New research suggests evening may be best time to exercise

Dear Doctor: I always heard that the best time to exercise is the morning because it gets your metabolism going. I don’t like working out so early, though. What about that new study that says working out later in the day is actually better for you?

Dear Reader: Getting regular exercise is so important to health and well-being that whenever you can fit a workout into your day is the best time. That said, the human body is a complex mechanism. Our skeletal, musculature, circulatory, digestive, respiratory and nervous systems are all intricately interconnected. Add in the fact that that our bodies are attuned at a cellular level to the 24-hour cycle of light and dark, and it would be surprising if time of day didn’t play a role in how exercise affects us. Researchers have long been interested in this very question. And it turns out that the answers depend on a number of variables. These include the type of exercise you’re doing, the goals of that exercise, as well as the sex, age, general health and fitness level of the individual. There’s no definitive answer yet.

The study you mentioned, which was recently published in the journal Diabetologia, asked very specific questions. Researchers were interested in whether or not the time of day that someone exercises will affect fat metabolism and the way the body regulates blood sugar, all while on a high-fat diet. The results suggest that time of day may indeed make a difference. And before we get into the specifics, it’s important to note that the study participants were all overweight or obese, did not exercise regularly and were men between the ages of 30 and 45. (Women were excluded due to potential metabolic variables arising from their menstrual cycles.)

Elizabeth Ko, MD and Eve Glazier, MD

Before the study began, the men were screened for body fat composition, aerobic fitness and specific information about their metabolisms. During the 10 days of the study, the men ate meals in which 65% of calories came from fat. After the first five days of the study, blood tests revealed metabolic markers of increased risk of heart disease. In the final five days, participants were also divided into three different exercise groups. One group worked out at 6:30 a.m., another at 6:30 p.m. and the third did no exercise at all. After five days of exercise, the morning group’s blood work showed no significant changes. Neither did the sedentary group. However, cholesterol levels in the evening exercisers dropped, and the metabolic signs of heart disease risk had lessened. The evening group also exhibited better blood sugar control than the morning exercisers and the sedentary group.

It’s an intriguing study that, because it was so small and short, with a highly specific group of participants, calls for further investigation. The researchers say that in future research, they want to add variables such as sleep into the mix, and study a more diverse group of participants. Meanwhile, it’s important to note that exercise of any kind, at any time, is important. Morning exercisers shouldn’t use this study as a reason to stop working out.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)


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