What Is Fat Adaptation & How To Become Fat Adapted? (How to, Benefits, Symptoms…)

by | Last updated: Aug 16, 2020 at 4:00PM | - Published on: Nov 15, 2019

Being fat adapted means you can burn lots of fat for energy. It usually results in metabolic flexibility, meaning you can dip in and out of burning fat or sugar. This ability to burn fat and be metabolically flexible is a big part of what makes you healthy.

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What is fat adaptation?

We have metabolisms that are set up to burn fat and glucose to varying degrees. Most of us nowadays are on a Western diet, which locks us into relying on glucose for most of our energy. We’re metabolically inflexible, in a sense. But metabolic flexibility can be gained through fat adaptation. It breaks our reliance on glucose and it’s not hard to do: replace starchy and sugary carbs, like bread, orange juice, and rice, with a lower carb, higher fat, higher protein foods like almonds, shrimp and steak. These sorts of lower carb, higher fat, higher protein foods tend to be both fillings (i.e. induce satiation) and keep hunger at bay for longer thereafter (e.g. prolong satiety) [1]. This improved appetite makes it easier to avoid snacking and so shift into a healthy pattern of intermittent fasting.

Extending our daily fasting windows through intermittent fasting removes the body’s access to glucose, leaving body fat as the only substantial alternative. Calorie restriction, which may or may not happen during intermittent fasting, burns off the carbs we store in our liver and muscle (i.e glycogen). This also causes us to be more reliant on stored body fat.

How long does it take to get fat adapted?

Getting fat-adapted takes time, whether it’s by removing dietary carbs to attain nutritional ketosis and/or by restricting calories. The fastest way to become fat-adapted is by fasting. However, fasting may not be the most convenient or even appropriate way, depending on your goals and current context.

One study found that only 5 days were necessary to fully upregulate fat-burning genes [2]. However, fat adaptation is much more than just fat-burning genes.

What about studies looking at more than the levels of fat-burning genes? In a study lasting 35 days, athletes went from a standard diet to a ketogenic one without changing their caloric intake. A lot of metabolic adaptations took place in the first 7 to 10 days [3]:

Protein balance. This crude measure of net lean mass loss or gain – largely assumed to be muscle mass – was initially negative but then rebounded to a positive baseline. It remained positive for the next 3 weeks of the study. At the 35 day mark, the athletes were estimated to have gained an average of 400 g of net lean body mass.

Natriuresis. Most participants noted “mild lethargy”, also known as the keto-flu. They were given sodium to supplement with. Some supplemented up to 7 g of sodium daily, equivalent to about 14 g of salt!

Blood marker improvements. On average, within the first week:

  • blood glucose levels dropped 19% from 4.57 mmol/L (82 mg/dL) to 3.72 mmol/L (69 mg/dL)
  • blood ketones (BhB) jumped up nearly 40-fold from 0.06 mmol/L to 2.34 mmol/L
  • blood triglycerides dropped 27% from 107 mg/dL (1.2 mmol/L) to 84 mg/dL (1 mmol/L)

You can become more fat-adapted without going keto, even on a low-carb diet where blood ketones don’t surpass 0.2 to 0.3 mmol/L. Getting into ketosis, however, whether by eating keto foods until you’re full or by fasting, is a deeper level of fat adaptation.

For anyone significantly dropping their dietary carbs to somewhere between low-carb and keto, there’s a significant degree of fat adaptation taking place. The full adaptation, like returning to or even surpassing one’s best athletic performance, may take much longer in the order of months to a year. This is observed most evidently in athletes who push the limits of their metabolism, thereby magnifying any small new gains or lingering deficiencies.

What are the signs that you are fat-adapted?

There are signs telling you whether or not you’re fat-adapted, some of which are objective and others subjective.

