Metabolic Flexibility - Get Real

(Michael - When reality fails to meet expectations, the problem is not reality.) #1

Several frequent posters to this forum continually talk about consuming carbohydrates to gain and/or maintain so-called ‘metabolic flexibility’. A few others bring it up occasionally. All these posters talk about ‘metabolic flexibility’ as something desirable and healthy. So what exactly are these people talking about?

From what I can determine from their posts, to all these folks ‘metabolic flexibility’ means eating carbohydrates for the purpose of - eating carbohydrates. In other words, they are using the term ‘metabolic flexibility’ to justify eating carbohydrates.

Now, I stand to be corrected if I am interpreting this incorrectly. And I started this topic to give these folks the opportunity to correct me.

My understanding of so-called ‘metabolic flexibility’ is this. When glucose levels are low, especially over time, most cells will switch to using fatty acids and/or ketones for fuel. Ketones allow cells to be metabolically flexible. Eating carbohydrates not so much. Since gluconeogenesis synthesizes all the glucose we need for those cells and organs that can’t utilize fatty acids and/or ketones there is zero need to eat carbohydrates, which are simply sugar molecules of various descriptions.

Do you build tolerance
I'm disgusted and could scream!
Paul Saladino + "metabolic inflexibility" (+rise in morning BS?)
Metabolic Flexibility?
Sugar flu?
(Bob M) #2

I do eat higher carbs at times, but it has nothing to do with “metabolic flexibility”, whatever that is. If anything, it’s low carb/keto and IF or long term fasting that provides “metabolic flexibility”, not periodic eating of carbs.

(Lazy, Dirty Keto 😝) #3

This. I don’t “carb up;” however, I think those that do typically say that they are able to do so because they have metabolic flexibility from doing keto. I may be wrong but that’s how I interpret it.

(Mame) #4

I don’t eat carbs to increase my metabolic flexibility but when I do eat more carbs by choice than is normal for me I like to see my BG numbers come down in 60-90 minutes after not rising more than 20 pts in the interim. (say 85 before meal and 105 post meal and back down to 85 within 2 hours) For me that is metabolic flexibility. I want my metabolism to be so resilient that it can handle carbs/sugars easily and quickly.

(Windmill Tilter) #5

I enjoy eating keto, and I enjoy being in ketosis. I’ve been doing it for a year and I’ve lost quite a bit of weight, and gained quite a bit of muscle. I also enjoy eating a slice of deep dish Chicago style pizza and drinking beer. If those two things are not mutually exclusive, why on earth would I voluntarily restrict my choice to only one or the other?

For me, Keto is an incredibly powerful metabolic tool with the potential for both good and harm. I use it because it facilitates extended fasting, which is another incredibly powerful metabolic tool with the potential for both good and harm. I don’t presume that too much of either is prudent in the long term.

There is also the bizarre and daily occurrence of ketonians posting about “discovering” new and terrible intolerances, allergies, and metabolic problems. It kind of freaks me out. WTF is all that coming from? Were these actually latent allergies, or has restricting diet to fat and protein for years at a time fundamentally changed their metabolism, immune system, endocrine system, and gut flora? Given that metabolism, immune system, endocrine systems are profoundly influenced by gut flora, my money is on the latter. Maybe metabolic flexibility isn’t as trivial as simply “liking pizza”, it might actually have serious health implications.

At the turn of the previous century before food became a manufactured chemistry experiment, people did just fine. My grandfather is 97 and my grandmother is 94, and they have been thin, happy, and fit as fiddles their whole lives. They ate plenty of potatoes, bread, bacon, and steak their whole lives and they still do. When someone shows me their 97 year old ketogenic grandparents who’ve eaten keto their whole lives, they will have my full and undivided attention.

Until then, I’ll be waiting in quiet corner with the rest of the heretics… :yum:

(Michael - When reality fails to meet expectations, the problem is not reality.) #6

That’s my point. Ketosis creates metabolic flexibility by habituating cells to utilizing fatty acids and ketones. Because gluconeogenesis always synthesizes glucose for those cells and organs that can not utilize fatty acids and/or ketones, we never lose the ability to metabolize glucose. So there is zero need to eat carbs to ‘insure metabolic flexibility’. A ‘hit’ of excess glucose once in a while just passes through elevating BG and insulin for a while and then subsiding. It’s when metabolic disorders and/or insulin resistance are present that it gets messy.

(Murphy Kismet) #7

^^^The word “because” is important here, methinks.

