While there is no conclusive evidence that fasted cardio is the best method for maximizing fat loss there is quite a bit of scientific literature that supports the theory that performing cardio exercise prior to breakfast will yield a higher rate of fat oxidation. But is performing cardio in a fasted state the best method for exercise induced weight control?
A substantial portion of energy production during prolonged light or moderate exercise comes from fat oxidation (Gollnick 1985; Romijn et al. 1993). The increased use of lipid fuel induced by such exercise is largely the result of increased fat mobilization from lipid stores and results in an increased extraction of fatty acids from the blood by working muscles (Groop et al.1991).
Fat is also the main oxidative substrate in energy balanced individuals after an overnight fast. However, after a carbohydrate-rich meal (Acheson et al. 1984) glucose becomes the principal oxidative substrate. And there is an increasing body of evidence demonstrating that fat oxidation is inhibited during exercise following carbohydrate ingestion.
Research has shown that total fat oxidized in a fasting condition is 23% greater than under pre-fed conditions. An individual will oxidize significantly more fat during exercise when the exercise is performed prior to either a low or high glycemic index meal. Further, the total amount of fat oxidized during exercise AND the 2 hours post training is also significantly greater in the fasted state.[i]
Based on the literature, it would appear that individuals wishing to maximize their efforts for exercise-induced weight control could benefit from cardio exercise if it is performed soon after waking in the morning, before breakfast.
However, one could argue that benefits of performing cardio in a pre-fed condition are insignificant and may be negated by the catabolic effects of exercise in a fasted state.
Performing cardio in a fasted state has been shown to have a catabolic effect on muscle. Studies show that training in a glycogen-depleted state substantially increases the amount of tissue proteins burned for energy during exercise[ii]. Protein losses may exceed 10 percent of the total calories burned over the course of a one-hour cardio session –which is more than double that of training in a pre-fed state[iii]. Since lean muscle mass is the major determinant of metabolism, contributing anywhere from 60% to 80% of BMR (Muller et al., 2009), sacrificing lean tissue would be detrimental to overall BMR and total caloric expenditure.
Additionally, the benefits of fasted cardio may not be as significant as they first appear in the literature. In a 1999 study, trained subjects who exercised at 50 percent of their max heart rate, demonstrated no difference in the amount of fat oxidized–regardless of whether the subjects had eaten. Only after 90 minutes of exercise did fasted subjects begin to yield a favorable result in the amount of fat oxidation. What this means is you would need to exercise for a minimum of 90 minutes to provide an additional fat-burning benefit. [iv]
Based on the evidence, it appears to be true that performing cardio exercise in a fasted state does result in higher rates of fat oxidation. However, in order to improve body composition, spare lean tissue, and maximize total daily caloric expenditure, performing exercise after a meal may be more beneficial.
Train Smart and Good Luck
[i] A Bennard, Patrick Doucet, Éric..Acute effects of exercise timing and breakfast meal glycemic index on exercise-induced fat oxidation Applied Physiology, Nutrition & Metabolism Oct 2006, Vol. 31 Issue 5, p502
[ii] Blomstrand E, Saltin B. Effect of muscle glycogen on glucose, lactate and amino acid metabolism during exercise and recovery in human subjects. Journal of Physiology. 514:293-302, 1999
[iii] Lemon PW and Mullin JP. Effect of initial muscle glycogen levels on protein catabolism during exercise. J Appl Physiol 48: 624-629, 1980.
[iv] Horowitz JF, Mora-Rodriguez R, Byerley LO,and Coyle EF. Substrate metabolism when subjects are fed carbohydrate during exercise. Am J Physiol 276(5 Pt 1): E828-E835, 1999.
Shoenfeld, Brad The Myth Of Cardio Before Breakfast—Debunked! 2011