I read on a post recently where someone was suggesting to rest or don't do physical activity during IF. My personal take is I feel just fine doing any level of activity. I regularly do long hikes the day of an IF, having eaten the night before and eat the evening I get back. My only precaution is heavy strenght training, which I would only do at the end of an IF, right before eating.
I am curious as to other people's thoughts on this subject. Do you rest during IF? Do you have the same energy level? Etc. Or does it depend on the duration. For example I consider IF at least 16-18 hrs, and really not until closer to 24 hrs.
I do intermittent fasting and and I'm also a high school athlete. I find that exercising while fasting makes me more alert and quick on my feet. Most days, I have a good hard strength workout right before 12-1pm and then break the fast by eating a big meal. (Eggs with some ground beef and ghee is my all time fave.) Paleolithic hunter gatherers often had to hunt or search for food on an empty stomach so I figure exercising a little while intermittent fasting is just fine. Just be sure to eat a big meal when you're done working out so you muscles can recover & rebuild.
I have two types of IF I go through: Natural and Planned. With Natural IF I just don't eat if I'm not hungry. Probably 3-4 days a week I won't eat breakfast because I am simply not hungry. With planned IF I plan ahead that after dinner on day 1 (usually 630-7) I won't eat again until the following dinner. I never do planned IF on days that I lift heavy.
In terms of activity levels, I maintain the same activities. I've seen that my longer runs and temp runs are more difficult on days that I IF, but that hasn't stopped me. I think of it like having a weight vest on -- I am purposely putting a load on my system so that it becomes better at burning fat for fuel. Then when I take the vest off (i.e. put some carbs in my system before a race) I feel much faster.
If you have extremely low body fat naturally like I do, coupled with possible adrenal issues, be careful of water fasting and exercising hard during the fasted period. I've just about killed myself doing HIIT in the morning then trying to fast the rest of the day.