Why the Time of Day You Exercise Really Does Matter

Sheri Colberg-OchsBy Sheri Colberg-Ochs, PhD

September 2013 — A recent research study published in the British Journal of Nutrition addressed how timing of exercise relative to when you eat a meat influences what happens to your after-meal blood glucose and blood fat levels. In that study, ten sedentary, young (average age of 28 year), overweight men walked moderately for an hour either before or after eating breakfast. Not surprisingly, exercising at either time (compared to just resting) lowered how much insulin their bodies released for breakfast. Their blood fat levels (that is, triglycerides) also remained lower following pre-breakfast exercise, so the authors of the study concluded that exercising before breakfast is better for managing body fat levels and blood fats.

Wait a minute!  Before you change the time of day you exercise to before breakfast, listen to my take on this study first. Conclusions like this personally drive me crazy. First of all, this study was done on overweight young men without diabetes, so you can't assume that it applies to anyone with diabetes. Second, the only conclusion you can reach about body fat is that walking before breakfast caused these participants to use (slightly) more fat during the activity—not surprisingly—than walking after their morning meal when your muscles have more fuels (like glucose coming from breakfast) available to use. One of the researchers also said that exercise undertaken before breakfast is extra beneficial for weight loss because it forces the body to rely on its stores of fat for energy—a statement with which I totally disagree!

That study was obviously not performed or reported on by anyone with a background in exercise physiology (like I have). Conclusions like these perpetuate the myth that you have to burn fat to lose body fat, which is completely and utterly not true! While you need to use more calories than you eat to lose weight, the operative word is calorie, not fat. You will lose more body fat when you use more calories, regardless of which fuels (like fat or blood glucose) your body uses during the activity. Using a little bit of extra fat during a pre-breakfast walk is not going to make you lose more body fat—it's your total calorie deficit that matters to weight loss.

What's more, exercising when your liver's carbohydrate stores are limited after an overnight fast will likely make you walk slower and use fewer calories since you can't exercise as long or as hard in the morning without eating first. Normally when you exercise more briskly, carbohydrates (both glycogen stored in muscles and blood glucose) are your body's preferred fuel, not fat, because carbs are actually more fuel efficient (meaning that you get more calories released per liter of oxygen your body uses to convert fuels to useable energy). Having to rely more heavily on fat during exercise causes you to slow your pace and/or limit your total activity.

Fat (both in your diet and released from your fat cells), however, is your body's primary fuel during the time that you are recovering from being active. Using fat as a fuel then spares blood glucose to restore muscle carbs (glycogen) and protein to rebuild and strengthen muscles. As a result, most of the calories your body uses during the day just for basic maintenance and repair of tissues and basic functioning come from fat, which causes the greatest loss of body fat. The exact fuel your body used most of during the exercise is really irrelevant.

What you also need to know is that when you have diabetes, exercising before you eat breakfast can actually elevate your blood glucose. When you exercise pre-breakfast, you are prolonging your overnight fast, which makes your body release more glucose-raising hormones like cortisol that make you insulin resistant. A usual response for anyone with diabetes who exercises before eating breakfast is a rise in blood glucose levels, not the usual decline you get when you exercise moderately any time later in the day. I can't tell you how many times I have advised people with diabetes to simply eat something first before doing early morning exercise to prevent elevations in their blood glucose levels!

Exercising after any meal (breakfast, lunch, or dinner) is actually much more effective at lowering your blood glucose levels than doing it before. Especially for anyone with type 2 diabetes who releases some insulin naturally, having a meal prior to exercise enhances its glucose-lowering effect. So, when it comes to both body weight and blood glucose management, exercising at any other time of day besides pre-breakfast is good advice. Remember, regardless of what it says about being in a "fat-burning range" on that cardio machine in the gym, you do not have to focus on using fat during exercise to effectively lose body fat—quite the contrary! 

Read Sheri's bio here.

Read more of Sheri Colberg-Och's columns.

NOTE: The information is not intended to be a replacement or substitute for consultation with a qualified medical professional or for professional medical advice related to diabetes or another medical condition. Please contact your physician or medical professional with any questions and concerns about your medical condition.

Last Modified Date: December 02, 2013

All content on dLife.com is created and reviewed in compliance with our editorial policy.

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