The FastDay Forum

OK, probably a daft question, but fundamentally this is what I understand and don't.

Fasting works as it switches your calorie burning from Carbs to Fat, when you exercise when fasting, or otherwise you use up most of your carbs then switch to fat.

When you eat again you switch back to your carbs, and this stops you taking muscle mass as energy.

You shouldn't starve yourself as this causes the body to shut down a bit.

Not starving means eating near your BDR when not fasting??

Does this work on total calories, or Net calories after exercise???

So what I mean is, if I was exercising and burning off all my 600 calories on a fast day, as well as say half of my normal calories on a feast day, meaning net calories would be <1,000, does this constitute "starvation" and will have a negative impact or not.

I hope that makes sense, in my mind it's fine, as I am getting all the nutrients etc as I am eating and digesting the food, and as I have plenty of fat to make up the short fall, it's got to be ok?

I keep a weather eye on the way I am feeling, and have learnt in a couple of weeks that I MUST eat the next day as soon as breakfast comes, or I get an energy crash on the second feast day, is this sort of marker enough to signpost that I am eating enough or not?

[carorees]

OK, probably a daft question, but fundamentally this is what I understand and don't.

Fasting works as it switches your calorie burning from Carbs to Fat, when you exercise when fasting, or otherwise you use up most of your carbs then switch to fat.

When you eat again you switch back to your carbs, and this stops you taking muscle mass as energy.

No it's the release of growth hormone during fasting that spares muscle. You switch back to glucose for fuel when you eat carbs because your fat represents long term storage.

You shouldn't starve yourself as this causes the body to shut down a bit.

Not starving means eating near your BDR when not fasting??

TDEE

Does this work on total calories, or Net calories after exercise???

Total calories including exercise

So what I mean is, if I was exercising and burning off all my 600 calories on a fast day, as well as say half of my normal calories on a feast day, meaning net calories would be <1,000, does this constitute "starvation" and will have a negative impact or not.

Fast day calories are 25% of TDEE so that includes exercise. If you exercise shoo much that your TDEE is above 2500, your fat day calories can be adjusted upwards. Feast day calories likewise. We want around a 3500 cal deficit over the week to lose 1 lb a week.

I hope that makes sense, in my mind it's fine, as I am getting all the nutrients etc as I am eating and digesting the food, and as I have plenty of fat to make up the short fall, it's got to be ok?

I keep a weather eye on the way I am feeling, and have learnt in a couple of weeks that I MUST eat the next day as soon as breakfast comes, or I get an energy crash on the second feast day, is this sort of marker enough to signpost that I am eating enough or not?

more likely to be due to eating too much on feast days, especially if you are having sugary/high GI foods.

Not starving means eating near your BDR when not fasting??

Sounds like something from MyFitnessPal forums.

Starvation is a severe deficiency in caloric energy, nutrient, and vitamin intake. A very obese Scotsman fasted for over a year, and lost a great deal of weight.

Low intensity or endurance exercise, and sitting around, is fuelled at least 50% by fat, high intensity exercise primarily by carbs until reserves are depleted. In either case the effect on the overall energy balance is to create a deficit which has to be met from somewhere - if you deplete your carb reserves then when you next eat the calories are used to replenish the glycogen reserves rather than being stored as fat.

See I told you I hadn't a clue!

Crack on then Bobby Boy!
Thanks for the replies.

A little off topic but I think also worth pointing out that many of the benefits of IF are also achieved by exercising eg glucose tolerance, insulin sensitivity, reliance on fat as an energy source, increased BDNF levels (for brain health), increased energy levels and sense of wellbeing.

Whilst I agree with all the above I am having difficulty training hard on the day after a fasting day but then tremendously well the day after that. I am in the gym by 0630 and do not really have time to sufficiently reload my carbs. My solution at the moment is to do lower intensity long-distance runs which I manage but they are far more difficult than they should be.

Any ideas?

My view is that your glycogen levels are naturally low the day after a fast, especially first thing in the morning, and you're not as comfortable burning fat as you could be. Personally I prefer to exercise in the afternoon the day after fasting but if I can't then I'm like you and find it harder than usual. Not sure how you fast exactly but I've been 5:2ing since aug last year basically doing 36 hr fasts with 600 or less kcal. Exercising on fast day is fine as long as it doesn't involve high intensity/sprints late in the day. My view is that it should get easier training with low glycogen levels, many folks seem to adapt ok when on low carb diets. I think I have found it tough to adapt due to eating too much confectionary on feed days. If you are doing real high intensity first thing in the morning after a fast maybe you will never fully adapt as your body will the primarliy be using solely glycogen/glucose/carbs rather than fat. Maybe low carb paleo diet would help the adaptation process. There's a chap called Peter Attia who blogs about his experiences in the low carb area with exercise, the blog is v scientific and makes interesting reading. Blog is http://eatingacademy.com/quick-faq
In line with your experiences plus my own I also think the comment in the FAQ section about not doing a marathon on a fast day is wrong. My advice would be not to do one the day after fasting.