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?

When 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,

    I have a few questions I was hoping you knowledgable people could answer for me…

    I’m relatively new to this style of eating (had pulses and whole grains before, but otherwise mostly paleo). I feel like I’m some way down the Fat Adapted road – don’t get hungry between meals so much, can train without carbs etc. but I would like to become fully fat adapted. My questions are these:

    – how to accelerate that process? Just continue as-is (reasonably low carb, high protein, moderate fat) or move towards a higher fat diet?

    – what happens when you eat carbs? I am vegan one day a week – will these additional carbs have any effect on fat-adaption?

    – I do bouts of quite intense exercise, and normally eat carbs (banana or sweet potato) straight afterwards – if I am fat-burning, do I not need to do this, or do I still need to replenish muscle glycogen stores?

    Thank you in advance for any advice you many have!

    Fi wrote on July 7th, 2012
    • Get an oil lamp. Fill it with protein. Try to get it going. Now fill it with coconut oil and watch it burn baby, burn!

      Don’t be afraid of fat. Fat, as we have just demonstrated, is your fuel. Primal is not high protein, it is high fat.

      If your vegan day consists of vegetables and green leafys with olive oil and/or coconut oil, no problems mate. Consider, however, that one way to eat vegan is to – not eat. How about a fast day instead?

      Consider also the possibility that a vegan day is an affectation. You aren’t actually a philosophical vegan and there is no dietary or environmental reason for it. Let it go.

      Unless you are are engaging in multi hour, multi times a day, intense exercise in a competitive environment there is no reason to even think about your glycogen stores. It’s a complete irrelevancy. Your body will manufacture what it demands. From stored fat. While you sleep. “Worry” about getting enough high quality sleep.

      Note that an Ironman is NOT a multi session intense exercise just because it is nominally three events. It is a single moderate effort.

      If you have a psychological need to eat something try a hard boiled egg.

      kfg wrote on July 16th, 2012
  2. Mark and all… At our Center, we test our client’s resting and exercise metabolisms using the New Leaf Metabolism Assessment System. We have found it to be extremely important in determining whether the client is “fat adapted” and to what degree. In fact, it does give you an EXACT RQ value that you speak about in your article above and ultimately tells us what percentage the client burns fat versus sugar while at rest. Then, we design an exercise and weight loss nutrition program to help them improve all aspects of their metabolism and lose excess body fat as well. Thanks for posting this article – it truly describes the “real” issue when it comes to why the general population can’t lose and then maintain their weight.

    Richard Farina wrote on July 7th, 2012
  3. Thanks for this article!I am sugar-addict without any doubt and want to change it.I admit that it might seem really strange, but I don’t know how to do it. Just buy some primal book and start to eat this way? And what book would you suggest? Or should I somhow gradually do it? I would be really grateful if someone would give me a short advice.
    Thanks!

    elfriide wrote on July 7th, 2012
    • As someone fairly new to paleo eating, I would recommend just going for it. There’s enough stuff on this site that you could read here and then just try things; I found Mark’s 21 Days book particularly useful because it gave me specific tasks for each day, which kind of got me started.

      DEFINITELY clear the junk out of your pantry; if you have to get dressed and leave the house to acquire a craving food, you can better resist ithan if it’s just down the hall.

      I really like the stuff on this website, the rest of Mark’s books, Loren Cordain’s books, Robb Wolf’s book and It Starts With Food.

      Good luck !

      BJML wrote on July 8th, 2012
  4. Ahhh, thanks. That was just what my brain needed after reading someone’s blog post “Ten Reasons I’m Not Primal/Paleo”.

    “……paleo is a fad with no evidence supporting it……”

    She even adds links to one of Mark’s interviews.

    Next blog entry you see CheesyGal and her grain fed mid-western hubby and think, “Gosh, I’m looking at two fairly big reasons why I eat primal”.

    Keep up the good work and thanks.

    Kenny wrote on July 8th, 2012
    • Millions of years of evolutionary eating: fad diet.

