Marks Daily Apple
Serving up health and fitness insights (daily, of course) with a side of irreverence.
4 Jul

What Does It Mean to Be Fat-Adapted?

fat adaptedWhen describing someone that has successfully made the transition to the Primal way of eating I often refer to them as “fat-adapted” or as “fat-burning beasts”. But what exactly does it mean to be “fat-adapted”? How can you tell if you’re fat-adapted or still a “sugar-burner”? I get these and related questions fairly often, so I thought I’d take the time today to attempt to provide some definitions and bring some clarification to all of this. I’ll try to keep today’s post short and sweet, and not too complicated. Hopefully, med students and well-meaning but inquisitive lay family members alike will be able to take something from it.

As I’ve mentioned before, fat-adaptation is the normal, preferred metabolic state of the human animal. It’s nothing special; it’s just how we’re meant to be. That’s actually why we have all this fat on our bodies – turns out it’s a pretty reliable source of energy! To understand what it means to be normal, it’s useful examine what it means to be abnormal. And by that I mean, to understand what being a sugar-dependent person feels like.

A sugar-burner can’t effectively access stored fat for energy. What that means is an inability for skeletal muscle to oxidize fat. Ha, not so bad, right? I mean, you could always just burn glucose for energy. Yeah, as long as you’re walking around with an IV-glucose drip hooked up to your veins. What happens when a sugar-burner goes two, three, four hours without food, or – dare I say it – skips a whole entire meal (without that mythical IV sugar drip)? They get ravenously hungry. Heck, a sugar-burner’s adipose tissue even releases a bunch of fatty acids 4-6 hours after eating and during fasting, because as far as it’s concerned, your muscles should be able to oxidize them (PDF). After all, we evolved to rely on beta oxidation of fat for the bulk of our energy needs. But they can’t, so they don’t, and once the blood sugar is all used up (which happens really quickly), hunger sets in, and the hand reaches for yet another bag of chips.

A sugar-burner can’t even effectively access dietary fat for energy. As a result, more dietary fat is stored than burned. Unfortunately for them, they’re likely to end up gaining lots of body fat. As we know, a low ratio of fat to carbohydrate oxidation is a strong predictor of future weight gain.

A sugar-burner depends on a perpetually-fleeting source of energy. Glucose is nice to burn when you need it, but you can’t really store very much of it on your person (unless you count snacks in pockets, or chipmunkesque cheek-stuffing). Even a 160 pound person who’s visibly lean at 12% body fat still has 19.2 pounds of animal fat on hand for oxidation, while our ability to store glucose as muscle and liver glycogen are limited to about 500 grams (depending on the size of the liver and amount of muscle you’re sporting). You require an exogenous source, and, if you’re unable to effectively beta oxidize fat (as sugar-burners often are), you’d better have some candy on hand.

A sugar-burner will burn through glycogen fairly quickly during exercise. Depending on the nature of the physical activity, glycogen burning could be perfectly desirable and expected, but it’s precious, valuable stuff. If you’re able to power your efforts with fat for as long as possible, that gives you more glycogen – more rocket fuel for later, intenser efforts (like climbing a hill or grabbing that fourth quarter offensive rebound or running from a predator). Sugar-burners waste their glycogen on efforts that fat should be able to power.

Being fat-adapted, then, looks and feels a little bit like the opposite of all that:

A fat-burning beast can effectively burn stored fat for energy throughout the day. If you can handle missing meals and are able to go hours without getting ravenous and cranky (or craving carbs), you’re likely fat-adapted.

A fat-burning beast is able to effectively oxidize dietary fat for energy. If you’re adapted, your post-prandial fat oxidation will be increased, and less dietary fat will be stored in adipose tissue.

A fat-burning beast has plenty of accessible energy on hand, even if he or she is lean. If you’re adapted, the genes associated with lipid metabolism will be upregulated in your skeletal muscles. You will essentially reprogram your body.

A fat-burning beast can rely more on fat for energy during exercise, sparing glycogen for when he or she really needs it. As I’ve discussed before, being able to mobilize and oxidize stored fat during exercise can reduce an athlete’s reliance on glycogen. This is the classic “train low, race high” phenomenon, and it can improve performance, save the glycogen for the truly intense segments of a session, and burn more body fat. If you can handle exercising without having to carb-load, you’re probably fat-adapted. If you can workout effectively in a fasted state, you’re definitely fat-adapted.

