Study finds that intermittent fasting is not effective for weight loss

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Looks like the study was self-tracked, which isn’t in and of itself an indicator of a bad study. But if we really want to understand the role that IF might play in weight management I’d be curious to see a study in a controlled setting where two populations get the exact same nutritional and caloric profile, except that the meal timing is either restricted or unrestricted.


I haven’t regularly been eating breakfast in years. I eat lunch super late in the day. And then dinner probably too late. I’m weird.

If I moderate snacks, I keep my weight stable. Cut most or all snacks I can lose weight. Too many snacks and:


hmm. seems to work really well for a lot of people despite the study saying they are all wrong.


It might be reasonable to ask why anecdotal evidence suggests that IF might work and this study suggests otherwise. There are lots of possible reasons: IF success stories have a big old dose of self-reporting and survivorship bias. This study could have some fundamental flaws (it too relies on self-reporting and is not in a lab setting).

But either way it is not reasonable to suggest that this study is saying that people finding success on IF are “wrong”. That’s not how science works.


The study had the IF group eat “ad libitum” (read: whatever they wanted) during their eating hours. I don’t think many proponents of IF are claiming that it negates the idea of calories. All this study shows if that if you eat the exact same amount of calories during your eating period as you would otherwise, you won’t lose weight - but that was already kind of a given! IF is a tool to help you manage caloric intake (reducing time spent eating, reducing mindless snacking, etc) and that’s why it works for some people.


This is an interesting data point, but a not-very useful headline, as IMO there is no mystery why it is anecdotally an effective strategy for maintaining or getting to a healthy weight: CICO.

I have been doing 16:8 for a couple years now as my default, without worrying overmuch about exact timing, motivated in equal parts by the claims made about impacts on daily metabolic patterns… and by the fact that it offers a no-brainer scaffold on which to moderate caloric intake.

For me, it’s this simple: having only black coffee before noon means I have breakfast/brunch/lunch after noon; and dinner.

Stick to two meals, and that’s a huge start to limiting calories… add some modest daily exercise/strength training, watch the alcohol/sugar, don’t snack excessively (or much at all)…

…that’s a pretty effective framework for someone like me.

Made a huge, huge difference.


Bingo - dead simple rule, easy to stick with, which improves mindfulness about intake.


If I could only eat 3 structured meals a day (the control group), I wouldn’t be overweight


same here. i believe that IF works for most people because of this reason. even modest reduction in caloric intake of 10-15% is easily achieved this way.
i will add that IF helps with maintaining a healthy glucose levels most of the day and increase in naturally occurring human growth hormone.

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I’ll second this. I’m less than a year in but I noticed when I had breakfast I’d mentally prompt myself to eat continuously through the day. I was curious to see if that would change if I skipped breakfast, so now I hold off eating until noon. I tend to snack once ( around 3:00, generally something like an apple ) and then eat dinner. Full stop after 8:00. The weirdest side effect is my appetite sort of shrunk so I get full faster. Been dropping a pound or two a month, down about ten pounds since I started.

tldr: yeah, it’s probably just about reducing the amount of time I have to put food in my gob, and then not compensating for time lost when I do eat.


All the comments about IF supporting calorie reduction seem valid, but also seem to contradict the core theory as fielded by the most visible proponents of the approach. I learned about IF - and decided to try it - from a couple of celebrities/sportsmen who are about my age (late 40s/early 50s). These men have high muscle mass and high caloric requirements… they are trying to stay lean, not lose weight. In fact, they might even be gaining weight as part of their training (muscle weighs more than fat, and they are actively maintaining/building muscle). They definitely are NOT restricting calories, because you can’t do that if you’re training heavily or participating in endurance sports. The theory behind IF as I understand it is that it forces ketogenesis by making the body tap its fat reserves every day to fuel activity during the fasting period. Perhaps if study participants are very sedentary, their caloric requirements are not sufficient to trigger this change. Also, I understand the IF protocol to be, roughly, “compress your already healthy diet, which has an appropriate calorie level for your activity level, into 8 hours/day” not “eat whatever you want 8 hours a day”. It’s not a quick fix diet, its a way to maintain fat-burning, and stay lean, for metabolically older people who are already eating healthy and exercising regularly. The last is key, since the big advantage of it vs. “dieting” is no additional caloric restriction, which means athletic performance can be maintained indefinitely while doing IF. Also, as I understand it, IF was never intended to be a weight-loss program per se, at least if you are male and already not obese - at least, the people I heard praise IF are not saying it caused them to lose weight or that they have a goal to lose weight. So the study results do not surprise me at all.


I thought the main benefit of intermittent fasting was supposed to be in maintaining healthy blood sugar - and avoiding wear and tear on the pancreas?