(Michael - When reality fails to meet expectations, the problem is not reality.) #8

@Don_Q In some ways it’s great that each of us responds somewhat differently to metabolic and environmental inputs and pressures. I presume it was survival enhancing during our evolution to have diversity rather than the risk of everyone being killed off by a single random incident. So good for that. On the other hand, having an uncle who smoked and drank a hell-of-a-lot of whisky every day of his 99 years doesn’t translate to a plan for good health and longevity to most other people. The uncle was a fluke, and good for him.

There are a couple of Michael Eades videos floating around this forum (I’ve posted them both a couple of times) where he discusses the health consequences to humans transitioning from very-lo/no carb hunter gathers to high carb farmers. The overall decline in health and stature is not a pretty picture. He shows the evidence of T2D, obesity, CVD and other so-called ‘modern’ diseases that were prevalent amoung the ancient Egyptians of 3000+ years ago via autopsies of mummies.

The evidence is clear beyond any shadow of doubt. We are far better off not eating carbs. If you have any evidence of any long term potential for harm from ketosis, let’s see it please. On the other hand, there’s a ton of evidence of the long term potential for harm from eating carbs. Any so-called ‘metabolic flexibility’ intended to justify eating carbs is nonsense. Eat carbs if you please, but realize they serve no useful purpose and pose very real risks. This is not fear-mongering, it is stating the reality.

(Eric - The patient needs to be patient!) #9

I chose to not engage in significant carb eating. While I believe my IR has improved, I just feel so much better on keto as a carnivore adjacent eater.

I may be wrong but I believe that I respond better to the failure resistance training I do as well when I am fully keto. I’m losing body fat at a rapid rate and gaining muscles, as evidenced by a lot of loose skin and other visual evidence.

(Windmill Tilter) #10

It’s not actually. Smart and thoughtful scientists on both sides of the fence are doing compelling research as we speak. I don’t think it will be a settled question for decades at the earliest.

There are plenty of perfectly rational, well informed people who elect to eat more than 20 carbs a day. I’m not one of them yet, but I can see the logic of doing so.


(Michael - When reality fails to meet expectations, the problem is not reality.) #11

Smart and thoughtful scientists brought us the disaster of the current world-wide T2D, obesity and CVD epidemic. Until someone can show me that the ancient Egyptians didn’t really suffer from T2D, obesity, CVD and a host of other metabolic disorders and all those mummies that Eades illustrates as examples are just flukes - I will continue to consider the question settled. The evidence points pretty conclusively to eating carbs as the problem.


“Metabolic flexibility” in the scientific literature has a very specific non-ambiguous definition…

The capacity for the organism to adapt fuel oxidation to fuel availability

Insulin resistance for example is the opposite of being metabolically flexible because insulin resistance implies glucose metabolism is impaired, meaning you can’t tolerate carbs anymore.

The Keto diet actually increases insulin resistance, due to the “muscle sparing glucose for the brain” effect. This is called physiological insulin resistance and it does make the blood sugar spike whenever people go off keto for the occasional treat.

This was noticed in tightly controlled trials like this:

And also fatty acids play a role in inducing skeletal muscle insulin resistance:

It’s a normal effect, but it does make one less metabolically flexible by definition.

Diabetics can also suffer from “impaired 1st phase insulin response to a sudden rise in blood glucose level”. So unfortunately T2 diabetes is a doubly edged sword as insulin secretion can also be a problem and a low carb dietary pattern may actually impair this first phase response, making the ketoers even less tolerant of carbs, this being one of the reasons why official recommendations recommend 200 g of daily carbs, which sounds a little insane given the potential for hyperglycemia.


Also fat burning is equally important and people on keto do adapt to burning fat more efficiently for anaerobic effort. However what’s missing from the conversation on ketosis is that in healthy people that are not doing Keto:

  • glycogen stores deplete over night enough that many people are in ketosis before breakfast
  • ketosis also happens due to a caloric deficit, many people can actually get away with more than 60 grams of carbs per day and still remain in ketosis if maintaining a caloric deficit

And some trivia:

  • the 0.5 mmol/L threshold btw is completely arbitrary
  • ketosis has many benefits but there are no observed hunter gatherer populations to have lived in a chronic state of ketosis; even the Inuit that everybody loves referencing have CPT-1a deficiency, a genetic disorder that prevents them from entering ketosis suggesting that evolution really did not want us to stay in ketosis 24/7

In other words cycling in and out of ketosis and keeping your ability to tolerate carbs might actually be healthy.

