STOP Doing Long Fasts — A New Study Reveals

Last week one chap advised my friend to try 24-hour fasting for weight loss. I shall call him Jacob. Since that was his name. When I heard that I thought to myself:

So I took an evidence-based dive into the literature to find out how long fasts affect weight loss. Since the whole of my existence is dedicated to being a geek I quickly stumbled upon this 2021 study (and it doesn’t look promising for people who do long intermittent fasts):

Researchers randomized lean, healthy subjects into two groups:

Contrary to a bunch of crappy low-carb studies, this one gave fair conditions — both groups had the same calorie deficit throughout the study:

And here comes the part that will make Jacob’s advice seem ridiculous in every possible way — the fasting group lost less fat:

Since both groups had the same calorie intake, different weight loss seems weird.

The thing is, 24-hour fasts turned subjects into wastoid couch potatoes — they had nothing left in them. They moved around less and burned fewer calories from physical activity during fasting days. This resulted in less weight loss.

The fasting group also lost more fat-free mass:

This is how the ACTUAL weight loss looked like:

And that’s my biggest beef with long fasts whether it’s long intermittent fasting, utterly worthless water fasts, detoxes, or cleanses. You see, the goal of losing weight is twofold:

Loss of muscle mass when dieting is a major reason why metabolic rate drops after losing a significant amount of weight. Which makes weight loss maintenance harder.

Also, as Alan Aragon, nutrition researcher observed:

With that, comes my recommendations:

Don’t fast for more than ~16 hours (even shorter for women). Fasting for up to 16 hours seems to have no impact on muscle loss when paired with resistance training.
If you’re still going to ignore the above for no explainable reason, do it safely — try protein-sparing modified fast. Have a high protein intake during the fasting window to prevent muscle loss.
Don’t fast at all unless you genuinely enjoy it, you’re pressured by time, or prefer fewer but larger meals. Intermittent fasting may work for some people but it DOES NOT cause any special effects compared to continuous calorie restriction: