Limit fructose and eating high-glycemic foods prior to strength training to burn more fat. If your goal is fat loss and strength gain for a lean body composition, you’ll achieve better results if you eat more protein, low-glycemic foods, and avoid fructose prior to working out. A new study in the European Journal of Applied Physiology found that eating fructose with a low-glycemic meal will result in the body burning more carbohydrates than fats during moderate exercise. In comparison, a low-glycemic meal without fructose does the opposite, leading to more fat burning and the use of less carbs during exercise.
Carbohydrates are classified by the glycemic index based on their ability to elevate glucose in the blood rapidly or slowly. High-glycemic foods elevate glucose rapidly, triggering a quicker and larger insulin response from the pancreas. For example, if you eat a piece of white bread, it will be digested very fast and all of the carbs will quickly be turned into glucose, raising the overall glucose level rapidly and triggering more insulin to keep up with the high amount of glucose in the blood. In comparison, a low-glycemic bread such as multi-grain bread with fenugreek (an herb that naturally lowers glycemic index) will be digested more slowly and the carbs will more gradually be turned into glucose, raising the overall glucose level slowly and triggering a smaller more measured insulin response. Lower glycemic foods are better if your goal is weight management and fat loss.
Fructose changes things because it does not stimulate insulin secretion from the pancreas. In previous studies, individuals who ate fructose carbs had a lower metabolic rate and more fat gain than a group that ate glucose carbs. There are still unknowns about how fructose modifies metabolism, but we do know that it is turned into fat in the liver where it is metabolized more easily than glucose. This leads to greater fat gain, and this new evidence provides additional data that the presence of fructose will result in less fat burning during exercise.
This study used active young men and gave them either a low-glycemic meal, a low-glycemic meal with fructose, or a high-glycemic meal without fructose. Calorie and macronutrient makeup was the same for all three. The exercise was very brisk walking for one hour.
The high-glycemic group and the low-glycemic with fructose group both burned significantly more carbs than fat immediately after eating and throughout the exercise trial. Levels of fat and carb burning were almost equal. In comparison, the low-glycemic index group burned primarily fat and much fewer carbs immediately after eating and throughout the exercise trial. Results indicate that both the glycemic index and fructose content of a pre-exercise meal dictate the use of substrates (fat or carbs) burned during exercise.
High-glycemic index foods result in higher insulin concentrations, which will suppress fat burning and is the reason for the shift in substrate use with this meal. Low-glycemic index foods don’t lead to this high insulin and allow the body to burn more fat. Researchers are unclear why the presence of fructose in low- glycemic meals changes substrate utilization but they think it has to do with the fact that it raises lactate levels, which in turn suppresses fat burning in a similar way as insulin.
Take away from this study an understanding that fructose should be generally limited in your diet and avoided pre-workout. A small amount of fructose can be metabolized by the liver, but high quantities will be turned into fat, which are then sent into the blood stream, elevating blood triglyceride levels and leading to cholesterol build up. I generally suggest limiting your intake to 5 to 10 grams of fructose a day, with very active individuals maxing out at 20 grams. Lower fructose fruits and vegetables include most berries, nectarines, grapefruit, avocado, and tomatoes. Bananas, apples, and pears are on the high end of the scale.
Sun, F., Wong, S., et al. Substrate Utilization During Brisk Walking is affected by Glycemic Index and fructose Content of a Pre-Exercise Meal. European Journal of Applied Physiology. November 2011. Published Ahead of Print.
Cox, C., Stanhope, K., et al. Consumption of Fructose-Sweetened Beverages for 10 Weeks Reduces Net Fat Oxidation and Energy Expenditure in Overweight/Obese Men and Women. European Journal of Clinical Nutrition. September 2011. Published Ahead of Print.
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