Fasting and Exercise

Hi there @JustinP. My own personal experience with using muscles/lifting heavy things, and aerobic exercise, whilst fasting is really simple - I can't do it! without recent 'fueling' with food, preferably protein. Maybe yeah - the first day. So, I think - if you can do it, and are even contemplating it - all the power to you! In the true sense of the word.

I personally absolutely needed to be fueled up with food to do lifting/muscle movements, and aerobic exercise. But with anything to do with metabolism, we all know the drill - hugely individual! In the broad strokes.

A fantastic book which goes into the ins and out of activity, sports and athleticism and low carb, and fasting too I think? is Volek and Phinney, ‘The Art and Science of Low Carbohydrate Performance’. Another great researcher and exercise expert re folks with diabetes is the magnificent Dr Sheri Colberg.

And not to forget - Prof Tim Noakes, and for the popular approach - Dr Michael Mosley. (Both of these who are also people with type two diabetes.)

I think you will enjoy a deep dive into these writers and researchers - what are they also called? Ah yes - sports scientists and physiologists.

I can understand fasted running for fat burning, but wouldn't strength training and continuing to fast for a long time afterwards just break down the muscles with no way to repair them?

JustinP said:

Good Afternoon,

Looking for some advice on the practicalities and approach to Exercise and training whilst longer fasting.

Exercise is 5KM run and strength training.

I currently do 16:8 twice a week and probably a 24 hour three times a month.

I really want to do a 48-72 hour 'deep' fast

I have no problems exercising on a 24 hour fast - did it this morning - but i am concerned about going to the gym having fasted for 36-48 hours

Does anyone have any thoughts / tips / practical experience or advice?

Thanks in advance

Click to expand...

I'm not sure how useful this will be to you, it's by no means scientific and is based on my personal experience alone. I'm also quite overweight still, so our capabilities and goals are significantly different. I only do 16:8 fasting, 4 or 5 times a week. I'm just offering this so you get a collection of responses, more than anything. I also suspect this is one of those topics with a large dose of "it's different for each individual" involved.

On keto, I find that although my general energy levels are far more stable, it doesn't offer much "burst" capacity. I keep my exercise firmly within aerobic heart rate zone (82% of my exercise is in this zone), with the rest being in the next heart rate zone level down - weight control. I can sustain aerobic zone without issue and my fitness has massively improved already doing so. I can do this level of exercise on 16:8 fasting before I eat that day.

However, if I push intensity into anaerobic zones, I don't last long, tire very easily, and I can literally feel my muscles filling with lactic acid very quickly. I also find this is more pronounced when I'm fasting. From a physiological perspective, burning fat for energy is a slower process than burning carbs. With lower carb intake, the risk of burning lean mass and muscle also increases as exercise intensity increases, so that's something to be mindful of too.