Some say it is better to do cardio activities in the morning, on an empty stomach. Others say you should do cardio later during the day when your glycogen stores are elevated. What time of the day is best for preventing muscle loss, whilst providing the greatest amount of fat utilization when exercising? Thanks.

You asked a good question that does not come up often enough.

Frankly, for most patients the issue is not the time of day but simply getting enough exercise in the first place, so I emphasize finding the time that will work the best. Any exercise is better than none.

A second factor might be your energy level at different times of day. If you are a “morning person”, you might do best at that time rather than later. Many people have job or family constraints. You might be able to get away for a brisk walk or a trip to the gym at lunch but not in the evening. There are other considerations. As the weather warms up, for example, a workout in the early morning or evening may be more practical than mid-day.

Assuming you have the luxury of choosing your time and no time is much better than another, let me mention what I was able to find on the choice of times. There does not seem to be a clear answer but rather a range of opinions. One argument is that by exercising in the morning, you increase your metabolism during the day. Another is that your muscles warm up as the day goes on and that you can do more exercise in the afternoon or evening with less risk of injury.

The University of Chicago Hospitals Web site mentions a study from their Clinical Research Center testing exercise performance at four times of the day. The study found that “Subjects who exercised at night had much larger drops in glucose levels in response to exercise than at other times of day. Exercise in the evening and at night elicited large increases in the levels of two hormones important for energy metabolism, cortisol and thyrotropin. Exercise at other times of day had much smaller effects on these hormones. In contrast, marked increases in growth hormone levels in response to exercise were not affected by the time of day.”

Another consideration is that exercise releases brain transmitters called endorphins that are associated with a sense of well-being and improved mood. So exercising in the morning may help your mood and performance during the day.

There is a lot that can be said on this interesting topic. I hope this is helpful. (Personally, I exercise in the morning and get it over with.)

Thanks for your question. Keep up your exercise whenever you get the chance.

Dr. Joshua Rassen

-- Joshua Rassen, MD, is a board-certified internist and geriatrician in northern California. He has a private practice in geriatrics and internal medicine and is the Chief of the Division of General Internal Medicine at California Pacific Medical Center.

First published March 29, 2005
Last updated October 28, 2008
Copyright © 2005 Consumer Health Interactive