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Research Review
Lyle McDonald
Melby CL et. al. Effect of carbohydrate ingestion during exercise on post-exercise substrate oxidation and energy intake. Int J Sport Nutr Exerc Metab. 2002 Sep;12(3):294-309. Links
Thirteen physically active, eumenorrheic, normal-weight (BMI < 25 kg/m2) females, aged 18-30 years, completed 4 experimental conditions, with the order based on a Latin Square Design: (a). CHO/Ex: moderate-intensity exercise (65% VO2peak) with a net energy cost of approximately 500 kcals, during which time the subject consumed a carbohydrate beverage (45 g CHO) at specific time intervals; (b). CHO/NoEx: a period of time identical to (a). but with subjects consuming the carbohydrate while sitting quietly rather than exercising; (c). NoCHO/Ex: same exercise protocol as condition (a.) during which time subjects consumed a non-caloric placebo beverage; and (d). NoCHO/NoEx: same as the no-exercise condition (b). but with subjects consuming a non-caloric placebo beverage. Energy expenditure, and fat and carbohydrate oxidation rates for the entire exercise/sitting period plus a 90-min recovery period were determined by continuous indirect calorimetry. Following recovery, subjects ate ad libitum amounts of food from a buffet and were asked to record dietary intake during the remainder of the day. Total fat oxidation (exercise plus recovery) was attenuated by carbohydrate compared to placebo ingestion by only approximately 4.5 g. There was a trend (p =.08) for a carbohydrate effect on buffet energy intake such that the CHO/Ex and CHO/NoEx energy intakes were lower than the NoCHO/Ex and NoCHO/NoEx energy intakes, respectively (mean for CHO conditions: 683 kcal; NoCHO conditions: 777 kcal). Average total energy intake (buffet plus remainder of the day) was significantly lower (p <.05) following the conditions when carbohydrate was consumed (CHO/Ex = 1470 kcal; CHO/NoEx = 1285 kcal) compared to the noncaloric placebo (NoCHO/Ex =1767 kcal; NoCHO/NoEx = 1660 kcal). In conclusion, in young women engaging in regular exercise, ingestion of 45 g of carbohydrate during exercise only modestly suppresses total fat oxidation during exercise. Furthermore, the ingestion of carbohydrate with or without exercise resulted in a lower energy intake for the remainder of the day.
My comments: For years, there have been a bunch of prevailing beliefs regarding exercise and fat loss including (but probably not limited to):
a. The idea of an optimal fat burning zone
b. The idea that fasted/unfed cardio burns more fat (some have claimed 300% more) than cardio done when you've eaten
c. The idea that you must do morning fasted cardio to lose fat or you're just wasting your time
And other amusing chestnuts.
I'm not going to address them all in detail. The fat burning zone is basic nonsense, based on a total misunderstanding of percentages versus absolute values. That is, while you may burn a higher percentage of fat at lower intensities, you often end up burning more total/absolute fat (in terms of grams) at higher intensities. Put simplistically, 100% of 5 calories is still less fat burned than 70% of 10 calories.
Additionally, it's questionable whether burning fat during activity is very important under most circumstances (one possible exception is getting rid of stubborn bodyfat). If you had to burn fat during activity, interval training (which relies solely on muscle glycogen) would not cause the fat loss that it does.
Finally is the idea that doing cardio fasted/unfed is critical to one's fat burning efforts. Various studies have found that consuming food before or during cardio (usually carbs) tends to decrease fat oxidation, that much is true. A question might be how much of an impact really exists which is what this addresses.
And it found a very small effect; importantly it looked at fuel oxidation for 90 minutes after the exercise bout. The difference between groups (both burned 500 calories, taking an average of 75 minutes to do it, and one consumed 45 grams of carbs while the other didn't) in terms of total fat oxidation was a whopping 4.5 grams. That's a 45 calorie difference in fat burned compared to the women having consumed 180 calories during activity.
Additionally, and perhaps more interestingly, the women who consumed carbs during training compensated by eating less at the meal given after the workout/the rest of the day. Which, arguably, has less relevance to bodybuilders and athletes who are eating a fixed amount but is still important overall. If training on an empty stomach ends up making you hungrier later in the day, that's a negative.
As an additional factor, which this study didn't address, I'd add this: intensity of training (and thus calorie burning potential) is often compromised when cardio is done on an empty stomach. Consuming even a small amount of carbs during training often lets folks maintain a higher intensity, meaning that they can burn more calories. And since caloric expenditure is a far greater determinant of fat loss down the road, you might make the argument that fasted cardio is worse than doing it while fed (or at least consuming small amounts of carbs).
Finally, I would add that a 500 calorie aerobic bout is actually a lot more than most people think. Someone walking slowly on the treadmill for 30 minutes isn't burning nearly that many calories, I'm not saying that they need the big bottle of Gatorade to fuel it. But when you start getting folks doing an hour at a decent clip, issues of blood glucose and maintenance of intensity become a bigger issue.
