Fasting before exercise increases fat utilization and lowers the rate of muscle glycogen depletion. Since a 24-h fast also depletes liver glycogen, we were interested in blood glucose homeostasis during exercise after fasting. An experiment was conducted with human subjects to determine the effect of fasting on blood metabolite concentrations during exercise. Nine male subjects ran (70% maximum O2 consumption) two counterbalanced trials, once fed and once after a 23-h fast. Plasma glucose was elevated by exercise in the fasted trial but there was no difference between fed and fasted during exercise. Lactate was significantly higher (P less than 0.05) in fasted than fed throughout the exercise bout. Fat mobilization and utilization appeared to be greater in the fasted trial as evidenced by higher plasma concentrations of free fatty acids, glycerol, and beta-hydroxybutyrate as well as lower respiratory exchange ratio in the fasted trial during the first 30 min of exercise. These results demonstrate that in humans blood glucose concentration is maintained at normal levels during exercise after fasting despite the depletion of liver glycogen. Homeostasis is probably maintained as a result of increased gluconeogenesis and decreased utilization of glucose in the muscle as a result of lowered pyruvate dehydrogenase activity.

Source: http://jap.physiology.org/cgi/content/abstract/61/4/1363

Notice the "lowers the rate of muscle glycogen depletion" statement - that is important.

Something I had bookmarked and noticed. Please discuss here with your opinions and any personal experience! I love exercising fasted and even feel stronger, but I recover within 1-2 hours with a "protein + very small amounts of good carb" meal or drink before bed.

Can somebody break this down into English, please? It sounds potentially fascinating.

How did they define "fasted"? No food at all? No water? No juices?

How did they define "fasted"? No food at all? No water? No juices?

Fasted is no food in the gut. The subjects were put on a 23 hour fast, although I'm sure that water and calorie free drinks (probably just black coffee) were allowed - once you start introducing juices and (probably) artificial sweeteners into the mix, you have sugars and other chemicals to metabolize.

Can somebody break this down into English, please? It sounds potentially fascinating

As requested, my "english" breakdown... :)

Fasting before exercise increases fat utilization and lowers the rate of muscle glycogen depletion. Since a 24-h fast also depletes liver glycogen, we were interested in blood glucose homeostasis during exercise after fasting.

Fasting before exercise increases fat burning and lowers the usage of glycogen stores in the muscle, but depletes liver glycogen readily.

An experiment was conducted with human subjects to determine the effect of fasting on blood metabolite concentrations during exercise. Nine male subjects ran (70% maximum O2 consumption) two counterbalanced trials, once fed and once after a 23-h fast. Plasma glucose was elevated by exercise in the fasted trial but there was no difference between fed and fasted during exercise. Lactate was significantly higher (P less than 0.05) in fasted than fed throughout the exercise bout.

The nine males ate as normal up to their workout, then worked out (running at 70% of V02max) and were monitored and tested. Then, another time, the nine fasted for the 23 hour window before their workout, then worked out (running at 70% of V02max) and were monitored and tested. No differences were noticed in exercise between the two trials other than more glucose in the blood plasma and lactate (lactic acid) amounts.

Fat mobilization and utilization appeared to be greater in the fasted trial as evidenced by higher plasma concentrations of free fatty acids, glycerol, and beta-hydroxybutyrate as well as lower respiratory exchange ratio in the fasted trial during the first 30 min of exercise.

The fasted group entered lipolysis (the break down of fats into the bloodstream to use for energy) more readily and in greater amounts than the fed group.

Hmm... Makes sense now, thanks. :tu:

Please discuss here with your opinions and any personal experience!

I did a lot of fasted exercising when I started out, and I think it was helpful in building a good endurance base and burning fat. I also don't think it cost me any muscle. Sometimes I exercise fasted these days, usually fed. I think once you get trained up for good endurance nutrient timing makes less difference than otherwise.

I notice (if I am correct), the "workout" was "running" not pumping iron.

I have a problem with that as far as burning fuel sources go.

I've tried fasted a.m. cardio...didn't like it. While it may have helped with fat burning, I generally felt weaker and 'bitchier' later on through the day.

As a rule I don't do a.m. cardio or a.m. lifting per se. There are lots of days when I will work out about 11.30 a.m. though.

My strongest days are emphatically the days of a good breakfast and pre-workout nutrition.

However I never took up lifting to lose weight because I've almost never had a weight problem of any importance.

I basically lift to increase muscle.

I absolutely believe that the fastedcardio ideology has a lot of true merit and that it works however.

Thanks for the feedback everyone! Just an interesting study. I believe endomorphs (or mesos with some strong endo characteristics - namely fat retention) would be the best candidates for fasted weight-lifting.

It works well for me (and zen as well I see,) but of course it's a "do what your body likes best" type of thing for everyone. :)

It works well for me (and zen as well I see,) but of course it's a "do what your body likes best" type of thing for everyone. :)

I think the "do what works best" is really the ultimate guide. It sounds stupid, but everyone is at different states of adaptation. No matter what is best, if you keep doing it, eventually something else will be better.