If you’re exercising to lose body fat, fasted cardio can help you reach your goals faster.
Fasted cardio refers to activity performed after an overnight fast or at least four hours after a meal. There are two reasons to consider this type of training.
The first is behavioral. For people with busy lives and traditional work schedules, exercise can be performed first thing in the morning before breakfast and a shower. If you typically avoid exercise because it interferes with other activities, it may be easier to start your day with cardio. Exercise before you do anything else and get on with your day.
The second reason to consider fasted cardio is metabolic. The big picture idea is that the potential for fat oxidation is greater when exercise is performed in a fasted state. The details of exercise metabolism complicate this issue somewhat, which explains why fasted cardio is such a controversial topic. The key variables to consider are exercise intensity, duration, and fuel mixture.
Here’s why these details matter. During exercise, stored and circulating carbohydrate and fat are the primary sources of fuel. As the intensity of exercise increases, the proportion of energy from carbohydrate increases. With intense exercise (e.g., interval training) the majority of energy is provided by carbohydrate. As the intensity of exercise decreases, the fuel mixture shifts, and fat becomes a more significant fuel source.
The combination of moderate-intensity (higher proportion of energy from fat) and extended duration (greater energy expenditure) exercise is appealing to those who wish to oxidize as much body fat as possible. This is the rationale most frequently cited for doing cardio.
This effectiveness of this protocol is enhanced when performed in a fasted state. During fasted training, the proportion of energy derived from fat increases. It therefore seems logical to perform fasted cardio if maximal fat oxidation is the goal.
In a previous article, I made the argument that training at 50% VO2max for 60 minutes is ideal for balancing several goals: energy expenditure, fat oxidation, and program compliance. A recent study demonstrated that fasted training makes this protocol that much better.
Shimada et al. (2013) used whole-room calorimetry to measure 24-hour energy expenditure and substrate utilization in response to fed or fasted exercise. Participants were 12 endurance-trained athletes who exercised on a cycle ergometer at 50% VO2max for 60 minutes either before or after breakfast (measured a week apart). In both trials, participants spent the entire 24-hour period in the laboratory setting. Food intake and activity in the two conditions were identical. The results indicated that, although there were no differences in energy expenditure during exercise (approx 550 kcal) or at 24 hours, fat accounted for 34% of energy expenditure during exercise in the fasted state and 19% in the fed state. At 24 hours, total energy from fat continued to be greater for the fasted condition (720 vs. 608 kcal). The graph below demonstrates differences in fat oxidation between the fasted (black circles) and fed (white circles) conditions during exercise and at all other times.
Is fasted training ideal at higher intensities or for athletes with performance goals? No. But if fat loss is your goal, you can improve the effectiveness of better cardio by exercising in a fasted state.
Source: Shimada, K., Yamamoto, Y., Iwayama, K., Nakamura, K., Yamaguchi, S., Hibi, M., Nabekura, Y., & Tokuyama, K (2013). Effects of post-absorptive and postprandial exercise on 24 h fat oxidation. Metabolism, 62, 693-700.