I'm curious about this as well. I do Zumba (pretty high intensity) 3 days a week, and have started training for a half marathon so run 3 days a week as well, with the long run on Saturday. My fast days are Tues/Thurs which are my running days as well. Will this work - will I have the energy to run? Will probably run in the afternoons.

I'm not a lifter, so I can't give you advice on that. I do a lot of HIT spinning and running, though.

General comments first.
- When phasing into 5:2 your performance will go down a bit. Give your body time to adjust. Only do LIGHT workout or none on FD in the beginning. When you feel like it, you can start incorporate fasted exercise on FD.

After a while I returned to normal and HIGHER performances. This morning I took 10g protein powder and 10g fiber before going to the gym. But I just had two BIG carb days, so muscle glycogen was pumped! I was 24 mins in orange zone (88-92 % of VO2 max) and a new record 9 mins in red zone (93-100%) 9 mins in yellow. All of this during a 55 mins group class with 20 mins warmup and cooldown. I also started running during the 5:2 diet, sometimes fasted.

@feistybucket

If you're feeling weak or lightheaded during the exercise itself, it's likely your blood sugar crashing. If you're eating enough fat and low-carb regularly, that shouldn't be happening.

Agree. My guess is that a lot of the time when people say being lightheaded, dizzy, almost fainting, headaches, feeling too hungry etc. blaming whatever diet they're on or tried, it's because they're carb crashing. Or they try to keto and don't take more salt. Or they just don't take into account that they're sometimes doing a nutritional U-haul. The body is conservative, it doesn't like abrupt changes.

Got it this week myself. I had a lunch (I don't eat breakfast) that was mainly carbs and a bit of fat, avocado and fried pork rinds. Went to the city and felt so dizzy I almost had to sit down. I realized what it was and grabbed a cucumber in the nearest grocery store, wolfed down half of it on the way out...crisis averted. Maybe a protein in the meal would have helped, I don't know. Look up "atkins flu" if you wanna know more.

@feistybucket

I do 5:2 and lift. In my understanding and experience, the best time to purposefully eat carbs is post-lifting to restore your glycogen stores. The approach is called Carb Back-Loading (CBL) if you want to look it up.

I thought CBL is more designed for lifters who are bulking and need muscles to grow. If you're lifting to maintain LBM, I would guess CBL isn't necessary. But if that is your experience, then you're probably right:)

I think that when doing 5:2 and combining with lots of (higher intensity) workout, it's best to avoid FD and pushing hard starting out. Over time you can do both, if you wish. On FD, I find low intensity or housekeeping works well. Whenever I've burned too much in the morning (300-500 kcal), I eat dinner too early on FD. Leaving me guilt snacking towards the evening, breaking the 500:(
Trial and error what works for you!

Following are MY experiences, which may or may not apply to YOUR body.

a) FD are naturally very low in carbs, since I want to feel fuller for longer. So I generally only have brothy soups, vegs and protein on FD. Being so strict on carbs kinda migrated into my TDEE days. I try to limit my intake of low nutrition carbs all days.

b) 1x a week or so I've done a carb refeed BEFORE a planned HARD workout. Still pretty frugal at about 100g uncooked pasta or a stew with lots of vegs and added sweet/normal potatoe. I know I will burn those carbs away during my workout anyway, but they're just enough to give my gym time a bit of boost.

Since I'm pretty low carb all of the week, the carb-refeed works very well for me. Not only can I perform slightly better, but the day after I often experience a whoosh too, with new personal record low scale weight. Body dumping water weight it's been holding on to. I eat 1 portion of complex carbs with a good fiber count like plums and apples most days. For some reason I've got hangup on fresh flat leafed parsley since 5:2...maybe they contain something I need, lol.

I feel that I can perform well fasted too atm. But maybe not quite THAT well, like it's just a bit harder to push through the pain (into red zone).

I've been loosing 1kg/week since 5:2. 2 weeks ago I was cycling for 11 hours in the span of 4 days without loosing energy. If this is because of 5:2, LCHF or what not, I don't know. But the combined effect is great!

TRIAL AND ERROR FTW

Interesting to hear your experience. I'm a huge advocate of taking what we can learn from a variety of sources and then TWEAK and ADAPT "recipes" to our own bodies. Great to hear you found something you feel comfy with and that works for you:)

@surfingbodi. I don't have a system, lol. I just noticed that carb loading pre WO gives a slight edge. Whether that's in a dinner type meal 3 hours before WO or loading the day before. (I almost throw up if I eat closer to WO)

For ketosis, I would think carb loading the same day is better, since you burn off the carbs that day.

I only carb load on the days I have planned HEAVY gym day, though.

Trial and error FTW

My main advice would be patience. Fasted training (weights or cardio) gets easier and easier over time. At first on 5:2 I was close to "bonking" / "hitting the wall" (symptoms of glycogen depletion) after very gentle exercise / workouts on a fast day. Had to abandon a few fasts as I was so wobbly!

Now fasted training is just the same as fed training both in how it feels and performance. Today I did a really tough simulated mountain climbing training session on a Spinning bike 16 hours after eating and have ended the day with negative calories and feel absolutely fine.

With weights I've got tons of energy and hit exactly the same weights fasted or fed, it's just not a factor in performance for me at all.

It's only for exercise way over two hours I bother with pre-loading or feeding as I go now.