Lyle McDonald
Melby CL et. al. Effect of carbohydrate ingestion during exercise on post-exercise substrate oxidation and energy intake. Int J Sport Nutr Exerc Metab. 2002 Sep;12(3):294-309. Links
Thirteen physically active, eumenorrheic, normal-weight (BMI < 25 kg/m2) females, aged 18-30 years, completed 4 experimental conditions, with the order based on a Latin Square Design: (a). CHO/Ex: moderate-intensity exercise (65% VO2peak) with a net energy cost of approximately 500 kcals, during which time the subject consumed a carbohydrate beverage (45 g CHO) at specific time intervals; (b). CHO/NoEx: a period of time identical to (a). but with subjects consuming the carbohydrate while sitting quietly rather than exercising; (c). NoCHO/Ex: same exercise protocol as condition (a.) during which time subjects consumed a non-caloric placebo beverage; and (d). NoCHO/NoEx: same as the no-exercise condition (b). but with subjects consuming a non-caloric placebo beverage. Energy expenditure, and fat and carbohydrate oxidation rates for the entire exercise/sitting period plus a 90-min recovery period were determined by continuous indirect calorimetry. Following recovery, subjects ate ad libitum amounts of food from a buffet and were asked to record dietary intake during the remainder of the day. Total fat oxidation (exercise plus recovery) was attenuated by carbohydrate compared to placebo ingestion by only approximately 4.5 g. There was a trend (p =.08) for a carbohydrate effect on buffet energy intake such that the CHO/Ex and CHO/NoEx energy intakes were lower than the NoCHO/Ex and NoCHO/NoEx energy intakes, respectively (mean for CHO conditions: 683 kcal; NoCHO conditions: 777 kcal). Average total energy intake (buffet plus remainder of the day) was significantly lower (p <.05) following the conditions when carbohydrate was consumed (CHO/Ex = 1470 kcal; CHO/NoEx = 1285 kcal) compared to the noncaloric placebo (NoCHO/Ex =1767 kcal; NoCHO/NoEx = 1660 kcal). In conclusion, in young women engaging in regular exercise, ingestion of 45 g of carbohydrate during exercise only modestly suppresses total fat oxidation during exercise. Furthermore, the ingestion of carbohydrate with or without exercise resulted in a lower energy intake for the remainder of the day.
My comments: For years, there have been a bunch of prevailing beliefs regarding exercise and fat loss including (but probably not limited to):
a. The idea of an optimal fat burning zone
b. The idea that fasted/unfed cardio burns more fat (some have claimed 300% more) than cardio done when you've eaten
c. The idea that you must do morning fasted cardio to lose fat or you're just wasting your time
And other amusing chestnuts.
I'm not going to address them all in detail. The fat burning zone is basic nonsense, based on a total misunderstanding of percentages versus absolute values. That is, while you may burn a higher percentage of fat at lower intensities, you often end up burning more total/absolute fat (in terms of grams) at higher intensities. Put simplistically, 100% of 5 calories is still less fat burned than 70% of 10 calories.
Additionally, it's questionable whether burning fat during activity is very important under most circumstances (one possible exception is getting rid of stubborn bodyfat). If you had to burn fat during activity, interval training (which relies solely on muscle glycogen) would not cause the fat loss that it does.
Finally is the idea that doing cardio fasted/unfed is critical to one's fat burning efforts. Various studies have found that consuming food before or during cardio (usually carbs) tends to decrease fat oxidation, that much is true. A question might be how much of an impact really exists which is what this addresses.
And it found a very small effect; importantly it looked at fuel oxidation for 90 minutes after the exercise bout. The difference between groups (both burned 500 calories, taking an average of 75 minutes to do it, and one consumed 45 grams of carbs while the other didn't) in terms of total fat oxidation was a whopping 4.5 grams. That's a 45 calorie difference in fat burned compared to the women having consumed 180 calories during activity.
Additionally, and perhaps more interestingly, the women who consumed carbs during training compensated by eating less at the meal given after the workout/the rest of the day. Which, arguably, has less relevance to bodybuilders and athletes who are eating a fixed amount but is still important overall. If training on an empty stomach ends up making you hungrier later in the day, that's a negative.
As an additional factor, which this study didn't address, I'd add this: intensity of training (and thus calorie burning potential) is often compromised when cardio is done on an empty stomach. Consuming even a small amount of carbs during training often lets folks maintain a higher intensity, meaning that they can burn more calories. And since caloric expenditure is a far greater determinant of fat loss down the road, you might make the argument that fasted cardio is worse than doing it while fed (or at least consuming small amounts of carbs).
Finally, I would add that a 500 calorie aerobic bout is actually a lot more than most people think. Someone walking slowly on the treadmill for 30 minutes isn't burning nearly that many calories, I'm not saying that they need the big bottle of Gatorade to fuel it. But when you start getting folks doing an hour at a decent clip, issues of blood glucose and maintenance of intensity become a bigger issue.
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