If your fasts are shorter, don’t worry about it and just make sure to get a varied, healthy nutritional boost post-fast to get them back up. The body is great at not burning through them too quickly
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If you’re going a long time (like more than a week), a multivitamin is enough to keep you healthy. I think unless you’re already close to deficient somewhere, though, it’s a non concern. and you don’t even need to worry about it in these cases, either. But the multivitamin won’t hurt - your body simply wont use what it doesn’t need anyway so if it makes you feel better, go for it!