The balance between weight gain and weight gain loss is predominantly determined by what you eat, how much you eat, and by how much exercise you get. But another important factor is often neglected... Published February 27 in the open-access journal PLOS Biology, research conducted by Kevin Kelly, Owen McGuinness, Carl Johnson and colleagues of Vanderbilt University, USA shows that it's not just how many calories you eat, but WHEN you eat them that will determine how well you burn those calories.

Your daily biological clock and sleep regulate how the food you eat is metabolized; thus the choice of burning fats or carbohydrates changes depending on the time of day or night. Your body's circadian rhythm has programmed your body to burn fat when you sleep, so when you skip breakfast and then snack at night you delay burning the fat.

The researchers monitored the metabolism of mid-aged and older subjects in a whole-room respiratory chamber over two separate 56-hour sessions, using a "random crossover" experimental design. In each session, lunch and dinner were presented at the same times (12:30 and 17:45, respectively), but the timing of the third meal differed between the two halves of the study. Thus in one of the 56-hour bouts, the additional daily meal was presented as breakfast (8:00) whereas in the other session, a nutritionally equivalent meal was presented to the same subjects as a late-evening snack (22:00). The duration of the overnight fast was the same for both sessions.

Whereas the two sessions did not differ in the amount or type of food eaten or in the subjects' activity levels, the daily timing of nutrient availability, coupled with clock/sleep control of metabolism, flipped a switch in the subjects' fat/carbohydrate preference such that the late-evening snack session resulted in less fat burned when compared to the breakfast session. The timing of meals during the day/night cycle therefore affects the extent to which ingested food is used versus stored.

This study has important implications for eating habits, suggesting that a daily fast between the evening meal and breakfast will optimize weight management.