There are plenty of studies showing that alternate-day calorie restriction gives comparable benefits to alternate-day fasting or every day caloric restriction. For example, Improvements in LDL particle size and distribution by short-term alternate day modified fasting in obese adults, Modified alternate-day fasting and cardioprotection: relation to adipose tissue dynamics and dietary fat intake.

So, I wouldn't say it's all or nothing.

I've read on Leangains that as long as you stay under 50 calories total during your fast, you're still in a "fasting" mode.

I have some coconut milk in my coffee and don't count that as breaking my fast.

PHD mentions a ketogenic diet is what's important so a diet of coconut oil and leafy greens during the fast is ok.

This simply can't be answered with a yes or no. Think of it this way...

Why are you fasting?

If your answer to this is to get the benefits of kicking yourself into Ketosis and lose body fat, the fast itself is not important. What's important is that you get into fat-burning mode and are able to tap into your stored body fat. This can be accomplished mainly with the elimination of carbohydrate and even protein for a prolonged period of time, but isn't inhibited with fat. Therefore, a "fast" with some fat calories isn't going to be detrimental. As long as there is a deficit during the fast, you will see benefits.

If your answer to this is to detox or at least get that feeling of what I like to call "rebooting", the important thing isn't necessarily the Ketosis, but the clearing out of the bowels, improving digestion, cell regeneration and overall just giving the body a break. For me, I'm extremely active, I eat a good amount of calories from different sources and I'd say my digestive function is very good, but also constantly going. When I fast, I do it primarily to give this part of my body a break.

To me, these are two completely different reasons to fast. If your goal is specifically one or the other, that is going to alter the definition of what a "fast" is to you. Most people seem to want the benefits of both, and if that's the case, your best bet is to completely eliminate caloric intake entirely, and even go as far as not putting ANYTHING into your body that will need digesting or even cause the secretion of digestive juices or enzymes.

Paul Jaminet makes a good case for the benefits of different kinds of fasts. Might be worth an experiment or two! Note: be sure to scroll down and read Paul's response in the comments to Don Matesz.

I know Mat Lalonde mentioned on one of Robb Wolf's podcasts that he'll have coconut milk for breakfast to continue his protein/carb fast so it might have something to do with glucose restriction. I don't have any literature but Mat definitely knows his biochemistry. Worth looking into a bit more.

Balor 123 and nutritionator your comments make sense to me. I see IF as a functional continuation of ketosis. One is always warned if you do not eat enough your body will begin to store rather than burn fuel (fat) as it thinks no fuel is available. thus..keep the food coming..eat fat to burn fat.

I have not researched IF to understand why this does not happen. Glucose carb restriction makes perfect sense to me. I do not know how long one can go with no fat intake (if no other) before the body responds by storing.

This seems contradictory to me [IF benefits] until I have a better understanding of how/when and why it works the way it does.

I'm no expert, but everything I've read about the benefits of ketosis seems to mimic the biochemical and physiological effects of starvation. (Meaning that by going very low carb and high fat, you get the "benefits" of fasting/starvation without actually having to starve.)

So if you were fasting and had a little bit of pure fat, like a Tsp of coconut oil, butter, or cream here and there, I don't think you'd negate the benefits of fasting. It probably does more psychologically than physically, in terms of you feeling like you broke the fast or "cheated" or whatever.

There might be a point at which even pure fat would start affecting things though. I wouldn't go eating 4-5 tablespoons of fat and still call it a fast. I think, especially for people who are new to fasting, there's a psychological lesson in just proving to yourself that you can go x number of hours without food.