Signs you are fat adapted

Physical parameters and blood markers. Since fat-adaptation follows on form factors reducing the overall secretion of the hormone insulin, there tends to be a generalized knock-on effect. Having less circulating insulin for the average person is generally positive [4,5,6,7,8]. These are some changes that are easily measured at home or with a blood test:

  • Blood sugar levels tend to drop down and stabilize
  • Triglycerides drop
  • T3 thyroid hormone (triiodothyronine) readjusts to a lower level following newfound tissue sensitivity
  • Weight loss occurs to some degree (especially excess body fat)
    • Water weight is part of that weight loss as full glycogen stores are used up
  • More fat is used for energy at rest and during exercise
  • Cholesterol ratios improve, despite total cholesterol levels increasing

Subjective changes. These can include [9,10,11,12]:

  • Appetite stabilization, fewer cravings and less snacking (i.e. longer fasting windows)
  • Keto-breath! When deeply fat-adapted, you get this fruity breath from the ketone body acetone that accumulates and is then released in your breath
  • Energy levels, especially athletic performance, return to baseline once the keto-flu is over
  • Cognitive function improvements (i.e. more ‘mental clarity’ is commonly mentioned)

The most objective and direct measure of being fat adapted is to measure your respiratory coefficient (RQ) at rest. It’s the ratio of how much carbon dioxide (CO2) you breathe out to how much oxygen (O2) you breathe in. If your RQ is 1, you’re a full-on carb burner. If you’re RQ is 0.7, you’re fully ‘fat adapted’. RQ testing isn’t commercially available so institutes with exercise physiology equipment are your best bet.

What are the benefits of being fat-adapted?

Our biology is geared towards using fat for fuel. We can meet the majority of our energy needs with it, and we have the unusual ability to store large amounts of energy as body fat. Compared to other primates, we’re the fattest [13]!

Fat is an efficient fuel source. Without getting too technical, here’s a simple comparison of how much usable energy (ATP) you can get from fat versus sugar. For every molecule of glucose that your body burns you get between 30 to 32 molecules of ATP. For every fatty acid, over 100 molecules of ATP are obtained – between 106 to 129 (estimates vary). Organs with extreme energy demands have preferred fuels. Our heart is one such example, it’s an energy field that preferentially burns fat whenever given the opportunity. Interestingly though, ketone bodies are considered even ‘more efficient’ than fatty acids when it comes to supporting the heart (as measured by its hydraulic work output) [14].

Fat supports our uniquely energy-hungry brains. The human brain is about 2% of our total weight, yet it consumes 20% of our total energy needs! It needs an uninterrupted, large supply of energy to carry out its basic functions. Ketones can supply nearly 2⁄3 of that need [15]. The duo that is our fat tissue and liver succeed in delivering all the needed energy: fat tissue releases fat, it makes it to the liver, it’s turned into ketones and glucose (with a little help from protein), both of which are happily taken up by the brain. In a fat-adapted state, the fuel mix appears to improve something very important about the brain: its general redox state. By analogy, think of it as improving the hiring and firing of an electric circuit. In scientific jargon, that’s called an improved NAD+/NADH ratio [16]. The point of this is that it’s tremendously promising with regards to treating neurodegenerative and mental disorders.

It’s easier to lose fat and stay lean. Fat-adaptation largely hinges on keeping insulin low, something low-carb diets do pretty well. Many studies are converging towards the idea that hyperinsulinemia is a causal factor in obesity [18]. One controlled study with obese subjects found that every 10% decrease in calories from carbohydrates resulted in a so-called ‘metabolic advantage’. This is where you waste a few more calories every day – compared to the control, non-low-carb group. How much exactly? About 52 kcals/day, which is less than 3 bites of an apple [17].

Fat-adaptation can be anti-inflammatory. As mentioned before, whenever insulin is lowered good effects tend to follow – especially in our world where most of us suffer from too much insulin (i.e. hyperinsulinemia). One such improvement is the lowered inflammatory state that can follow from the deeper kind of fat-adaptation called keto-adaptation. The ketone body D-beta-hydroxybutyrate (BhB) has powerful gene regulatory effects. For instance, it can dampen NLRP3 inflammasome activity, a big cellular machine that dispenses inflammation throughout our body [19]. Furthermore, the deep state of fat-adaptation reached by those on a ketogenic diet has lowered inflammatory markers like IL-6 [20].

Our cardiovascular risk factors improve. When we shift our metabolism from burning predominantly sugar to one where we’re deeply reliant on body fat, this induces a series of beneficial changes in cardiovascular risk factors [21]. Triglycerides, blood glucose, blood pressure, body weight, small LDL-P, LDL particle size, hsCRP, white blood cell counts and HDL-C all improve to varying degrees. Intriguingly, a few clinical case-reports show coronary calcium scores (CAC scores) regressing in people after going on low-carb ketogenic diets [22].

Is being fat-adapted and being in ketosis the same thing?