      A few decades of eating according to propagandized by marketing and political concerns: standard diet.

      Sometimes the stupid is so great the only thing you can do is point and giggle.

      kfg wrote on July 16th, 2012
  5. Something to note for people who still consume a fair amount of carbs, like me – if you are feeling constantly hungry, check your water intake. Glycogen stores require as much as 3g of water to store 1g of carbs, so if you are dehydrated all the carbs will leave your body without filling up your glycogen.

    Alex wrote on July 8th, 2012
  6. Great article. The one additional thing I would like to add regarding there is no “fat adaptation home test kit.” You can easily test blood ketone levels (bohb) via an inexpensive blood glucose and ketone monitoring system. I read that measuring one’s blood is more accurate than via a urine sample, so I ordered a testing kit with some blood ketone test strips and watched how my bohb levels are around 2-2.5 mmol after I became fully keno adapted (for me, I define that as <= 50 grams of carbs per day max — usually in the 40s for me). The Volek/Phinney book on LC performance was very beneficial for me. I tend to look at any home test kit like this as more of a validation tool that I am in the general range of being in a keno-adapted state, rather than over-analyzing individual test result strips down to the tenths.

    I have found the benefits of LC to be fabulous. I'm training for my first marathon, and I can wake-up early, no food/breakfast, and go out and run 6-8 miles and not feel tired, shaky, etc., something I could never do when I wasn't LC. Moreover, after I'm done with a long run, I'm not even hungry most of the time, and usually wait a few hours to eat, it's great not having to scarf food down for some supposed window for recovery as well as being really hungry. I know other local keno-adapted long-distance runners who have run 50 milers total totally LC leading up to and including the race, etc.

    chris wrote on July 8th, 2012
    • “I read that measuring one’s blood is more accurate than via a urine sample”

      A point often overlooked is that when one becomes adapted to ketones there will be few, if any, ketones in the urine.

      The reason is simple; you’re using them for fuel!

      kfg wrote on July 16th, 2012
  7. This weekend I went on a bike ride with my elite-road-racer boyfriend and some of his friends – 30 miles in about 2.5 hours – fasted, refusing their offers of Gu every 45 minutes, and finished strong! Later the boyf and I discussed fat adaptation, and he really can’t believe it. He’s definitely one of those people who gets cranky if he doesn’t eat every few hours. Fat adaptation and intermittent fasting come easily to me, but what about people who are so strongly sugar-adapted?

    Alexa wrote on July 9th, 2012
  8. Mark,

    I really like much of what you promote and enjoyed this article as well. Happily I am a fat burner. My biggest problem with Primal living is your belief in evolution. Because of your belief you give not credence to the Creator’s injunction not to eat the unclean foods such as pork and shellfish. To promote them is a disservice to your readers and followers. May I suggest you look at the scientific basis of how Biblically unclean foods digest and how they impact human health? ~Jon

    Jon Frank wrote on July 9th, 2012
  9. Some of you mentioned headaches and brain fog symptoms.

    Sounds like low carb dieting to me.

    Since I’ve increased my natural carb intake (of taro, sweet potatoes, normal potatoes, rice and quinoa) my headaches, muscle aches in the chest, brain fog, mood swings, heart racing, high cholesterol, anxiety, low libido and arrhythmia have stopped!

    Having too few carbs can make you excrete minerals, vital to keep your mental health and heart healthy. So if you’re getting same symptoms I did after going on PB, try increasing natural carb intake, increase mineral intake (potassium, magnesium, selenium, copper) and eat more liver to mitigate any loss of vitamins.

    So safe carbs aren’t evil (only wheat & sugar are evil).

    Sylvia wrote on July 10th, 2012
  10. 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
  11. 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

Leave a Reply

If you'd like to add an avatar to all of your comments click here!

© 2016 Mark's Daily Apple

Subscribe to the Newsletter and Get a Free Copy
of Mark Sisson's Fitness eBook and more!