One dilemma facing athletes is whether to eat anything before cardio, and if so, what. A recent study sheds some light on this and suggests the best option. (1).
The rationale for cardio on an empty stomach is obviously to increase fat burning. When carbs are taken in before exercise the carbs are preferentially used for fuel, sparing fat. On the other hand exercising on an empty stomach elevates cortisol levels which break down not only fats but muscle for fuel.
The other option is ingesting protein before exercise. This may spare muscle, but does it inhibit fat burning? Surprisingly, according to the study, it depends on the type of protein.
Rats were exercised under 4 different conditions. (1) Fasting. (2) Glucose meal before exercise. (3) whole milk protein before exercise (4) lactalbumin enriched whey before exercise.
At the end of the study, the glucose and milk protein fed rats gained fat mass, showing that these diets blunted the fat burning from exercise. The fasted rats lost both muscle and fat, whereas the whey fed rats lost just as much fat as the fasted rats, but gained muscle.
The moral is to burn fat and actually build muscle while doing cardio, ingest whey protein beforehand.
Am J Physiol Endocrinol Metab 2002 Sep;283(3):E565-72
A preexercise alpha-lactalbumin-enriched whey protein meal preserves lipid oxidation and decreases adiposity in rats.
Bouthegourd JC, Roseau SM, Makarios-Lahham L, Leruyet PM, Tome DG, Even PC.
Unite Mixte de recherche de Physiologie de la Nutrition et du comportement alimentaire, Institut National de la Recherche Agronomique, Institut National Agronomique Paris-Grignon, F75231 Paris, France.