There are degrees of fat adaptation. Ketosis, in a sense, is a full-on fat adaptation. Once we’re fat-adapted we’ve gained better access to stored body fat. This happens because insulin was low enough to allow the outflow of fat from fat cells (adipocytes).

When more and more fat is liberated in this way, our cells end up seeing more and more of a molecule called acetyl-CoA. Once there’s enough acetyl-CoA compared to this other molecule (oxaloacetate), your body starts handling acetyl-CoA differently: before it would use it directly to make ATP (i.e. to meet energy needs), but it now turns some of it into ketone bodies. The latter can then go on to serve other functions beyond generating ATP.

This is all to say that, yes, ketosis and fat-adaptation have a lot of overlap. However, you don’t have to be on a keto diet to derive many of the benefits of being fat-adapted. In fact, it would make more sense to refer to fat-adaptation for someone on a low-carb diet, and keto-adaptation for someone on a ketogenic diet (or who’s doing a prolonged fast). Keep in mind, just because fat-adaptation is good, it doesn’t mean that the more deeply ketogenic you are the healthier you are.

As mentioned before, but it’s worth repeating, you can become fat-adapted without changing the macronutrient composition of your diet or your caloric intake. For example, you could be on a standard American diet eating 3 meals a day plus snacks but compress all that food into a smaller eating window. This would put you into a pattern of intermittent fasting, where you’ve effectively extended your fasting window. This forces your body to access stored body fat (i.e. start fat-adaptation). Lastly, you could gain a little metabolic flexibility by focusing on food quality. In large part, this means avoiding sources of fast-absorbing sugars, such as bread and fruit juice. If you replace them with carbs, go for slower absorbing sources such as berries, carrots, and parsnips.

List of foods to kick-start fat adaptation

If you’re going to kick-start fat-adaptation by making changes to your diet, you should prioritize foods that are between Moderate and Good on our insulin index and that have a nutrient density score between OK and Good.

Recall that timing also matters, meaning that for a given set of foods, eating them within a compressed feeding window can rev up fat adaptation (as your fasting window is lengthened) [23]. You can even play around with food order to limit blood sugar and insulin spikes by eating carb-rich foods at the end of meals rather than starting out with them [24].

Animal foods

  • Ribeye (OK nutrient density at 6/10 and a Good insulin index of 19%)
  • Bacon (OK nutrient density at 6/10 and a Good insulin index of 14%)
  • Chicken roast (OK nutrient density at 6/10 and a Good insulin index of 20%)
  • Mackerel (Good nutrient density at 9/10 and a Good insulin index of 23%)
  • Shrimp (Good nutrient density at 10/10 and a Moderate insulin index of 42%)
  • Chicken eggs (Good nutrient density at 8/10 and a Good insulin index of 17%)

Plant foods

  • Creamed spinach (OK nutrient density at 4/10 and a Good insulin index of 20%)
  • Raab broccoli (Good nutrient density at 7/10 and a Good insulin index of 21%)
  • Roasted almonds (OK nutrient density at 4/10 and a Good insulin index of 10%)
  • Kale (Good nutrient density at 7/10 and a Good insulin index of 14%)
  • Pistachios (OK nutrient density at 5/10 and a Good insulin index of 14%)

Feel free to include more or less plant foods while making animal foods a priority. The reason why is because animal foods are more nutritionally complete foods compared to even the best plant foods.


Fat adaptation is an important concept to know about. It represents a system-wide change in how our body handles moment to moment energy needs and is generally considered healthy. Most people nowadays are heavily reliant on carbs and aren’t fat adapted. In fact, according to a recent survey, less than 12% of us are metabolically healthy nowadays [25]. This implies that virtually everyone stands to benefit from becoming fat adapted.

As explained above, fat adaptation is not the same as the metabolic state is known as ketosis. However, they overlap greatly. Keto adaptation is a deeper state of fat adaptation requiring a full-on ketogenic diet and/or fasting. People on low-carb diets are fat-adapted but not to the same extent as people on ketogenic diets. And although fat-adaptation is good, that doesn’t mean the more ketogenic you are the healthier you are.

Written by Raphael Sirtoli, MSc Biology

Raphael is self-learner and self-experimenter. A decade before starting on his scientific path he started adopting various low-carb diets and many new lifestyle practices, such as cold exposure, meditation, barefoot running and Wim Hof breathwork.

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