Fat Adaptation

The term fat-adaptation gets thrown around a lot in the low-carb/keto space but I find that many people are confused on what it means, how it is achieved, and how they know if they are fat adapted. So in this blog post I am going to try and answer some of the questions you may have and also provide a clearer definition around what it means to be fat adapted.

What does it mean to be fat adapted?

I first heard the term “fat-adapted” when I read the book Primal Endurance. It is honestly hard to find a concrete definition for what being fat adapted means. Even the Primal Endurance book does not lay out a concrete definition. Mark Sisson’s, article on fat adaptation also does not lay out a concrete definition. Another article from Virta Health’s says

….it implies you have restricted carbs enough to induce an increase in fat burning


Dr. Andy Gustin says from Perfect Keto says

When you’re in this metabolic state, you’re able to burn your stored body fat reserves for energy.


I don’t really like any of the definitions above because everyone is burning fat (body fat or dietary fat) to some degree all the time. So at what level of fat burning am I considered fat-adapted? It is not black and white, it is not like if you are not fat-adapted that you are burning no fat at all and only carbohydrates. The opposite is also true, if you are fat-adapted you are not burning exclusively fat.

Dr. Andy Galpin does a nice job explaining how the body burns a mixture of fat and glucose (whether fat-adapted or not) at all times in the video below if you want more information

So what do I think being fat-adapted means? I consider someone fat-adapted when they begin to use fat for energy in the correct scenario. Let me explain.

Many people today are getting a large portion of their energy from glucose when they shouldn’t be. When you are sitting in front of the TV watching Game of Thrones you should not be burning much glucose. Yet for many people today they are doing just that. Why? Because they are eating more carbohydrates than they need. What does carbohydrates get turned into? Glucose. Too much glucose in your blood can be deadly, so your body will try to remove it from your blood ASAP by burning it off and whatever it can’t burn off it will store as fat. Your body is preferentially burning glucose when it shouldn’t because there is simply too much of it around for it to do anything else. It is doing it to save your life.

Unfortunately, years and years of eating this way causes a whole bunch of health issues, but it also causes the body to “forget” how to burn fat for fuel, which is what should be your primary fuel when doing low intensity activity (like watching TV).

If you are fat-adapted, that means you are no longer depending on glucose all the time to fuel your next Netflix binge, instead you are burning fat, either from your body or the food you are eating. This is exactly what you should be doing and the way your body has been designed to work from millions of years of evolution.

So Carbs Are Bad Then, Right?

No. Bet you didn’t expect me to say that 😉 Carbs are not bad, they have their place in a healthy diet. What is bad is an over-abundance of carbs all the time, especially from highly processed “frankin foods”. I like the saying of “you should eat the carbs you earn”. If you are sedentary, not moving all day, staring at a computer, you do not require a ton of carbs. If you are a highly trained athlete working out multiple times a day training for a competition, you should not fear carbs at all.

The great part about being fat-adapted is that you will require less carbs. The ratio of fats/carbs we are burning is pliable. Highly trained fat-adapted athletes burn a large percentage of fat at some pretty intense efforts. The FASTER study was a great example of this.

Peak fat oxidation was 2.3-fold higher in the LC group (1.54 ± 0.18 vs 0.67 ± 0.14 g/min; P = 0.000) and it occurred at a higher percentage of VO2max (70.3 ± 6.3 vs 54.9 ± 7.8%; P = 0.000). Mean fat oxidation during submaximal exercise was 59% higher in the LC group (1.21 ± 0.02 vs 0.76 ± 0.11 g/min; P = 0.000) corresponding to a greater relative contribution of fat (88 ± 2 vs 56 ± 8%; P = 0.000). Despite these marked differences in fuel use between LC and HC athletes, there were no significant differences in resting muscle glycogen and the level of depletion after 180 min of running (− 64% from pre-exercise) and 120 min of recovery (− 36% from pre-exercise).


These low carb athletes in the study burned far more fat even at higher intensities than their high carb counterparts. This means they did not require as many carbs to do the same amount of work. Pretty cool. This is important for certain athletic sports like endurance sports where the less fuel you need the better. I will note that this study did not prove anything except that you can train the body to be more fat-adapted. It does not prove that the low carb athletes performed better than the high carb athletes.

