Can Intermittent fasting slow metabolism?


I’v been keto for about 7 weeks. I started intermittent fasting about a month ago. My concern is since doing both of these my appetite is so small I’m wondering if I’m actually slowing my metabolism way too much. I eat twice a day (usually between 11-7 window) and I eat rich satisfying and mostly organic foods and my carbs are 20 grams and below. Honestly most evenings I hardly want to eat dinner even though I do. Any wisdom on this?


(George) #2

Listen to the “Live Q and A” episode of the Obesity Code Podcast with Jason Fung and Megan Ramos. There’s a segment within the first 30 minutes or so that covers this question in detail from what I remember

But, as long as you’re eating to satiety and consuming enough caloric dense fat macros, I think you’ll be fine.


Thanks so much. I’ll be checking this out.

(Michael - Don't expect miracles and you won't be disappointed.) #4

@Remy5555 Are you losing weight? If so, do you want to? If not, you’re probably OK until you start to. Getting your energy and calories primarily from fat tends to boost metabolism, not slow it. But you are correct to be concerned about total input, since your appetite is reduced. Tracking weight will tell you if/when you’ve reached a deficit energy imbalance. At that point you can decide whether or not to increase input to stop the loss. Or if you want to continue losing, slow it to about 1-2 pounds per week, which will not result in slowing your metabolism while on keto.

Also, I find it curious that you are doing IF 7 weeks into keto and actually started “about a month ago”. Shifting your lifestyle and diet to keto is a BIG change and requires time to allow your body to adjust to the new environment. Adding IF on top of that so soon doesn’t make much sense to me.


Yes, I’m trying to lose weight and have been. There was a dramatic result in the beginning and now it’s been slow. Hopefully I’m on track. Also I didn’t jump into keto from a standard American diet and had already been off regular sugar for almost 2 months. I found keto to be a very easy transition and didn’t experience and of the “flu” like symptoms. Thanks for the feedback and I’ll be tracking my weight better.

(George) #6

Mind sharing your stats? sex, height, starting weight, etc.

I’ve been on Keto since January 1st and i also do intermittent and extended fasts and find it to be working well.

(Michael - Don't expect miracles and you won't be disappointed.) #7

That makes a difference. To lose weight, even on keto, you have to maintain a negative energy imbalance. On keto, however, it doesn’t mean simply fewer calories. It means eating less carb calories and more fat calories. Fewer carb calories means both glucose and insulin go down which enable more fat metabolism (AND at a higher rate) to occur.

Initial weight loss is water. Glucose and glycogen are hydrophilic and carry as much as 3 times their weight in water. Once you burn through the glucose and glycogen stores, the water goes with them. After that initial water purge things settle down. Fat burn is a slow process whether you do it in keto or not.

It is still a BIG metabolic change. Ketosis is very different from not. Every cell and organ in your body is habituated to using glucose and now you are giving it ketones and fat instead. This takes time. Some folks on this forum report that it took them months to transition. And really, efficient fat burn is a sliding scale. Your metabolism gets more efficient at it the longer it does it.

Don’t fool yourself. At 7 weeks you have not transitioned yet. Getting into ketosis is quick, getting fat adapted is not. The goal is not simply ketosis, it’s efficient fat burning. Glad you didn’t experience any carb withdrawal symptoms. They can be very uncomfortable. I didn’t either, so it doesn’t really mean anything other than we were lucky. Luck of the genetic draw? :wink:

Anyway, it’s good that you’re aware of potential issues regarding your appetite and how to insure you stay on track. Welcome aboard.


47 year old Male, 6 feet and I started at 270 lbs and as of today I’m at 246.6 lbs but I’ve been at the 246.6 for about 2 weeks now. I lost 20 lbs immediately but I’m assuming that’s water weight.


I really appreciate all the help here. I’m feeling inspired and willing to keep this up.

(George) #10

Ah very good. I’m 5’9, started at 297 and was 243 this past Sunday.

How are your protein and fat macros looking? Maybe too much protein and not enough fat can be causing the weight loss to pause. Are you consuming a lot of dairy? I had to do a lot of tweaking to find what worked best for me.

If your fat and protein macros are good (and obviously your net carbs are under 20) then the first thing I’d do is eliminating any and all artificial sweetener and dairy for 1 week. If the scale moves, you found your problem.

I’d normally recommended getting a little stricter on your fasting windows but you might be too new to this to jump right into that, although that is a bit hypocritical of me since I jumped from starting keto to 16/8 IF 3 weeks later, to 18/6 the next week, 20/4 the following week, and have been OMAD from the next week till today, so you can give that a go if you want.


I was thinking about the dairy. I’m gonna start there. Good suggestions. My Macros seem to be okay.

(Bacon is the new bacon) #12

Fasting means different things to different people, so correct me if I’m wrong: it sounds as though you are gradually eating fewer meals, but still eating every day, is that right?

If so, it shouldn’t be a problem, as long as you are keeping your carbohydrate intake under 20 g/day and satisfying your hunger when you do eat. As long as you are letting your body dictate the quantity you eat, and not dictating to your body, you should not trigger what I call the “famine reflex,” in which the body adapts the metabolism to the food intake.

Previous posters appear to be assuming that you were talking about extended fasting, which is much eaiser to do once we are fat-adapted. At seven weeks in, you are probably on the verge of fat-adaptation, however, if not already there. How are your energy levels doing? Do you excercise? If so, how is it feeling, now?

If you are enjoying eating only one meal a day, you might find yourself quite naturally forgetting to eat on a given day, and that would be a natural introduction to extended fasting, which is basically anything over 24 hours. If you let your body be your guide, you should be fine. For various reasons, fasting does not trigger the famine reflex the way reduced calories do.

As far as weight loss is concerned, you seem to be doing fine. I would be willing to bet that at least some of the intial 20-lb. loss was fat, not just water, but do bear in mind two points: first, fat loss is never a linear process, and second, the scale is not always a reliable guide to what’s going on. What we want is to lose excess fat, but to preserve or even gain muscle and bone density. Lean gains can happen at the same time as fat loss, and this confuses the scale. So if your scale doesn’t change, but your clothes keep getting looser, keto is still working!


This is great and inspiring. Yes I’m eating every day. I’ve just been amazed since going keto how my hunger cravings have diminished. I have no desire to over eat.