What Does It Mean to Be Fat-Adapted?
When describing someone that has successfully made the transition to a Primal or keto way of eating I often refer to them as being "fat-adapted" or "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?
Fat-adaption describes the ability to efficiently burn both fat (through beta-oxidation) and glucose (through glycolysis). 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 fuel ourselves. That's actually why we have all this fat on our bodies—turns out it's a pretty reliable source of energy.
Here's what you need to know about the benefits of becoming fat-adapted and why it works with your biology.
The Disadvantages of Primarily Burning Sugar for Energy
To understand why you want to become fat-adapted, it’s useful examine what it means to depend on sugar for energy.
A Sugar Burner Doesn't Readily Utilize Dietary Fat or Stored Body Fat for Energy
When your body is primarily looking for sugar for fuel, your skeletal muscle doesn’t as readily oxidize fat for energy. As a result, more dietary fat is stored than burned, and the fat that is already in storage is more likely to stay trapped there. As we know, a low ratio of fat-to-carbohydrate oxidation is a strong predictor of future weight gain.1
Sugar Stores Deplete Quickly And Need to Be Replenished Often
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). Even a 160 pound person who's visibly lean at 12% body fat still has 19.2 pounds of fat on hand for oxidation, while our ability to store glucose as muscle and liver glycogen (stored glucose) are limited to about 500 grams (depending on the size of the liver and amount of muscle you're sporting).2
If you're unable to effectively beta oxidize fat (as sugar-burners often are), you'd better have some quick snack options on hand.
Sugar Burners Use Stored Glucose 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.
The Benefits of Being Fat-Adapted
There are some compelling advantages to being fat-adapted, which may move you to make the switch if you haven't already.
People Who are Fat-Adapted Often See Improved Insulin Sensitivity
A well-constructed, lower-carb (but not full-blown ketogenic) Primal way of eating that leads to weight loss generally improves insulin sensitivity.3
Being Fat-Adapted Means You Go Longer Between Meals
A fat-burning beast can effectively burn stored fat for energy throughout the day. If you are fat-adapted, chances are, you can handle missing meals and are able to go hours without getting ravenous and cranky (or craving carbs).
You Can Better Utilize the Fat You Eat for Energy
A fat-burning beast is able to effectively oxidize dietary fat for energy. If you're adapted, your post-prandial (after mealtime) fat oxidation will be increased, and less dietary fat will be stored in adipose tissue.
You Always Have a Fuel Source
A fat-burning beast has plenty of accessible energy available in the form of body fat, even if he or she is lean. If you're adapted, the genes associated with lipid metabolism will be upregulated in your skeletal muscles.4 You will essentially reprogram your body.
You Can Burn Fat While Exercising
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.5
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.
Yes, You Can Still Burn Glucose When Fat-Adapted
It's not that the fat-burning beast can't burn glucose, it's that we do not 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.
A fat-burning beast will be able to burn glucose when necessary 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. When you're fat-adapted, you 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.
How Do You Know if You're Fat-Adapted?
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.
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?
Yes? Then you're probably fat-adapted. Welcome to the human metabolism you were wired for!
Are Being in Ketosis and Being Fat-Adapted the Same Thing?
One last thing. Fat-adaption does not necessarily mean you’re in ketosis. Ketosis and ketogenic diets are a huge topic that I’ve covered in detail on the blog,* but briefly: ketone bodies are a “third fuel” derived from fat that the body can use for energy when glucose is scarce, as happens when you eat a very low-carbohydrate diet. When you’re in ketosis, you can usually detect ketones in your bloodstream.
Being keto-adapted specifically means your body has built the metabolic machinery to burn ketones for energy. If you’ve never spent any real time in ketosis, you won’t be keto-adapted.
You can still be fat-adapted—i.e., be a good fat-burner—but until you undertake a concerted period of ketogenic dieting, you probably aren’t keto-adapted.
The opposite isn’t true. Folks who are keto-adapted must be good at burning fat, almost by definition. Keto dieters get a significant proportion of their energy from fat oxidation, even if they are producing relatively high levels of ketones.
Ok, that’s it for today, folks. Send along any questions or comments that you have. I’d love to hear from you guys.
References
https://ajpendo.physiology.org/content/259/5/E650.abstract ↩
https://onlinelibrary.wiley.com/doi/10.1111/j.1748-1716.1970.tb04764.x/abstract ↩
https://www.ncbi.nlm.nih.gov/pubmed/19407076 ↩
https://www.ajcn.org/content/77/2/313.short ↩
https://www.ncbi.nlm.nih.gov/pubmed/18801964 ↩
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* This blog reflects my personal views and opinions and isn’t intended as medical advice, but I hope it will be informative and inspiring as you pursue a healthy, fulfilling life.