Actually, not so much. We don’t eat calories, we eat food - and the number of calories we extract from that food varies wildly, both between individuals but also for one individual over time. One’s metabolism impacts the calories we get from food, and IF should impact one’s metabolism, not just change habits and the amount of food consumed. (Although the changes to metabolism don’t necessarily aid in weight loss…)


In the end, it all boils down to “do what works for you.” I don’t find IF changes my weight at all, although I generally don’t eat anything until 10:00 each day, and I never eat after 6:00. My weight stayed pretty steady for years. Once the pandemic hit, though, we started baking a ton and I gained a lot of weight early this year because apparently 10,000 peanut butter cookies are bad for your waistline no matter what time of day you’re eating them. Who knew!

Now I’m eating much smaller portions and far less sweets, snacking on handfuls of nuts if I need a little extra something. Since beginning the shift in early July, I’ve shed the 25 I gained from the beginning of March to the end of June, and will continue a while longer since I’d always maintained a bit heavier weight than I should have.


This has been shown to be false time and time again in scientific studies. Yes, armchair nutritionists will say again and again that “losing weight is as simple as calories consumed - calories burned”, but “calories consumed” and “calories burned” are just not that simple. We don’t fully understand how the calories actually taken in translate into calories metabolized. For example:

Non-science version: in a tightly controlled dietary study, increasing salt in the diet (but otherwise keeping dietary structure the same) boosted food metabolism and weight loss (and also made people ravenously hungry).

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If cutting a meal is the best way for someone to cut their energy intake it’s going to help them lose weight. But the point is to cut energy intake which the study sort of demonstrates.

So reading the study, it looks like the real conclusion is “the study wasn’t big enough to make a clear determination” (which isn’t surprising given that >16% of the people instructed to intermittent fast self-reported that they didn’t adhere to it, compared to ~8% of the “3 structured meals” group).

First part of the outcomes section on weight:

There was a significant decrease in weight in the TRE group (−0.94 kg; 95% CI, −1.68 kg to −0.20 kg; P = .01) and a nonsignificant decrease in weight in the CMT group (−0.68 kg; 95% CI, −1.41 kg to 0.05 kg; P = .07). Importantly, there was no significant difference in weight change between groups (−0.26 kg; 95% CI, −1.30 kg to 0.78 kg; P = .63)

If real, an extra half pound over twelve weeks is not a small difference. More people, or a longer timeframe, and maybe they would have learned a lot more.

They also conclude, “This indicates that participation in a weight loss study alone (even in the control group) is sufficient to lead to short-term weight loss and highlights the importance of including a control arm in weight loss studies.” Which is just not supported - there’s nothing to suggest that the people in the “3 structured meals” group were eating that way beforehand, which means they don’t really have a true control group.

And not for nothing, there’s a reason IF is often associated with low-carb and keto diets: those make it easier to fast, since you don’t feel as hungry, your blood sugar is more stable when not eating, and your body has built up the metabolic machinery to get more of your calories from fat, including body fat. Basically, you’re combining a diet that lets your body not think it’s starving in terms of blood sugar when you miss a meal or in terms of leptin when you lose weight, with concentrating your food in fewer meals to provide more of a feeling of fullness. Whether that’s effective or not, this study completely ignores the only explanations I’ve ever read about why IF is worth trying from a metabolic point of view.

It would’ve been great if they’d measured body fat and lean muscle mass before and after, too, especially since they claim there’s evidence that the IF group ate less protein than the “3 meals” group (which wouldn’t happen with IF on a keto diet). If they lost more lean muscle as a result (possible, not sure how likely that is), and still lost more weight, that would be interesting. If that happened, did the IF group lose their weight closer to the beginning of the study (aka might the difference between groups have been statistically significant after the first six weeks)?

I hope they do some bigger follow-ups to learn more, but I’m guessing it’s more likely this’ll just get marked down as “negative result, let’s not bother revisiting it.”

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Exactly. Let’s say a person fasts 23.5 hours a day, from 12:30pm one day to noon the next. But then, each day they absolutely ravenously gorge on 3000 calories if they can fit it all in there. Obv “fasting” will have zero effect on weight loss for that person. CICO, as you say. Fasting is not a license to gorge oneself in the in-between times. That’s not what it’s for. It’s for regulation. It’s for giving someone a structure to avoid that late night eating that blows any dieting out of the water with another 500 extra calories before bed.


The factor which people are not talking about is muscle mass. Muscles are expensive to feed, and having more muscle should divert energy from fat reserves.

The basic Physics Diet actually works. Consistently consume more calories than expended and body mass increases. Consistently consume fewer calories than expended and body mass decreases. Exercise appropriately for desired muscle tone.

Put some determination into it. If you don’t really wanna, you ain’t gonna, pardon my vernacular.

Meanwhile, do not think of LOSING weight. We usually don’t like to lose stuff; we try to get it back. Rather, think of DROPPING weight. By exhaling, actually – what you burn-off is blown out as CO2. So you’re ridding yourself of unwanted carbon, that’s all.

Normal nutrition is your friend. McFood and fad diets ain’t.

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