Examine publishes rigorous, unbiased analysis of the latest and most important nutrition and supplementation studies each month, available to all Examine Members. Click here to learn more or log in.

In this article

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

Obesity research typically focus on what you eat, but less frequently touches on when you should eat it. Since animal models have shown strong results for meal timing, this study looked at potential weight-related benefits of eating earlier in humans.

Study under review: Is the timing of caloric intake associated with variation in diet-induced thermogenesis and in the metabolic pattern? A randomized cross-over study


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.

Who and what was studied?

Become an Examine Member to read the full article.

Becoming an Examine Member will keep you on the cutting edge of health research with access to in-depth analyses such as this article.

You also unlock a big picture view of 400+ supplements and 600+ health topics, as well as actionable study summaries delivered to you every month across 25 health categories.

Stop wasting time and energy — we make it easy for you to stay on top of nutrition research.

Try free for two weeks

Already a member? Please login to read this article.

What were the findings?

Become an Examine Member to unlock this article.

Already a member? Please login to read this article.

What does the study really tell us?

Become an Examine Member to unlock this article.

Already a member? Please login to read this article.

The big picture

Become an Examine Member to unlock this article.

Already a member? Please login to read this article.

Frequently Asked Questions

Become an Examine Member to unlock this article.

Already a member? Please login to read this article.

What should I know?

Become an Examine Member to unlock this article.

Free 2-week trial »

Already a member? Please login to read this article.

Other Articles in Issue #15 (January 2016)

  • DASH plus fat equals ...
    The DASH diet is one of the most studied diets of all time, and was specifically formulated to curb chronic disease. But will DASH still do it’s thing if you add extra fat each day?
  • Wine and dine with diabetes
    For some, wine is a daily or weekly indulgence. As those with type 2 diabetes must pay extra attention to the blood sugar and lipid impact of what they consume, this trial puts red and white wine to the test.
  • Better living through cherry juice
    Cherries and berries (the former is not a type of the latter, by the way) have increasingly shown cognitive benefits. This trial specifically explores cherries for Alzheimer’s disease.
  • Interview: Victoria Prince, MD, PhD
    Victoria Prince is passionate about ancestral health and evolutionary medicine, and has a particular interest in dietary fats and the role they play in health and disease, especially liver disease. She writes at principleintopractice.com.
  • The chocolate fountain of youth
    Cocoa contains high levels of beneficial phytochemicals called “flavanols”, which may provide a variety of health benefits. This randomized trial tested cocoa for the specific purpose of wrinkle reduction and other skin-related improvements.
  • Beyond ‘eat less, move more’: treating obesity in 2016
    By Spencer Nadolsky, DO
  • A fishy depression treatment
    With many trials already conducted on the topic of fish oil and depression, the question of overall impact still remained. This is the latest update to the Cochrane systematic review on the topic.
  • A bit of D for CVD
    Vitamin D is touted for pretty much every health condition out there. While observational evidence has strongly linked optimal vitamin D levels to cardiovascular disease, the trial evidence has been more mixed. This trial attempts to strengthen that literature base.
  • Your probiotic may be lying to you
    Take a gander at a probiotic bottle label and you may be astounded at the number of live bacteria, as well as the variety these supplements contain. But the labels may not be entirely accurate