How come fasting doesn't slow down the metabolism, but calorie-restriction does?

#1

When the calorie-intake may be the same during a day or week anyway?
Jason Fung said it and I don’t understand why.

(Chris) #2

Because Fung wants you to FEAST between fasts in order to keep your metabolism up.

#3

Your body “averages out” the calorie intake. I.e. it respond to the general intake rather than a local fast/feast,
which also makes a lot of sense from an evolutionary point of view.

#4

It’s because metabolism is driven not only by how much you eat but on the hormonal responses to what and when you eat. The responses of key metabolic hormones (most notably insulin, epinephrine/norepinephrine, and growth hormone) to severe restriction in food intake to fasting (or near fasting) levels is quite different from when you lower calories to the modest levels of typical calorie-restriction diets. The cyclic nature of fasting also makes a difference in the long term effect of these changes, lowering weight set point rather than lowering metabolism, so long as plenty of feasting is done in between. It’s complicated but to really understand, I think a book like The Obesity Code is really helpful, as are his various talks that are available online.

#5

is it the amount of calories per meal that keep the metabolism up? ( I think it was him saying that a lot of snacking was the worst for weight loss. )

#6

I have a simplistic way to think of it, a little oversimplified but gets the job done IMHO. I think of the presence of an insulin response (like, to a meal, not just the normal baseline level that’s always there) as a switch that is either on or off.

Anytime you eat, no matter what you eat, it turns on. Eating keto (when feasting) will keep that response lower during this time, which is good, but let’s set that aside for a moment. When it’s on, your body turns to what you’ve eaten for fuel, shifting away from fat stores, so give your body no reason to think your fuel intake is being decreased. Eat enough for your body to think there’s plenty coming in and there’s no reason to shift energy output (metabolism) down.

A few hours after eating, the insulin response dies away, and the switch turns off. When it’s off, the mechanisms for turning to your fat stores for energy begin to switch on. The longer the insulin switch stays in the off position, then longer your body is using up stored fuel. When it has to do that for a few days, the other hormones (epinephrine, norepinephrine, growth hormone) kick up a notch, which not only maintains metabolic rate but also maintains the lean body mass that you have (sparing muscle from loss).

The reason he discourages snacking is because every time you eat, you’re turning the insulin switch back on, even if you only ate a snack and not a full meal. So time-restricted eating has hormonal benefits leading to weight loss. And it’s getting enough calories for your body’s needs when insulin is active anyway that will prevent metabolism from slowing down.

I hope that helps!

(Michelle) #7

[quote=“Callisto, post:6, topic:48366”]
Eating keto (when feasting) will keep that response lower during this time, which is good, but let’s set that aside for a moment. When it’s on, your body turns to what you’ve eaten for fuel, shifting away from fat stores, so give your body no reason to think your fuel intake is being decreased.
[/quote] I think this is the key. If your body doesn’t know it is not being fed because it is seamlessly using body fat, your metabolism isn’t affected.

(Chris) #8

Per meal? Eh, I wouldn’t really worry so long as you keep your meals at 3 or less a day. Eat to satiety each time you eat. The rest works itself out.

(KetoQ) #9

Let’s have Jason Fung speak for himself …

https://idmprogram.com/difference-calorie-restriction-fasting-fasting-27/

And thanks for the question Davida, I’ve been thinking about the same thing lately.

#10

Good article.Thank you.

(Alan Williamson) #11

During fasting, adrenalin and HGH levels rise. This will make a person jacked…it can be hard to sleep. All mammals have a similar response to a famine. A person’s metabolism will not slow down. With CICO, the body will adjust to the calories in. A person will feel tired, lethargic, and hungry. It is crazy. It is almost impossible to maintain.

https://idmprogram.com/fasting-physiology-part-ii/

https://idmprogram.com/fasting-and-growth-hormone-physiology-part-3/

#12

Maybe when we eat small meals that doesn’t satisfy us with enough calories the body understand that as starvation and think it is not enough food around so it slows down the metabolism to reserve the most energy it can? So big meals is better for metabolism. As long as the meals it gets are big and satisfying it is happy even though the calories per week is the same as someone who eats often and little?

(karen) #13

I’m not sure if I can back this up, but my perception is that a certain amount of calorie restriction leaves the body saying “ok, I can handle this, I’ll just slow down metabolism.” An extreme amount of calorie restriction leave the body saying “Oh :poop:, I’d better up the energy level and get out there and find some food before I die.”

Like … if your salary suddenly dropped by 15%, you’d go on a stricter budget. If you actually lost your job, you’d hustle your keto buns pretty quick to find a new one.

#14

My question is, how long is the right amount to fast now that I’m at my ideal body weight? Do I just keep keto and an eye on my weight/waist size and add in fasting when I’ve put on a few lbs (only weigh weekly)

#15

Love this analogy, Karen!!

#16

I imagine this is a really individual thing. You may not need to fast, but others find that IF (24 hours, for example) makes it easier to stay at goal weight. If you don’t have a lot of excess body weight to spare though, my understanding is that EF is harder and not necessarily the best thing for you. I’m only reporting what I’ve heard and read, since I’m not at goal weight yet.

(charlie3) #17

I eat two times a day Sunday-Friday and no food on Saturday every week, as a routine. To gain or loose weight I vary calories and activity. I can always fast more for faster weight loss but I don’t believe I’ll ever need to do that.

(karen) #18

Fasting seemingly has other benefits, including autophagy. The people I know who fast and don’t want to lose more weight seem to fall into three camps: they limit their fasts to maybe 48 hours, they don’t mind getting too thin and then putting weight back on (but it might be muscle they’re losing, I don’t know), or they do modified fasting where they’re intaking a bit of fat to make up for the body fat that’s no longer available.

(bryan vandyke) #19

Your metabolism can’t distinguish between calories coming from food and calories coming from fat stores.

On fasting days your fat cells provide all the calories your metabolism requires. As far as it’s concerned you ate a full days worth of food. No action required.

Under calorie restriction on high carbs the insulin from food and the act of eating inhibits things like Hormone Sensative Lipase (HSL). Which inhibits the release off triglycerides. Your metabolism only gets the calories from what you ate. Nothing from fat cells. The result being, it doesn’t receive it’s required calories. As far as it’s concerned you’re in starvation mode. It will then slow down and do other bad things in attempt to match the lower incoming calories.

(Katie the Quiche Scoffing Stick Ninja ) #20

I’m sorry to hijack the thread
I had this question myself but I wanted to add further;

I am 10 weeks keto and 5 weeks fat adapted and am trying my first 24 fasts.
I will continue with extended but am just getting used to it for now.
Should I treat my fasting as OMAD? And try and get as many calories in when I break my fast to when I begin fasting again? Or should I just eat a regular meal to satiety?

Last night I had two lamb cutlets, brocolli doused in butter and a fat bomb - chocolate mousse. This is about 1000 calories and I can eat this comfortably. I will eat the same again to break my fast tonight 24 hours after my last meal. Thoughts?