Eat Right to Burn Fat: Low Carb Meal Before Workout Shifts Substrate Utilization During Exercise More Towards Fatty Acids

In view of everything, you as regular visitor of the SuppVersity have learned about nutrition and exercise science, you probably would not need the results of the following study, to know better than to consume a sugary pre-workout drink, if your main goal is to lose fat and get ripped. Just in case, however, your friends or relatives belong to the group of unconvertible carbohydrate addicts, who still (want to?) believe that "fat burns in the fire of carbohydrates", printing this blogpost and sticking it to their gymbag may in fact turn out to be a "fat burning" endeavor...

Figure 1: Respiratory Exchange Ratio (RER) following the consumption of a  low carb (LC) or low fat (LF) mealt; remember: a lower RER is associated with a higher percantages of fatty acid oxidation / lower carbohydrate oxidation (data adapted from Gregory. 2011)

Sara Gregory and her colleagues from the Department of Exercise Science and Sports Studies at Springfield College recently published the results of a study (Gregory. 2011) which investigated the effects of high and low carbohydrate meals on substrate utilization, i.e. the predominant use of fat or carbohydrates to fulfill the energetic demands of a successive exercise bout. They fed their previously (12h) fasted subjects, 8 active, premenopausal females, either a low fat (LF, 396 kcal @ 78% carbohydrate, 7% fat, and 15% protein) or a low carb meal (LC, 392 kcal @ 15% carbohydrate, 68% fat, and 18% protein), and measured Respiratory gas exchange (RER) for 55 minutes in the post-prandial window, as well as in the course of the ensuing 30 minutes of treadmill exercise at 60% to 65% of the subject's VO2peak. Assuming that the lower RER values are, as it is generally accepted, indicative of a greater percentage of fat being "burned", the results are quite unequivocal:

Postprandial RER from min 0 to min 25 was significantly higher (p < 0.05) following the LF meal compared to the LC meal. Postprandial RER from min 25 to 55 was also significantly higher (p < 0.05) following the LF meal. RER during the 30 min exercise period was significantly higher (p < 0.05) following the LF meal compared to the LC meal.

So, even if the carbs wont make you fat, because you "exercise them away" they will hinder your fatloss, because no matter how much "calories you burn", as long as as there are still enough carbs (and likewise higher insulin levels) your lazy body will take the easier route and use those to fuel your workout.