It IS 800 calories on fast days

Published 8.16.2024; reformatted on 8.26.2026: for the third time in a row, I am writing about the 5:2 eating plan. I'm doing so because I was googling around today looking for data on it (actually I was looking for a documentary on it), and I found a 2019 website post from Michael Mosley (RIP). In that post, he announced that he was increasing the calorie allowance from 500 or 600 calories to 800 calories on fast days.

So I had remembered correctly… though I don't think I'd ever read that particularly post. Rather, I think (as Mosley did) that the 800 calories I had in mind was based on Roy Taylor's work on type 2 diabetes. 800 calories in the amount that they assign diabetics to eat (in the form of shakes) to lose weight quickly. Mosley and Taylor wrote a book together where they tested using 800 calorie meals of real food to lose the weight and cure type 2 diabetes.

After that book, Mosley wrote a new book called the Fast 800, in which he goes through all the research of the 800 calorie diet and the benefits, and then altered his weight loss way of eating. I am reading the book now, and will write it up properly when I've finished. This is a more recent podcast/video with Michael Mosley discussing his Fast 800 (the updated 5:2). In essence, he applied Roy Taylor's results to his original diet, and found that people are more likely to stick with a diet that gives them 800 a day rather than 500.

Obviously, the 5:2 eating is a bit different (thought I'm sure you could do shakes on your "fasting" days) because you're only cutting calories to that level twice a week. Weight loss would therefore be slower. However, weight loss would come. One reason I was interested is that I have a gym friend who is thinking about doing the 5:2 (and who knew about it and had watched the documentary on it) because I am having results with eating this way.

That is my cue to include an update as to my progress. The scale has started to move, I've lost roughly 5 pounds since I've started, but more than that, I can tell that I am (at last) losing inches around my middle. This is important because it suggests that I may be losing visceral fat. I just know that for the first time in 20 years (and at a much higher weight) that my "pooch" which I've had since being pregnant is starting to shrink a bit.

A history note: In the 2008 time frame I was walking roughly 8 hours a day at a treadmill desk and my weight dropped to roughly 115 pounds— a weight I hadn't been at since Middle School. At that weight, my "pooch" did shrink. But the very low weight didn't last because it couldn't. Eventually life circumstances meant that I was walking less, and the weight returned, though not all of it. I'm not aiming to reclaim my Middle School weight, but I wouldn't mind dropping about 15 pounds. And I'd like to see if I can maintain it a bit longer this time, without having to do anything extreme.

My attitude has always been to begin as you mean to go on, so the real question will be if I can continue to leave Tuesdays and Thursdays as lighter days. Time will tell.

Addendum: So I rewatched the Fast Diet Documentary linked above, and much of it I remembered. However, what I didn't remember is that the researcher in Chicago (Krista Varaday) who developed the alternate day fasting diet ALSO lets people eat 400-500 calories (basically one mid day meal) on "fast" days. So in reality, it's not a true fast, which would mean no food. I remember that Varaday thought that Mosley "stole" her ideas with the 5:2, but I never realized that she let people eat on fast days. I thought alternate fasting meant every other day you didn't eat.

For the record, the 4:3 (or alternate day fast diet) holds no interest for me, because it doesn't ft in my lifestyle. One of the reasons I like the 5:2 is that it does. Others may have different experiences, and that's fine. You do you, I do what works for me. Varaday has also written books, but I have never read them.