During a period of fasting, the systems of your body are relying on fat, and the sugar that is stored in your liver for energy. Your muscles still have their own sugar that they need for exercising. The sugar in your muscles is used up quickly during high intensity exercises like weight training and sprinting.
Research completed back in 1987 found that a three and a half day fast caused minimal impairments in physical performance measures such as isometric strength, anaerobic capacity or aerobic endurance. In other words, they found that a three-day fast had no negative effects on how strongly your muscles can contract, your ability to do short-term high intensity exercises, or your ability to exercise at moderate intensity for a long duration.
I also pointed out in that post that people like Richard and Keith ( and myself actually) are often working out in a fasted state.
This study caught my eye today. An overnight fast in not really as long as the fasts that we might normally encounter as part of IF, however the findings of the study remain interesting and relevant nevertheless:
- "Research has demonstrated body fat-reducing benefits of exercise after fasting overnight." Interesting
- The researchers "hypothesized that substrate use during fasted-state submaximal endurance cycling would shift to greater reliance on fat." Fair enough. So they had some cyclists train in the morning before breakfast, incidentally on a lower calorie diet.
- The cyclists' power to weight ratio and body composition improved significantly, and their overall weight, fat weight, and body fat percentage decreased. Lean mass was maintained.
In addition to aerobic endurance and anaerobic capacity, high power-to-weight ratio (PWR) is important for cycling performance. Cyclists often try to lose weight before race season to improve body composition and optimize PWR. Research has demonstrated body fat-reducing benefits of exercise after fasting overnight. We hypothesized that fasted-state exercise in calorie-restricted trained cyclists would not result in performance decrements and that their PWR would improve significantly. We also hypothesized that substrate use during fasted-state submaximal endurance cycling would shift to greater reliance on fat.
Ten trained, competitive cyclists completed a protocol consisting of baseline testing, 3 weeks of caloric restriction (CR), and post-CR testing. The testing sessions measured pre- and post-CR values for resting metabolic rate (RMR), body composition, &OV0312;o2max, PWR and power-to-lean weight ratio (PLWR), and power output, as well as 2-hour submaximal cycling performance, rating of perceived exertion (RPE), and respiratory exchange ratio (RER). There were no significant differences between baseline and post-CR for submaximal trial RER, power output, &OV0312;o2, RMR, &OV0312;o2max, or workload at &OV0312;o2max. However, RPE was significantly lower, and PWR was significantly higher post-CR, whereas RER did not change.
The cyclists' PWR and body composition improved significantly, and their overall weight, fat weight, and body fat percentage decreased. Lean mass was maintained. The cyclists' RPE decreased significantly during 2 hours of submaximal cycling post-CR, and there was no decrement in submaximal or maximal cycling performance after 3 weeks of CR combined with overnight fasting. Caloric restriction (up to 40% for 3 weeks) and exercising after fasting overnight can improve a cyclist's PWR without compromising endurance cycling performance.
Granted these guys were also on restricted calories, but this is interesting and perhaps lines up with what Brad was saying.
Also it seems that there was no control of cyclist who were on lower calories but did not train fasted. Interesting stuff nevertheless and at least seems to demonstrate that training fasted is not a problem.