It is well known from scientific research that performing fasted cardio in the morning after an overnight fast is the equivalent of “running your car on empty”, which essentially means that something else will be broken down. In the case of human physiology, this “something else” is our precious muscle tissue. Exercising in a fasted state will promote catabolism (i.e. breakdown) of our muscle tissue, which is really detrimental to our metabolism and makes achieving a lean physique virtually impossible . Performing fasted cardio may utilize a greater percentage of fat for fuel, but eating prior to exercise burns more calories during the actual exercise session. In addition, consuming breakfast before exercising demonstrates more positive glucose and insulin responses in the meals later in the day. Recent evidence shows that exercising in a fasted state blunts fat oxidation and this blunted fat oxidation was even greater in women than in men! This means that working out on an empty stomach will actually hamper your ability to burn fat, especially in women!  Not at all what we want from our hard work in the gym. What about anaerobic activity?
In addition, performing any type of anaerobic workout (e.g. sprinting, weightlifting) on an empty stomach is a sure-fire way to guarantee a very bad training session and can possibly jeopardize future workouts. Our muscles utilize carbohydrates (i.e. glycogen) as fuel during anaerobic activity, such as an intense weight lifting session. If we perform this kind of activity following an overnight fast, we don’t have enough glycogen in the muscles to perform at our peak performance. In addition, low muscle glycogen levels can trigger increased cortisol levels in order to mobilize fuels (i.e. protein) to be converted to glucose and used as fuel for exercise. We don’t want high levels of cortisol in our bodies because it is a catabolic hormone and can contribute to immunosuppression. Summary
Human physiology is complex and we can’t dictate what fuel (i.e. fat, carbohydrate) we want our body to use for energy. If it were this simple, there wouldn’t be an epidemic in overweight/obesity throughout the world. Working out on an empty stomach causes a lot of detrimental effects that are backed by solid research. The question is why would you want to do this and be hungry during your workout? In addition, this fasted condition will also contribute to many detrimental effects in pursuit of our goal of achieving the health, energy levels and body shape we desire. There is a much better way through nutrient timing, knowing how to eat before and after your workouts and throughout the rest of your day. Let me know if you want to learn more about this. Connect here with WatchFit Expert Dr. Paul Henning! References
1. Biolo, G., et al., Increased rates of muscle protein turnover and amino acid transport after resistance exercise in humans. The American journal of physiology, 1995. 268(3 Pt 1): p. E514-20. 2. Henderson, G.C. and B.L. Alderman, Determinants of resting lipid oxidation in response to a prior bout of endurance exercise. Journal of applied physiology, 2014. 116(1): p. 95-103.