Why would my metabolism slow down if I’m getting cals from fat stores?

[Sharon_E]

Not trying to be a smart aleck, truly, but if our bodies have fat to burn (and assuming we’re in ketosis), why would lowering one’s calories cause your metabolism to slow down? If we’re burning our fat stores there should be no deficit, right?

Just my n=1 here, but I lose so much faster when I keep my calories to 1,200 or so. (5’4”, 47, 190#, female, sedentary, for reference)

Can someone help me make sense of this? I’m not a newb (been low-carb for a year) and I have devoured this site, but I have not run across anything that addresses this.

[Ilana_Rose]

It’s not smart alecky at all. To lose weight you have to be in a calorie deficit. The difference on keto is that once you’re fat adapted you don’t feel like you are restricting calories.

If you aren’t hungry constantly and if you are fully satiated following your meals then there is nothing wrong with consuming 1200 calories as long as you are getting your protein in and keeping carbs low enough so that you body can access your fat stores.

[carolT]

If we’re burning our fat stores there should be no deficit, right?

Short answer, yes.

Long answer, that’s a big “if”. Most likely variables limiting access to fat stores are elevated insulin (yes, even in ketosis if someone has severe IR) and the rate limit on accessing bodyfat in g/kg/day.

Longer answers will be along shortly…

[Sharon_E]

By that logic I could drop my calories to, say, 500/day and still not hurt my metabolism. But I know if I suggested that I would get shouted down pretty quickly.

It doesn’t happen every single day but there are many times when I go days only eating 500-600 calories and I’m full, and then I worry about my metabolism. And then I come back around to this idea that I’m really not in a deficit because I’m burning fat stores so I don’t need to worry.

I just don’t know what to think?

P.S. I would have liked your post but I’m out again.

[carolT]

Any idea of approximate bodyfat weight?

[Sharon_E]

I could tell you my (useless) BMI, but that’s about it. A LOT, let’s put it that way.

[carolT]

You could take some guesses and see what happens when you put different variables into this calculator for available energy from fat stores (at the bottom of page).

I’d say hunger would be a good indicator to go by, all other things being equal.

[Sharon_E]

Thanks for that, will check it out now.

[carolT]

Easy calculation example: 32 Kcal/lb x say 50lbs = 1600 Kcal available. Low intensity exercise may help liberate more by dropping insulin during the exercise period.

[Ilana_Rose]

It depends how much fat you have to lose. There is a limited amount of energy that can be pulled from your body’s fat mass per unit time. Someone lean can’t get 1200 calories out of their fat in a day while someone with plenty of body fat can.

I don’t think eating little or nothing when not hungry is a problem at all, but by the same token on days when you are extra hungry you shouldn’t be limiting to 1200 calories. I tend to eat anywhere from 800-2000 calories at maintenance based entirely on appetite. I don’t seem to gain more after hungry-eat-everything days. My body is burning it when it’s asking for it.

[David_Stilley]

By that logic I could drop my calories to, say, 500/day and still not hurt my metabolism.

I believe you can drop to zero calories as long as you feast every few days. EF is what it’s called. But then you already knew that I’ll bet!

[111]

Intermittent Fasting (IF) calls for not eating for hours or days. When in ketosis your metabolism does not slow down as explained in the Obesity Code. There is no need to worry about calories if you do Keto with IF.

[ZuleikaD]

Not trying to be a smart aleck, truly, but if our bodies have fat to burn (and assuming we’re in ketosis), why would lowering one’s calories cause your metabolism to slow down? If we’re burning our fat stores there should be no deficit, right?

Here’s how my non-sciencey brain understands it:

It’s the deficit in intake that’s sending signals to your body to slow your metabolism. Your body stored up fat in case of emergencies. Now you’re taking in less food/calories and your body needs more than the intake to run, so it uses the stores. You logically know that it’s not a food emergency. But your body’s signaling systems are designed to react as if it is when you start to use your stored fat. In response, your metabolism slows to use less energy and prevent use of your emergency rations. Your body’s goal is to be in stasis with your intake, so it’ll keep dropping your metabolic rate until it gets there. At the same time, a smaller body uses less energy to run, so your RMR will also drop because of this.

Like you, I lose steadily on low-carb if I keep my calories around 1400-1500 (I’m 5’8", so I’ve got more leeway than you do at 5’4"). But after reading studies about the metabolism slowing effect of under-eating calories and how fasting doesn’t have the same effect, I switched my approach. I’m still averaging about 1400 calories a day, but some days I eat 2400 and some days I eat nothing. I don’t totally understand the science behind why fasting doesn’t have the same effect. I think it’s because I have days where I’m eating way over my metabolic requirements, so signals about using the emergency fat stores never really have a chance to kick in.

[Karim_Wassef]

Metabolism slows down if your daily availability of energy is less than your demand for energy.

You get x from food and y from your fat. Your body has base metabolism m and activity metabolism n…

If x+y < m+n … the body will reduce m since it’s the only thing it can actually control. It’ll also make you more tired to get you to exercise less too - reducing n.

The y from fat is capped at 30-35 calories per lb of available body fat as long as your insulin is low. If insulin is high due to high carbs, then y is locked up and you lose m super fast - low metabolism and low energy. Then your body goes into “starvation mode” to make up the difference & consumes lean mass “s” to balance it out.

x+y+s = m+n

So s is starvation loss of mass to make energy as well as m & n are reduced. Those are the controls the body has

This is super simplified but directionally correct.

[carolT]

The y from fat is capped at 30-35 calories per lb of available body fat as long as your insulin is low.

Good point. That rate of calories was observed in metabolically healthy humans under starvation conditions, so it would be the most available. Likely a lot of us can’t achieve that much.

It’s the deficit in intake that’s sending signals to your body to slow your metabolism

That’s the opposite of what Phinney el.al. say about the initial phase of a ketogenic diet where bodyfat supplements dietary intake, in the context of eating to satiety.

[Karim_Wassef]

Also. That’s why eating fat allows your body to use its own fat. Fat has little to zero insulin effect so it allows maximal use of your own fat. Eating fat is the closest you can get hormonally to fasting… without fasting.

Protein is more complicated since it has an insulin effect but it’s a good one and short lived, but it can interrupt usage of your own fat for energy (or accelerate it - told you it was more complicated). But this is about calibrating for your own body.

Carbs just stop everything from working.

[CarlKeller]

By that logic I could drop my calories to, say, 500/day and still not hurt my metabolism. But I know if I suggested that I would get shouted down pretty quickly.

The trick is to not let your metabolism think that famine is a threat. You could do the 500/day for a while but your metabolism will adjust, especially if you start getting hungrier and ignore it. I wouldn’t worry about low calorie days as long as your hunger is not raising a red flag. That should be the primary warning that you are not eating enough.

[ODE]

Why would lowering one’s calories cause your metabolism to slow down?

Because its anti-survival to use fat storages, body doesnt like it at all. Theres ways to trick ur body to make it believe theres no famine, but it will always try to make u survive more rather than cater to “modern societys desires”.

[ZuleikaD]

It’s the deficit in intake that’s sending signals to your body to slow your metabolism

That’s the opposite of what Phinney el.al. say about the initial phase of a ketogenic diet where bodyfat supplements dietary intake, in the context of eating to satiety.

I’m not saying that body fat doesn’t supplement intake. It does. But doing so sends signals to your body in response to using these body fat stores. Also, we’re not really talking about the initial phase of a low-carb/keto diet. The metabolism drop happens over time.

I have read that switching to a low-carb/keto diet will increase your metabolic rate overall. But that doesn’t mean it won’t drop from that new, higher rate over the long term when responding to under eating total energy needed.

[andy71]

In general, my understanding of the way its supposed to work lines up with how Karim explained it. The problem is that this is a linear equation and its very clear that the way metabolism operates does not follow a linear model. it usually does in the short run (weeks/months), but in the long run (many months/years) that it operates in a dynamic fashion with feedback loops (e.g., the starvation response of slowing metabolism to a level that is below what would be suggested by the body composition at that time).

The real issue is that I haven;t seen anybody who has laid out a really good model of all the factors that affect metabolism and how they interrelate in feedback loops. Lets not forget that leptin was just discovered in 1994, gherlin in 1996. There is still so much we dont know about metabolism - its no wonder we get it wrong so often.

Read “The Secret Life of Fat Cells” https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6161053/