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You Don't Lose More Fat Exercising Before Breakfast

Aug 21, 2007
You will sometimes see it recommended to exercise before eating breakfast, so that you will burn more fat. This is based on a similar misconception to the idea that there is an ideal "fat burn zone" of exercise intensity.

Let's take a look a what exercise science has to say about this. If you compare two identical exercise sessions, one before breakfast, and the other an hour or so after a meal, it turns out you will indeed burn fat at a higher rate in the pre-breakfast session [Horowitz, 1997]. But what is most important during exercise is that you burn calories, not so much what they come from. If you burn more "sugar" (which actually means glucose in the blood from food, or glycogen in the liver or muscles) while exercising, you will just burn more fat later in the hours following the exercise session [Kiens, 1998; Calles-Escandon, 1996].

The body gives a high priority to restoring muscle glycogen levels in the hours after exercise, so it will bump up it's consumption of fat to supply it's energy needs, while using any carbs available from food to replace the glycogen you burned up [Kiens, 1998]. This makes sense because the body needs glycogen to exercise at high intensity levels- like in an emergency.

There are lots of good reasons for exercising at a certain time of day. Gyms can be quiter first thing in the morning, or maybe you want to fit it in before work. And some people just like to exercise before breakfast. My brother lives in Phoenix, and I've enjoyed many delightful desert runs and hikes with him in the early morning hours when I visit down there. But where I live at home it can be more pleasant to exercise later in the morning so I wait till after breakfast. There is nothing wrong with either of these. But when making this decision, I don't worry what percentage of my calorie burn is coming from fat during the exercise. That's irrelevant.

References

Calles-Escandon, J, et al, "Exercise Increases Fat Oxidation At Rest Unrelated To Changes In Energy Balance Or Lipolysis", Am J Physiol Endocrinol Metab, 270: E1009-E1014, 1996.

Horowitz, J, et al, "Lipolytic Suppression Following Carbohydrate Ingestion Limits Fat Oxidation During Exercise", Am J Physiol Endocrinol Metab, 273: E768-E775, 1997.

Kiens, B, and Richter, A, "Utilization Of Skeletal Muscle Triacylglycerol During Postexercise Recovery In Humans", Am J Physiol Endocrinol Metab, 275: E332-E337, 1998.
About the Author
I'm Richard King, 54 and a mechanical engineer with a Ph.D. from Stanford. Biking and fitness are my main hobby, and I am well versed in biomechanics and exercise physiology through many years of reading and research.My website is
www.bikeandfit.com.
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