Furthermore, a fat-burning beast will be able to burn glucose when necessary and/or available, whereas the opposite cannot be said for a sugar-burner. Ultimately, fat-adaption means metabolic flexibility. It means that a fat-burning beast will be able to handle some carbs along with some fat. A fat-burning beast will be able to empty glycogen stores through intense exercise, refill those stores, burn whatever dietary fat isn’t stored, and then easily access and oxidize the fat that is stored when it’s needed. It’s not that the fat-burning beast can’t burn glucose – because glucose is toxic in the blood, we’ll always preferentially burn it, store it, or otherwise “handle” it – it’s that he doesn’t depend on it. I’d even suggest that true fat-adaptation will allow someone to eat a higher carb meal or day without derailing the train. Once the fat-burning machinery has been established and programmed, you should be able to effortlessly switch between fuel sources as needed.

There’s really no “fat-adaptation home test kit.” I suppose you could test your respiratory quotient, which is the ratio of carbon dioxide you produce to oxygen you consume. An RQ of 1+ indicates full glucose-burning; an RQ of 0.7 indicates full fat-burning. Somewhere around 0.8 would probably mean you’re fairly well fat-adapted, while something closer to 1 probably means you’re closer to a sugar-burner. The obese have higher RQs. Diabetics have higher RQs. Nighttime eaters have higher RQs (and lower lipid oxidation). What do these groups all have in common? Lower satiety, insistent hunger, impaired beta-oxidation of fat, increased carb cravings and intake – all hallmarks of the sugar-burner.

It’d be great if you could monitor the efficiency of your mitochondria, including the waste products produced by their ATP manufacturing, perhaps with a really, really powerful microscope, but you’d have to know what you were looking for. And besides, although I like to think our “cellular power plants” resemble the power plant from the Simpsons, I’m pretty sure I’d be disappointed by reality.

No, there’s no test to take, no simple thing to measure, no one number to track, no lab to order from your doctor. To find out if you’re fat-adapted, the most effective way is to ask yourself a few basic questions:

  • Can you go three hours without eating? Is skipping a meal an exercise in futility and misery?
  • Do you enjoy steady, even energy throughout the day? Are midday naps pleasurable indulgences, rather than necessary staples?
  • Can you exercise without carb-loading?
  • Have the headaches and brain fuzziness passed?

Yes? Then you’re probably fat-adapted. Welcome to normal human metabolism!

A quick note about ketosis:

Fat-adaption does not necessarily mean ketosis. Ketosis is ketosis. Fat-adaption describes the ability to burn both fat directly via beta-oxidation and glucose via glycolysis, while ketosis describes the use of fat-derived ketone bodies by tissues (like parts of the brain) that normally use glucose. A ketogenic diet “tells” your body that no or very little glucose is available in the environment. The result? “Impaired” glucose tolerance and “physiological” insulin resistance, which sound like negatives but are actually necessary to spare what little glucose exists for use in the brain. On the other hand, a well-constructed, lower-carb (but not full-blown ketogenic) Primal way of eating that leads to weight loss generally improves insulin sensitivity.

That’s it for today, folks. Send along any questions or comments that you have. I’d love to hear from you guys.

You want comments? We got comments:

Imagine you’re George Clooney. Take a moment to admire your grooming and wit. Okay, now imagine someone walks up to you and asks, “What’s your name?” You say, “I’m George Clooney.” Or maybe you say, “I’m the Clooninator!” You don’t say “I’m George of George Clooney Sells Movies Blog” and you certainly don’t say, “I’m Clooney Weight Loss Plan”. So while spam is technically meat, it ain’t anywhere near Primal. Please nickname yourself something your friends would call you.

  1. Hi there Mark! Great stuff as always my man!

    had to smile that you reached the metabolic flexibility conclusion too.

    Awesome! I am biased as that has been my research focus for the past 6 years and my PhD dissertation topic. I agree with everything you noted here, but the only difference is that not all fat burners are metabolically flexible; but a vast majority are, so it is a fair assumption.

    For those looking at more research, check into metabolic flexibility and the formal name for the theory Mark is talking about is the “Glucostatic theory of appetite control” originally proposed about 50 years ago by Jean Mayer.

    Keep up the great work!
    Rock on
    Mike T Nelson PhD(c)

    Mike T Nelson wrote on July 11th, 2012
  2. Bizarre. I eat a whole food, starch based, low fat, high fiber diet and my answers to questions indicate I’m fat adapted. Fasted workout? No problem. Hours between meals? Cravings? Headaches? Nope. Maybe the Maffetone Method did it for me. I’ve been curious about this thanks to Ben Greenfield, but I still don’t get it.

