A calorie is a calorie ... or is it?

Is a calorie always a calorie? Technically, yes. One food calorie is equivalent to the amount of energy needed to raise the temperature of one kilogram of water from 15° to 16°C. The emphasis on simply lowering calorie intake for weight loss has long been stressed, and the FDA recently even began requiring chain restaurants, movie theaters, and pizza parlors across the country to post calorie counts on their menus.

However, over the past 15 years, the hypothesis of all calories being equal has been challenged, particularly with regard to lower-carbohydrate^[1] and higher-protein^[2] diets. There's some evidence that lower-carb and/or higher-protein diets increase calorie expenditure more than predicted, and therefore the amount of calories retained in your fat tissue could also be less than predicted.

An increasing number of studies in animals^[3] have shown that the timing of food intake has an influence on weight gain, glucose control, lipid levels, and inflammation, independent of total daily caloric intake. This is due to circadian rhythms^[4], or our internal body clock, which helps our body predict when we should be sleeping, eating, exercising, and so on. While research in this area has been on the rise, this is not a new concept. Daily fluctuations in metabolism were first observed more than 60 years ago, and for nearly 30 years it’s been known that a decrease in carbohydrate oxidation^[5] occurs in the evening, compared to the morning. While human studies are currently less conclusive than the animal studies, more research^[6] is being undertaken to see how closely these effects translate.

Resting metabolic rate (RMR) refers to the amount of energy used while the body is at rest, or to put it another way, how many calories your body would use if you were laying down all day. A related measure is diet-induced thermogenesis, which refers to the increase in RMR after consumption of food, and accounts for about 10% of daily energy expenditure. This is depicted in Figure 1.

Figure 1: How your daily calories are spent

While 10% may seem trivial, the thermogenic response may actually be blunted in obese individuals^[7] and these small differences over a long period of time can add up. In fact, it’s the approximate difference in calorie intake (and energy expenditure) between a lean person and an overweight person. Additionally, time of day may have an effect on metabolic rate, as the diet-induced thermogenesis after consuming a snack in the morning has been measured to be higher^[8] than after the same snack in the evening.

Due to a variety of reasons, including differences in activity, habitual diets, fasting times, and meal sizes, as well as low number of subjects and lack of multiple measurements, the data in the literature on metabolic rate throughout the day is somewhat variable. To study daily variations in diet-induced thermogenesis, measuring RMR under the same conditions at different times of day is required. Accordingly, this new study compared the caloric and metabolic responses to a standardized meal consumed in the morning (8 a.m.) and evening (8 p.m.) in healthy participants after controlling for diet, fasting duration and pre-measurement activity level.

Diet-induced thermogenesis refers to the increase in metabolic rate after eating, and may be decreased at certain times of day, as well as in some specific populations.