What you eat prior to training can have a significant influence on how much body fat you burn or oxidize during the workout. In those with low to normal body fat levels, consuming a high glycemic index or simple carb source just before a workout will blunt fat oxidation. The ingestion of quickly absorbed carb sources promotes a significant insulin release, and it's this increased insulin flow that blunts the fat oxidation process. This metabolic scenario pertains to those who are already lean. But what happens if you have an excess level of body fat? Those with higher levels of body fat are known to have higher resting and post-meal levels of insulin. Having too many, and too large fat cells results in insulin resistance, which the body responds to by increasing insulin release in an effort to overcome the insulin resistance.
In a new study, the effect of various meals ingested before exercise was studied. In the study, 20 obese subjects exercised on a stationary cycle for 30 minutes under the following conditions: 1) after a fast; 2) after a carb meal;3) after a protein meal. All the meals were ingested 30 minutes before exercise, except for one other carb meal, which was consumed 2 hours before the workout. They exercised at a low intensity level to maximize fat oxidation. The results showed that eating carbs shortly before training suppressed fat oxidation, as indicated by the levels of glycerol, a product of fat metabolism, in the blood. Fasting didn't affect fat oxidation. Eating the protein meal resulted in a slower, but still increased level of fat oxidation compared to fasting. Eating the carbs 2 hours prior to training resulted in a response similar to that of eating a protein meal before the workout: that is, a significantly greater level of fat oxidation during training.
Based on these results, the study authors suggest that those with higher levels of body fat not ingest carbs shortly before a workout if they want to maximize fat oxidation. If hungry, protein foods can be eaten before the workout without any negative effects on fat oxidation. Carbs can be eaten 2 hours prior to the workout because the insulin effect will have dampened by the time of the workout, thus not adversely affecting the fat oxidation process.
Erdmann J ,et al. Effect of carbohydrate and protein-rich meals on exercise-induced activation of lipolysis in obese subjects.Horm Metab Res 2010;42:290-94.
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