    Lance wrote on July 11th, 2012
  3. Hi, I know this is an old post, but only just read it!
    I have just read The Art and Science of Low Carb Living, and the performance version. In both books they say that it generally takes less than 50g carbs per day to get into ketosis and optimum fat burning. Also they say that if you have between around 50 to 150g carbs a day you wont be in ketosis, so wont produce ketones to fuel the brain, and wont be taken in enough carbs to get enough glucose for the brain.

    Any thoughts on this?

    Also, Mark makes the comment that you can have a high(er) carb meal regularly. But in the 2 books, and from interviews I’ve seen from the authors, they definitely dont recommend high carb meals, even after exercise.
    Conflicting info?!

    Cheers

    Dave wrote on November 3rd, 2012
  4. Hi Mark, this approach certainly makes sense biochemically, especially for older (going slower) endurance athletes. I’m having pretty good luck with it – see latest blog post http://goo.gl/1pzro.
    Nice site, great info. Thanks for the photo (see blog), which I linked back to your source article.
    -kevin

    FitOldDog wrote on January 12th, 2013
  5. Hello, I am VERY new to this; I haven’t even started yet; just doing the research, but desperate for my energy levels back, and to lose weight and honestly – just FEEL better. I am really nervous about giving up the “staples” in my diet that I have been accustomed to. How can I get my 3 kids and fiance to be on board with me? Is it difficult/expensive to change all the old habits to become fat-adapting? Side note: I also want to look GREAT in my wedding dress in October!

    daile wrote on February 5th, 2013
  6. Based on the signs, I think I am now fat adapted at 5 weeks into the programme. However when it comes to exercise, I can do low level intensity, such as a 3 mile run just fine without carb loading or even in a fasted state. I can even do the same for a bodyweight session in the morning.

    However, when it comes to sprint training with my sprint group and rugby training, I find it difficult to really push. E.g, when I try to sprint my hardest, I cant push my body hard enough to get properly out of breath, cos it just doesnt have the energy to do so. I can jog off fine but high bursts of energy are suffering. I am guessing this is down to low levels of glycogen stores? So, if i needd to up my glycogen stores before or during exercise? I have been trying to eat more carbs the day or two days before training in for m of sweet potatoes or squash and plantain but its not suffice for those intense bursts. Should I have dark chocolate? Honey, more fruit or what? Any advice will be much appreciated because I feel like my body now has the potential to be much more explosive and fast than before but is being limited by not being able to push.

    Hedge Monkey wrote on March 6th, 2013
    • I have the exact same problem, all the other indicators are good but when it comes to doing bursts of exercise like quick sets of jump rope, sprinting up the stairs, or sprints in my bike, my muscles just dont want to do it, they feel too drained!! So what gives?? How is the glycogen supposed to be replenished with less than 100g of carbs per day all being practically veggies? (i’m trying to lose weight, so I’m trying to stay on the “weightloss sweetspot” from Mark’s chart)

      Todtuga888 wrote on June 19th, 2013
  7. I’m fat adaptive, I’ve been an intermittent faster/no grain-eater for the past 22 months. I perform very well in a fasted state on long strenuous hikes and don’t feel all that hungry at the summit (though I do eat and always bring a clean protein and fresh veggies for my meal).

    This is my “problem”: I’ve got two big hikes coming up (one at the end of this month and the next at the end of August and both are beyond anything I’ve ever attempted). The group that I’m hiking with is planning a “spaghetti feed” the night before. I would surely like to stick with what works for me but I’m not sure I can defend my position properly in this crowd where I am clearly the novice hiker. I’ve taken all of their advice for safety, gear, etc but have avoided the carb-loading issue.

    I really like these guys but in all honesty, I believe them to be over-trained and out of shape (each sports a huge belly). They stop often to eat their cookies and granola bars, while I sip on my water. I’m sure their assessment of me isn’t all that flattering either (crazy anorexic or whatever).

    Anyway, I think they’ll be putting pressure on me to carb up and insist it’s for safety. I’d certainly do it if I thought it would help my performance on the hike AT ALL.

    So, could you tell me, would it help my performance??

    Laurie wrote on July 2nd, 2013
  8. I have gone six months of cyclical Keto/ carb backloading, and have only recently started to try and get fat adapted. Its only been three days, and I feel no lack of energy in my workouts, and can go more than 12 hours without feeling the need to absolutely devour food. I have never gone longer than 5 days totally keto/low carb, but is it possible that I am already fat adapted?

    Ani wrote on July 4th, 2013
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    I do not realize who you are however certainly you’re going to a well-known
    blogger when you are not already. Cheers!

    bodog88 wrote on December 27th, 2013

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