Read these two amazing articles:
The First Rule of Fast Club
by the amazing and highly recommended Krista Scott-Dixon
It's a great article, but possibly a little long in getting to the point for those with a short attention span. I recommend reading all the way through to the end – it's worth it.
and then jump into this *amazing* compilation of intermittent fasting experiences from most of my favorite kettlebell instructors:
Train Like A Man, Eat Like A Woman
By one of my favorite writers/kettlebell instructors ever, Lauren Brooks, SFG
The long and the short of both articles is this – Intermittent Fasting, while possibly the holy grail of fat loss for many men, is a bad idea for women. It seems to repeatedly be devestating for the metabolism.
I had a quick exchange with Lauren (author of the second article) about if she felt the same about partial day fasts as she does about full day fasts – and the short version is that partial day fasts (like 16 hour fast/8 hour eating) or Warrior Diet type semi fasts may be a fine as long as it doesn't turn into force-fed meals within the time allowed.
What You Should Learn
If you are female – skip full day fasts.
I've recommended some programs that included both 16/8 fasts, 20/4 fasts or 24/0 fasts. If I had it to do over again, I'd recommend those programs, but replacing the full day fasts with just more 16/8 fasts.
And again, 16/8 fasts work great for some people, and not for others. For some people, finding out that breakfast wasn't absolultey essential for their fat loss goals was super liberating. For others, they found any kind of fasting to be miserable. Both people are right.
On some level, you should eat when it works for your life and your body.
Total quantity always trumps timing. Quality trumps timing.
And, for some people, a 16/8 fast really works to reduce quantity, and to reduce food preparation time.
I Was Wrong
I haven't had any personal training clients that have done full day intermittent fasting, ever. I've had a couple (literally two) phone coaching clients that came to me after having done some serious damage to their metabolisms, and I wasn't able to help them in any short order.
So, I apologize for taking the experience of others, and some promising researching, and promoting something that sucks.
To be totally accurate – the intermittent fasting program I recommended was based primarily on 16/8 fasts, and designed to be short term only. But it did include one 20/4 fast per week, which now I would have all of my female readers drop completely, or replace with another 16/8 fast.
What To Do If Your Metabolism Has Taken a Hit
Eat more, for a while.
If you do feel like you might have done some damage to your metabolism through intermittent fasting, the only answer is to have a period of increased calories. During this time, it's very possible you're going to gain a little weight, so I'd use it to focus on a strength or performance goal.
This is where having a long term plan is important.
This video is long and depth: If you feel like you might have done some serious damange to your metabolism, you want to watch it. It's Layne Norton, Phd, talking about long term, incremental plans for repairing damaged metabolisms of fitness/bikini competitors: