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

[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/

[carolT]

But doing so sends signals to your body in response to using these body fat stores.

Specifically, which signals to where that would slow metabolism if enough energy is supplied via fat + diet? Are you suggesting to eat beyond satiety just to meet a calorie goal?

Also, we’re not really talking about the initial phase of a low-carb/keto diet.

True, but at one year in there is still a significant fat mass in this case. Now, if someone were intentionally limiting calories, even though hungry, to continue losing weight that would be a problem.

[Sharon_E]

I’m so confused. I don’t know why I thought my dumb brain would get it. Whatever, as long as it works I’ll just KCKO.

Thanks for trying y’all.

[monsterjuice]

Sharon, I just eat til full. I do not track. Some days I’m sure I’m under 1000 cals. Other days I may be 1700 cals. I just make sure I’m satiated. Your body will adjust especially if it needs to burn calories! I don’t stick to a amount to eat, I think eating different amounts different days, keeps my body confused and also keeping it out of starvation mode as I’m not constantly at a large deficit!

[BeStill]

I am grateful for your question because I think I have been overdoing the calories for the sake of not restricting and it is absolutely not working for me. I have lots of my own fat to burn, nearly 3 months in strict Keto with almost no weight loss, the only tweak I have left, I feel, is to stop worrying about under-eating calories and see if any loss happens with that change…

[PaulL]

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?

Dr. Fung explains it this way: if there is caloric input, the body matches energy expenditure to intake. Give it enough, or more than enough, and it will increase metabolism to match. Try to manipulate fat loss by reacting intake, and it will slow down the metabolism, making it necessary to cut intake even further. The Biggest Loser contestants did themselves permanent damage this way. This operates even in a low-carb milieu, which is required for fat loss.

But in a low-carb milieu, the leptin signaling to the hypothalamus is no longer being blocked by insulin, so the brain can set the appetite to a level at which both dietary and stored fat can be metabolized. Hence our advice to eat to satiety, so the body never goes into famine mode. But when we fast, Dr. Fung explains, there is no caloric intake to match, so the body simply switches fuel tanks, as it were, and fuels the metabolism out of the stored reserves; i.e., fat.

Makes sense, no? When we have a Mastodon carcass handy, we feast. When it runs out, we go out, fasting, and hunt another, metabolism running at full speed. But when game is scarce, we eat whatever is to hand, and the caloric restriction is the signal to the body that times are hard, and it needs to conserve resources, to get us through.

[Alecmcq]

I’m so confused

Sharon
I think you have asked a fantastic question and I am not sure we have a great and clear answer right now. I know I don’t. I have often wondered about the low calorie vs fasting thing and the differences to metabolic rate that we think/hope happen when we fast vs when there is severe calorie restriction.

But I think we do know that the body does react to lower intake by reducing metabolic expenditure. So it happens, we just don’t know the exact mechanisms. I am pretty sure that @Karim_Wassef and others experiments have shown that the metabolism definitely drops when we eat lower calories, even on keto.

The encouraging thing though is that it seems our metabolisms are pretty resilient unless you have done something crazy like Fastest Loser competitions.

[Karim_Wassef]

Let’s try an analogy. Imagine this:

You have a coal furnace that give you energy to do stuff.

You have one guy who can shovel coal into it from a huge pile. Just ONE guy.

You have 10 guys who can shovel coal into it if it comes from the outside… fresh coal is so special that the big boss put a whole team on it.

Now. There’s a supervisor. This guy is nasty and he checks all the incoming coal… if he sees any gray chunks of coal (instead of black coal), he chains up the ONE guy who can shovel coal from the big pile… even if all the new coal is finished… even worse, the furnace can’t handle more than one chunk of gray coal a day… so he just tells those 10 guys to move ANY incoming coal into the pile for that day.

Now… let’s say you don’t have any new coal coming in. All you get is one guy shoveling coal from the big pile… the oven gets cold even if you have a mountain of stored coal… you’re just going to have low energy.

Now… let’s say you get some new coal and it’s the gray kind. The ONE guy gets chained up and can’t shovel any coal from the pile AND you only get one piece of gray coal for energy… so if any new coal comes in, it adds to the pile. Now you really have no energy…

Now… let’s say you get some new coal and it’s the black kind. The team can keep shoveling it in and the ONE guy can shovel in old coal from the pile too. Now the pile gets smaller and you have TONS of energy.

The black coal is fat

The gray coal is carbs

The ONE guy is how your body processes it’s own fat.

The 10 guys is your digestive system and the liver.

The huge pile is your body fat.

The new coal is food.

The furnace is your body’s Metabolism.

The supervisor is insulin.

The big boss … leave it up to you to decide.

[Alecmcq]

But shouldn’t this mean if you are strict keto then you wouldn’t lower metabolism if you ate very low calorie chronically? Doesn’t Sharon’s question still stand?