Not a silly question at all. If you have a look at the section “How it Works”, you’ll see that frequent exercise will raise your Total Daily Energy Expenditure (TDEE), which you need to take into account on your non fasting days. However, on your Fast Days, you should use 600 as your general rule. This is a quarter of average male daily usage (2400), and that average too has been taken from across a week, or even a month, which allows for days when exercise is lower as well as higher. If you are a serious athlete who is training every day, or 6 days a week, then you may want to recalculate a bit, otherwise you should stick to 600. Have a look at the FAQ section too.

The point of this is not calories in and out merely, but rather what the body chemistry does when you drop them low on an intermittent basis. Otherwise you could lose some weight by just reducing your daily average by say a quarter in an even sort of way, but doing this wouldn’t have the advantages of 5:2 and has a number of disadvantages too.

The fact that many athletes are overweight (see some of the weightloss diets in triathletes mags, for instance) shows that you can exercise hard and still not lose it. There’s been plenty written about this eg by Dr Tim Noakes, Gary Taubes and more.

I started a new job in Jan, always commute to work by bike (approx 1000 cals/day) but unusually there is a free (cooked) lunch which I struggle to resist & since starting 5:2 3+ years ago I’ve never calorie counted, just have normal breakfast & lunch then nothing until breakfast next day. Am I getting any benefits from a 2 day if I have maybe 1000 intake even with the exercise?