In a past article, I explained that carbohydrate ingestion limits fat oxidation during exercise. Although I'm a big fan of lowed carbohydrates diets and fast exercise, this advice will not work for everyone, so as a coach I need to provide many options to help my clients towards their goals.
So what are your options when you are not on a carbohydrate restricted diet and can not perform exercise in a fasted state? There are many, and today I will discuss one such option. Let's say for example that you exercise at mid-day or in the evening after work. By the time mid-day rolls around you may have eaten once or twice, and by the evening possibly three to five meals if you were not careful about your carbohydrate selection and amount, your blood sugar may be high enough to inhibit fat oxidation during your exercise session later in the day.
A study using 8 healthy females examined the effects of pre-exercise mixed meals providing either a high glycemic (78) versus low glycemic (44) meal on substrate utilization during exercise. The woman was provided with a test breakfast 3 hours before performing a 60-min run at 65% VO2max on a treadmill. Both meals provided 2 grams of carbohydrates per kilogram of bodyweight (way too much carbs for fat loss goals. The only difference between the meals was the glycemic index (78 versus 44).
The results of the study demonstrated that the rate of fat oxidation during exercise was higher in the low glycemic group than the high glycemic group (18.7 grams per hour versus 8.3 grams per hour). That's twice as much fat oxidized for the low glycemic group! The amount of carbohydrates oxidized during the 60-minute run was 101.5 grams per hour in the high glycemic group and 70.5 grams per hour in the low glycemic group. The amount of carbohydrates you eat and its glycemic value will determine how much fat and carbohydrates you burn during exercise. Are you exercising to burn fat or carbs? If your goal is fat loss, then manipulating your macronutrient ratios (fat, protein, and carbs) is essential, and as this study shows, choosing low glycemic carbs over high glycemic carbs will also favor fat oxidation.
The researchers conclude “despite exercise in the fasted state promises optimal fat oxidation, many find it difficult or impractical to exercise while fasted. I agree. One more finding in this study was that the low glycemic group had a higher rate of feeling full post breakfast. Being hungry can destroy your fat loss efforts.
TIP: regardless of whether you're training in a fasted state or not, being familiar with the glycemic and glycemic load index will serve you well. If you're not training in a fast state, incorporating a lower glycemic index / load becomes even more important if you're trying to lose fat.