(Mike Glasbrener) #13

I’m not sure I like the NIH study. They took fat people (BMI 25-35) and fed them a high carb,50%, diet for 4 weeks. They’re likely fairly insulin resistant after this. They then go keto for 4 weeks. By then end of this maybe they’re getting fat adapted. Higher cholesterol is probably due the fat burning. LDL, HDL, APO-B etc need to be looked at. Longer term studies like VirtaHealth are compelling. It could be cycling carbs could be helpful. I’m not convinced yet.

(Todd Allen) #14

Physiological insulin resistance is a temporary effect that resolves in a few days of gradual reintroduction of carbohydrates. Virta Health has published their results of patients on ketogenic diets seeing stunning reversals of type 2 diabetes and the biomarkers of metabolic syndrome which strongly correlate to long term pathological insulin resistance.

I see physiological insulin resistance as similar to the alcohol intolerance of non-drinkers. One can improve short term alcohol tolerance by consuming more alcohol at least until one goes too far and trashes their liver. But it may not be a good strategy for optimum health. Likewise I see no reason to boost my carb tolerance by consuming carbohydrates. Knowing I have the option is sufficient.

(Empress of the Unexpected) #15

People before the 70s were not, as a general rule, fat. Yet our parents and grandparents had been eating bread and potatoes with every meal for a long time. Something very, very different is going on now. I just define metabolic flexibility as the ability to go in and out of ketosis easily. I can eat bread for two days and be in ketosis the next day. It is a good thing that our bodies can switch fuel so easily. Our bodies were designed to burn both glucose and ketones for a reason.

(Bunny) #16

One thing I can say if your not metabolically flexible then you are not metabolically fit?

Carb up for 5 days (or 5 days before a glucose tolerance test) and if your gaining weight rapidly or you fail the ogtt, your probably not metabolically fit nor flexible yet?

I would go stir crazy not knowing if I was metabolically fit and the only way to know that is to carb up to see the health reserve you built up?

(PJ) #17

Hmmn. I don’t know?

I feel better if I eat more carbs than usual (<100) about once every 7-10 days.

And I intentionally eat off-keto on 4th of July, and for six weeks from Thanksgiving (or my dad’s birthday around then) until the day after Christmas.

I don’t have any “so I can be metabolically flexible!” label assigned to any of that though.

I will say however that eating keto and low-carb consistently otherwise, and fasting, seems to have improved my ability to do the occasional carb-up without completely losing ketosis to the point where I gotta suffer for three weeks to get back to it.

When I went lowcarb – many times, off and on – it was this huge light-switch break. Low energy, carb cravings, waiting… then finally “shifted into ketosis” and and hallelujiah, I could taste feel smell ketones and had a sudden rush of energy.

Now, though, for example I was high carb up to 12/26. I fasted for one day, then resumed eating keto. I’ve had no issues with cravings, no issues with energy. And the last couple times I have done this off-then-back again, I don’t have a major “shift” point. It’s very gradual, there is no suffering. Yet I know it’s working because I’m dropping the gained water weight (which due to my size and issues is enormous) speed of light, even when trying to slow it.

Really though… I mean… does it matter? If Jane eats carbs now and then to be “metabolically flexiible,” whatever that means, even if she’s totally lost in space about the reality of it, in the end it’s just… Jane doing what she can, and it works for her or doesn’t.

I’m all for a dictionary of legit meanings for health/nutrition terms and phrases, but then I’d have to argue about who should be the person making the decision on each one. :wink:

(Jane) #18

My n=1 is this:

Pre-keto I was hungry all the time, eating every 2-3 hours, 40 lbs overweight even though I was still eating HALF what my husband ate, he weighed less than me and got less exercise.

Post keto I weigh less than him, still eat half what he does, but 40 lbs less now. I can eat 50+ carbs without storing fat. He can eat 100 carbs. I had my fasting glucose measured last summer and my HOMA score shows NO insulin resistance anymore!

Why would i care? Why not just keep eating keto? I do, mostly. But I am still employed in a higher-level corporate job where dinners are part of the gig. I need to be able to sip wine and eat a normal dinner for appearances without suffering ill effects.

(Empress of the Unexpected) #19

I was strict keto for a year and a half. Ate moderate carbs for two months. Because of kitchen remodeling somehow satisfied with one carb meal a day. I think keto prepares your body to accept more carbs. Even though I am temporarily eating one carby meal a day, I am still in ketosis.

(Jane) #20


You are closer to my husband than me when it comes to handling higher carbs but happy you have found your carb balance!!