When You Eat May Affect Weight Gain As Much As What You Eat

Reading time: 4 – 7 minutes

If there is one thing that the sugar free, low carb, low fat, and gluten free dieting trends of the past few decades have taught us, it’s this: a calorie is a calorie is a calorie. Right? Wrong! Whereas the source of the calories you consume might not have much bearing on the amount of weight you gain, when you consume them very well might. Research in both mice and humans demonstrates that eating whenever one pleases (mice) or later in the day (humans) causes significantly more weight gain than consuming the same diet in a time restricted manner, in keeping with the cyclical nature of the body’s energy metabolism.

Late night eating

Researchers often try to generate animal models of diseases in order to study them, and obesity is no different. The standard practice for creating diet-induced obesity in mice has been to feed them a high fat diet ad libitum – so they eat all throughout the day and night, disrupting the normal feeding cycle. Mouse models of diabetes as well as obesity often have altered circadian rhythms; conversely, disruption of circadian cycles in mice has been shown to promote glucose intolerance and metabolic syndrome. Scientists at UC San Diego and the Salk Institute wanted to see if the mice became obese because of the high fat diet or because of the observed disruption in their circadian rhythms. To do so, they “subjected” mice to either the high fat diet ad libitum or an identical high fat diet restricted to an 8 hour window [1].

Not only were the time restricted mice not obese, they were protected against many of the negative effects associated with obesity: metabolic syndrome, glucose intolerance, type 2 diabetes, inflammation, and fatty liver disease. Their circadian rhythms were not disrupted. They even had improved motor coordination. All of this despite the fact that the two groups of mice ingested the same number of calories, and the same percentage of them (61%!) came from fat. They were not in quite as good shape as mice who were fed normal chow — but they were not far behind.

Studies in humans are more difficult, as it is not really feasible to sequester people in a lab for 18 weeks and dictate exactly what and when they eat. In order to try to determine if when humans eat has any bearing on how much weight they gain, researchers in Spain had 420 people in who were already enrolled in voluntary treatment for obesity characterize themselves as “early eaters” or “late eaters” based on whether they lunched before or after 3 pm (this is Spain, after all) [1]. Lunch is the main meal in this group, accounting for 40% of total daily energy intake.

Over the 20 weeks of the study, the late eaters lost significantly less weight, and they lost it more slowly, than the early eaters. The participants had to report their weekly diets to the researchers, and according to these reports the total daily energy intake and dietary composition were similar between the two groups. The amount of time they slept and their levels of important appetite hormones were similar as well. Weight loss was independently correlated only with the timing of lunch, not breakfast or dinner – although people who ate lunch later tended to eat these meals later as well. These findings agree with those of previous studies showing that shift workers are more prone to obesity than day workers despite their similar energy intake over 24 hours. But the authors stress that this is merely an observed correlation, and further investigation is required to determine what might cause this observation.

Dr. Jonathan Waitman, a Clinical Instructor of Medicine at the Comprehensive Weight Control Program at the Weill Cornell Medical College-New York Presbyterian Hospital, noted: “It is not farfetched that adipose tissue can be functioning on a cycle — after all, the stomach secretes ghrelin (a hormone that makes us feel hungry) three times a day.” However, he cautions that “research on leptin (the hormone that makes us feel full) has shown us that findings in mice are not necessarily translatable to humans,” and since this human study is not as methodical as the animal study, he is not yet entirely convinced that when you eat dictates your weight gain as much as what you eat.

A calorie is a unit of energy, and thus describes the amount of energy our cells can get from the food we eat. Although all calories are thus equal, our bodies might treat them differently — shunting them into fat storage, for example — depending on when we ingest them. Perhaps we should give more credence to the old wives tale that eating late at night is not good for us. Those old wives get a bad rap — they were usually onto something.


  1. Hatori et al. Time-restricted feeding without reducing caloric intake prevents metabolic diseases in mice fed a high-fat diet. Cell Metab. 2012 Jun 6; 15(6):848-60. doi: 10.1016/j.cmet.2012.04.019. Epub 2012 May 17.
    View abstract
  2. Garaulet et al. Timing of Food Intake predicts weight loss effectiveness. Int J Obes (Lond). 2013 Jan 29. doi: 10.1038/ijo.2012.229. [Epub ahead of print]
    View abstract
About the Author

Diana Gitig, Ph.D., is a freelance science write based in White Plains, New York. She earned her Ph.D. in Cell Biology and Genetics from Cornell University's Graduate School of Medical Sciences.