following a 12 hour fast and 12 hour abstinence of exercise, and in the afternoon following a 4 hour fast and 12 hour abstinence of exercise. The measurements were taken on two consecutive days using the ventilated hood technique and body composition was assessed with dual energy x-ray absorptiometry. The afternoon measurements of resting metabolic rate (1593.5 and 1602.0 kcal/day) were significantly higher than the morning (1508.0 and 1511.4 kcal/day). The mean difference between the morning and afternoon session was 99.0 kcal/d.
These results suggest that resting metabolic rate is around 6% higher in the afternoon, compared to the morning. Previous research suggests that 60% of the thermic effect of food has attenuated after 3 hours, 78% after 4 hours and 91% after 5 hours. Other research suggests that 3 hours after a 1500 kJ meal, dietary induced thermogenesis has almost completely subsided. The afternoon measurements in this study were therefore far enough after the ingestion of food to negate any thermic effect from the meal interfering with the resting metabolic rate. Measurements of resting metabolic rate in healthy men and women appear to show good between subject repeatability and consistency between the morning and the afternoon. However, if exercise was performed in the proceeding 12 hours, resting metabolic rate could be expected to be significantly increased.
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