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I cycle to work weekdays - approximately 10 miles per day. I also do resistance training. For intermittent fasting such as 5:2, on calorie restricted days a common energy intake limit of 600 calories. As I am 6'6"/2m tall and 220lb/100kg I will burn more energy on the commute than the 600 calories I will consume each fasting day.

Should I modify the calorie limit to take account of that? Is anybody aware of research that suggests that the metabolic benefits of 5:2 intermittent fasting is affected by daily exercise? Also, are there any health reasons for a young, fit man not to do a 5:2 intermittent fast?

As an aside, I don't care about 5:2 intermittent fasting for weight control, gains or losing fat. The attraction is that research suggests effects include decreases in markers for cancer, diabetes and age related mental decline. Links are by no means proof but correlate with what I have read and heard over the years. 1, 2, 3, 4

  • How fast are you cycling? I think you are overestimating the calories burned there by quite a bit. – JohnP Jan 14 at 14:48
  • I don't cycle that quickly. Say 13-14 mph. I am usually around 100 kg. I cycle 10 miles. I cannot remember what I used at the time but a cursory glance online gave -cyclingweekly.com/fitness/nutrition/… - and 600 calories does not seem unreasonable given the fact I ride a hardtail MTB on tarmac with 300 meter climb 200 meter descent (and vice versa on return). If you have a better resource to estimate the calories burnt I would be very interested. – Flash_Steel Jan 15 at 11:44
  • Ok, that changes it a bit. I didn't realize that you were riding MTB. You're probably still overestimating a slight bit, but that's much closer than I initially thought. I would have put you around 500 calories for 40ish minutes of cycling, not "much more than the 6000 consumed". I still don't think you would be over your daily count on the cycle alone, however even considering the MTB. – JohnP Jan 15 at 13:48
  • Do you have any links to resources you are basing the estimate on? The link I posted really just tabulates the results for 8 kcal per kg per hour averaging 13 mph. For a 100 kg person that is 800 kcal per hour. 10 Miles is just over 3/4 of 13 miles therefore I can expect to burn just over 600 kcal on a road bike given my weight and speed using Cycling Weekly's calculations. Top athletes - i.e. much more efficient than I am, on better equipment - can expect to burn almost 500 kcal on the same run according to ehow.co.uk/about_5497335_calories-burned-biking-per-mile.html – Flash_Steel Jan 16 at 18:40
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Fasted training in endurance sports is generally discussed as beneficial, but this is often only related to training before breakfast, not 5:2.

If weightloss ins not your goal increassing the calories to cover your bikerides would make it more sustainable. Personally I tried longer periods with 0cal + excercise and it worked as well but wasn`t fun. Changing the type of intermittent fasting might be a good idea for you. The benefits you relate to are also mentioned with other styles. Maybe try the traditional 16:8 23:1.

  • Thanks. I am struggling to find research on 16:8 effects on cancer, blood sugar etc. I will have to give it a proper literature search when I have a spare afternoon. – Flash_Steel Jan 16 at 18:51
  • The principle/goals is always the same so the effects might vary but still be alike (btw. not really propper literature but a start : thetruthaboutcancer.com/how-to-intermittent-fast) – KathyWeb Jan 23 at 11:06

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