Intermittent fasting: Is it the calories or carbs that count? (The Carbs)

Medical Xpress / The Conversation / European Journal of Nutrition ^ | May 6, 2025 | Adam Collins et al

[ConservativeMind]

Intermittent fasting is not only a useful tool for weight loss, it's also shown to have many benefits for metabolic health.

But my latest study shows that you don't need to severely restrict your calories to get the metabolic benefits of intermittent fasting. Even just restricting the number of carbs you eat twice a week may be enough to improve your metabolic health.

My colleagues and I previously ran a study to demonstrate the effects of a fast on the body. We observed that following a day of both total fasting or severe calorie restriction (eating around only 25% of each person's daily calorie requirements), the body was better at clearing and burning the fat of a full English breakfast the next day. Fasting shifted the body from using carbs to using fat. This effect carried on both during the fast and the next day.

Our research has also compared the effects of intermittent fasting to a calorie-matched or calorie-restricted diet. Both groups followed the diet until they lost 5% of their body weight.

Despite both groups losing the same 5% of body weight, and at the same rate, the intermittent fasting group had greater improvements in their metabolic handling.

We recruited 12 overweight and obese participants. Participants were first given a very low-carb diet one day. Another day, they were given a severely calorie-restricted diet (around 75% fewer calories than they'd normally eat). After each fasting day, we gave them a high-fat, high-sugar meal to see how easily their bodies burned fat.

What we found was that the shift to fat burning and improved fat handling of the high-calorie meal were near identical following both the traditional calorie-restricted "fast" day and the low-carb day. In other words, restricting carbs can elicit the same favorable metabolic effects as fasting.

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TOPICS: Health/Medicine
KEYWORDS:

[ConservativeMind]

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[Jeff Chandler]

instead of restricting calories two days a week, you would restrict the number of carbs you consume twice a week

Restrict them to what? And what are the details of "fasting" in this article, which seems low on facts.

[Jeff Chandler]

three, one-day (36 h) dietary interventions in a random order, with a 5-day washout period between interventions

[Jeff Chandler]

Energy-balanced diet (nEB)
Diet served as control, included 100% of their estimated isoenergetic needs adhering to the principles of the Eatwell Guide. Carbohydrates comprised 55% of total caloric intake.

Low-carbohydrate energy-balanced diet (LCEB)
Diet included only 50 g of carbohydrate yet provided 100% of their estimated isoenergetic needs.

Low-carbohydrate energy-restricted diet (LC25)
Diet included only 50 g of carbohydrate and provided 25% of estimated isoenergetic needs.

[sauropod]

Bkmk

[dfwgator]

The key is also to trick up your body and not repeat the same eating patterns every day, keep it guessing.

[eyeamok]

I found that if you just stop eating you will lose weight fast. I lost 40 pounds in 3 months from something that made it extremely painful to chew. soup and ice cream for a couple months and now that I can eat I can’t seem to gain any weight back. They still don’t have all the answers.

[SomeCallMeTim]

Hmmmm. If you stop eating, you lose weight? What an interesting concept.

When I was 17, I went an entire week without eating anything. Nothing. It wasn’t that hard. Then I started eating a half sandwich each day. I lost 40 lbs in 3 months.

[Myrddin]

Prior to my ampullary cancer and Whipple procedure, I would eat 1250 calories except on Tuesday/Thursday I ate 600 calories. My macros were 30% protein, 30% fat, 40% carb. That approach dropped me from 193 lbs to 146 lbs. Following the Whipple where I lost all of my duodenum and gallbladder, I have to eat 2000 calories to hold my weight. I went very low carb on Wed/Thu this week in preparation for a PET CT scan. The "prep" said high protein. I ate under 500 calories. The prep was right for the PET scan, but caused awful diarrhea. That was rapidly remedied after the PET scan with a trip to Fuji for hibachi salmon and a large serving of fried rice. The "prep" dropped my weight from 150.5 to 148.2 even after the large hibachi chicken lunch.

Post Whipple, my weight stays around 150 lb. I aim for 100 to 120 grams of protein daily with 3 grams of HMB to protect from loss of muscle mass. It it easy to meet protein targets with Optimal Amino as a supplement. Carbs are easy to adjust.

[Openurmind]

The human body is built to process calories from fat and protein. MEAT. We lived on Mastodons and Mammoths for thousands of years... We are built for this... We didn’t get fat until we started eating roots and grain carbs...

And this is where Evolution takes a left turn. We are not direct from great apes or we would be vegetarians... We had some outside DNA manipulation in there somewhere which made our guts and metabolism more friendly to primarily meat.

[Jonty30]

I assume it’s both calories and carbs. If you skip a meal, that is 250-1000 calories you won’t be intaking and carbs that will not be stored by the body.

[ConservativeMind]

The study:

https://link.springer.com/article/10.1007/s00394-025-03646-5

[Redcitizen]

Bookmark

[Secret Agent Man]

I do it everyday.

Every once in awhile 3-7 days full fast.

[woodbutcher1963]

I have been intermittent fasting since December.
I started at 213, I am 186 this morning.
My wife and I do it together for morale support.
Twice a week from Sunday after dinner until Tuesday morning
Then again from Wednesday after dinner until Friday morning.
My total cholesterol dropped from 290 to 205.
My goal is 175.
I am 62 and 5’ 9”.

[Phoenix8]

I’m an intermittent faster. In my opinion it works.

I’ve been doing I.F.for maybe 20 years on and off. I just started using my first prescription drug (developing glaucoma) at 61. When I see nurses or doctors they seem amazed I’m not on any meds other than one. At first I was surprised “ isn’t everyone like that I asked” ?

No they said nearly all our patients over 60 are on multiple meds.

So yeah in my mind it works as that is the only health practice I follow.

My wife will be very happy to hear carb restriction works as well, possibly.

[C210N]

I do IF, and have gotten into a routine of detailed spread-sheeting of carbs, fat, protein, fiber, calories.

“carb restriction” imho requires data to know how restricted it is. I have days in the 30 gm area, and see my keto numbers go to pot. When carbs are less than 20, keto is possible. With carbs half that, 10-15, ketosis is attainable most days, especially later in the day, falling off to low ketones overnight.

[C210N]

Watching the actual number of calories I find essential to weight management. If I want to gain a few pounds, get the cal up in the upper 2000’s or low 3000’s. For maintenance, 2400.

When I was focused on getting ketones up, I lossed a LOT of weight. I wasn’t at that point monitoring daily calories, unfortunately. Went from 175 to 140 lbs over about 2 months period.

[Redcitizen]

Curiously enough , I came across this video before I read this FR article about intermittent fasting. Both the article, video and associated comments had some good info.

What Happens When YOU FAST For 36 Hours Straight?

https://m.youtube.com/shorts/Fr4wdirEbgI