Basically what this means is that once you are fat-adapted you not only are not dependent on carbs but also that when you do need carbs you will not need as much since a greater percentage of your fuel will come from fat. It does not mean you should not eat any carbs at all when they are warranted. You can if you want, but even the best fat adapted athletes in the world typically also consume carbs when the intensity warrants it.

How Do I Become Fat Adapted?

The best way to do this is via some form of carb restriction. The level of restriction is going to be dependent on the individual. Much of it is going to have to do with your activity level. A sedentary individual might have to restrict their carb intake to 50g a day in order to improve their fat burning capabilities. A high level athlete might be able to eat 200g of carbs a day and make the same improvements. (I should note here that if you are an athlete and concerned about performance I suggest you wait to make this transition to the offseason because the drop in carbs is almost always going to result in a temporary drop in performance. This might not be acceptable to occur in season.)

What happens when a carb dependent individual takes away the bodies primary fuel source? Those fat burning pathways that have largely lied dormant for a long period of time will suddenly be forced back to life. It may not be a pleasant few weeks while this happens as you body is still ramping up the fat burning pathways and there are no carbs coming in, but once you get past that point it should be smooth(er) sailing. Without the carbs coming in, your body will switch over to utilizing fat instead. Once that improves enough you will become fat-adapted!

How Do I Know I Am Fat-Adapted?

The best way I believe anyone can know if their fat-adapted is their ability to go between meals without food. If you crash consistently day after day of trying not to snack constantly you are probably not fat-adapted. Can you eat 3 square meals a day and not snack between them and perform fine? That is a great sign and you are well on your way to being fat-adapted. The ultimate test? Fasting for a longer period of time in the morning. After being asleep for 6-9 hours overnight, and you can extend that fast for a few hours in the morning and perform fine, it is safe to say you are fat-adapted.

If you are exercising (everyone should be), can you exercise without needing carbs before hand and not “bonk” during your exercise session? This is an excellent sign of fat adaptation. Again another high level test would be exercising fasted in the morning without eating before hand.

If you really want to know how fat-adapted you are you can go get a VO2 Max test and it will tell you exactly how much fat vs carb you are burning as your exercise intensity goes up. This is completely unnecessary, but would give you a definitive answer.

Fat-Adaptation Is The Gateway To Metabolic Flexibility

What we want from our metabolism is what is called metabolic flexibility. All that means is that you are able to efficiently use fat or carbs depending on the demands put on your body. In the beginning the focus is reducing carb dependency in an effort to achieve a fat-adapted state. After that is achieved to a suitable level for you, there is no reason you can’t eat the carbohydrate level that is appropriate for you. It is not necessary to never touch a carb for the rest of your life. You are not going to undo all the hard work of becoming fat-adapted by eating some carbs. Again as long as your carb intake is at an appropriate level for you, nothing is going to go wrong. In fact some people theorize that eating in low carb state for too long will make you inefficient at processing carbohydrates, basically the opposite end of the metabolic flexibility spectrum. Is that necessarily a bad thing? I don’t think it’s as bad as being carb dependent. However if you do plan on eat carbs, you want your body to efficiently process them.

Are you carb dependent? Interested in maybe trying to become fat-adapted and want some help? Let me know, always happy to help!

5 thoughts on “Fat Adaptation

  1. Hi Ryan

    I keep my carbs to below 30g…..eat a diet of meat,veg,good fats but don’t lose weight. I lost 20lb but need to lose more. Any tips please?

    1. Hi Julia, fat loss is effected by more than just diet. Sounds like you are eating a clean diet. You can most likely eat up to 50g of carbs and still be in ketosis if that is your goal. Sleep, stress, activity, and exercise all play an important role in fat loss as well. Also other medical conditions can effect fat loss too. It is not a black and white topic. If your diet is dialed in and you are confident you are in a slight caloric deficit take a look at your sleep, and stress levels and make sure they are at healthy levels as well.

  2. Jo Rockafellor June 22, 2019 — 8:05 pm

    Thank you Ryan! NOW I finally understand.

    1. You are welcome! Glad you found it useful!

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