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Dear Doctors: I heard on the news about a study that says you won’t get as hungry if you eat all of your meals earlier in the day. Does that have to do with the way the body clock works? It seems like circadian rhythms affect everything we do. Can you talk about that, and about this study?
Dear Reader: Your question returns us to the fascinating field of chronobiology. As the word suggests, it’s the study of biological rhythms in relation to the daily cycle of light and dark. While written references to the existence of a “biological clock” date back many centuries, the first known scientific observations on the phenomenon were made in the early 1700s. The modern branch of this science traces its roots to the 1950s, with fruit fly experiments and a few sleep studies. Today, chronobiology is a robust area of research.
What was once known as the body clock has been fine-tuned into the circadian cycle. With hundreds of academic studies in the last few decades, we have learned that the daily 24-hour cycle of light and dark not only influences virtually every biological function, but it also appears to be built into our bodies at the cellular level. Several recent studies have looked into the effects of our bodies’ circadian rhythms on diet and exercise. The latter generated some interesting results, which we’ll write about soon.
The research that you are asking about, which recently appeared in the journal Cell Metabolism, focused on diet. Specifically, the researchers wondered if the time of day at which we eat our meals is important. Sixteen adults who were either overweight or obese were randomly divided into two groups. One group ate their meals at 8 a.m., noon and 4 p.m.; the other group ate at noon, 4 p.m. and 8 p.m.
Each participant was served the same calorie-controlled diet, which was provided by the researchers. Information about each person’s level of hunger and satiety was collected 18 times per day. Blood tests and tests for body fat percentage, energy expenditure and body temperature were done on three different days. This part of the study lasted seven days. After a break of two weeks, during which all of the participants again ate the same diet, they switched roles: The “early” group time shifted to eating the controlled diet four hours later, and the “late” group was served their trio of meals four hours earlier.
Once the lab tests and daily self-reports were analyzed, it turned out that the early eaters felt half as hungry throughout the day as the later eaters. They also burned more calories than the other group did. Those who ate later in the day reported increased cravings for starchy and salty foods. Blood tests revealed that those who ate later in the day had levels of leptin, a hormone that causes hunger, that were 33% higher than in the early eating group. As with all studies, particularly those that are small and brief, further investigation is needed. But the results are certainly intriguing.