This is one of those questions that I get asked a lot and the short answer is, “yes, it seems to be fine to combine the two and may indeed be beneficial”. The long answer is a lot more complicated. I am just working my way through the academic literature on this, some of which is in the book, but the headlines are:
- Make sure you are well hydrated. We get a lot of fluid from food (some of it added during the manufacturing process to add weight, bulk and therefore value), so if you cut your calories to a ¼ during your fasting day you will be consuming less water. I recommend drinking lots of calorie free fluid during the day, whether you are exercising or not. This can be black tea, black coffee (the idea that coffee makes you dehydrate is a myth), water from the tap, herbal teas, whatever. I am not a fan of diet drinks for reasons i will write about later.
- I have had tweets saying things like, “i have a dry mouth” or “my mouth sometimes tastes funny on a fasting day” and this is almost certainly a sign of dehydration.
- Men seem to not only tolerate but respond better to exercise on fasting days. For women the picture is more complicated. I welcome feedback on this
One of the key benefits of exercise and fasting is they both increase insulin sensitivity, and insulin sensitivity is an independent predictor of future mortality. But they work in different, complementary ways. Exercise for example, particularly short burst of HIT (High intensity training) depletes the glycogen stores in the muscles, while Intermittent Fasting (IF) depletes the glycogen stores in the liver.
On a more general note I had a look at the government guidelines for the BBC R4 series, You and Yours, on exercise, 5 a day and alcohol. I attach links to features I wrote about them for the BBC