Fat adaptation is your body’s ability to burn fat as its main fuel source. When you’re fat adapted, you turn fat into adenosine triphosphate (ATP), the energetic currency that powers nearly every cell in your body.
Typical high-carb Western diets promote fat storage, but following a keto diet can help you start burning fat over sugar for energy to get you to the point of fat adaptation.
You don’t need extensive testing to know whether you’ve gotten there, either. There are mental and physical clues that let you know if you’re fat adapted.
Stable Energy Levels
When you eat carbs, particularly simple carbs like sugar, you often have swings in energy known as “sugar highs” and “sugar crashes.” (1) The same happens when you’re transitioning into fat adaptation. Your body is still looking for carbs as a fuel source instead of fat.
When you lower your carbohydrate intake and become fat adapted, you have plenty of body fat to burn at all times. You’re no longer relying on your glycogen stores for energy.
This affects energy crashes and your physical and mental endurance. Burning fat means your body has a more consistent, abundant fuel supply, allowing you to sustain activities even during fasting. (2)
If you go from energy swings to feeling stable energy throughout the day, even after exercise or fasting, you’re probably fat adapted.
Reduced Cravings
Of all the macronutrients, fat is the most satiating. There are several reasons for this: (3)
- Fat contains 9 calories per gram, while carbs and protein contain 4 calories per gram.
- Fat lowers circulating ghrelin, your hunger hormone.
- Fat lowers neuropeptide Y, another hunger stimulator.
- Fat boosts CCK, a neuropeptide associated with feeling full.
The net effect of all these changes is that a high-fat, low-carb diet reduces hunger and cravings.
Appetite control goes beyond fat, though. Ketones produced through ketosis also suppress hunger. (4) That’s why you can skip breakfast or lunch on keto with no problem. This is called intermittent fasting, and it’s easy when you’re fat adapted.
Hunger suppression and reduced cravings are two of the major reasons that high-fat diets like keto are so effective for weight loss. When you’re fat adapted, your sugar cravings effectively disappear. You’ll also see fewer blood sugar spikes that contribute to carb cravings.
It’s worth noting that in the beginning stages of fat adaptation, it’s possible you’ll experience carb cravings. Sugar is physically addictive, and you’ll likely have withdrawals and cravings for a few days once you cut it out of your diet. As soon as you’re fat adapted, the cravings will subside.
Better Endurance
We’ve hinted at this already with the effects of fat adaptation on energy levels. For the athletes out there, becoming fat adapted improves endurance by getting the body to use fat as a primary fuel source during prolonged physical activity. (5)
Unlike glycogen (sugar), which is limited and depletes quickly post-consumption, fat stores are abundant. They’re a more steady, efficient energy supply.
When fat is your primary fuel source for exercise, you spare stored glucose from a process called gluconeogenesis for longer. That glycogen can come in handy later when you need extra fuel at the end of long or high-intensity workouts.
The metabolic shift that happens with fat adaptation reduces your reliance on carbs and delays fatigue, which is so common with glycogen depletion. This enhances your metabolic flexibility, allowing you to switch seamlessly between burning fat and carbs when necessary. (6)
That last part is important if you cycle between strict carb counting and looser keto plans. (This includes the Targeted Keto Diet (TKD) or modified keto diets with less restrictive carb-counting around intense exercise, both popular with endurance athletes.)
Increased endurance is a good sign that you’re becoming fat adapted.
Mental Clarity
When you drop your carb intake, you may experience a temporary dip in energy and brain function. These symptoms are a sign of the keto flu and are most likely due to carb withdrawal.
Once the initial phase of fat adaptation is over, keto flu will subside, and the real health benefits of ketosis and consistent ketone levels kick in.
Ketones make excellent brain fuel. They’ve been shown to enhance memory, attention, and task-switching performance in the elderly. (7) They’ve been to improve markers of brain glucose metabolism to slow down mild cognitive impairment. (8)
Studies show this is realted to the effects of ketosis on your energy supply. (9) Your brain needs fuel to work at its best. Following keto and relying on more stable fat stores improves neurotransmitter function and mitochondrial metabolism while reducing inflammation.
That’s all good news for cognition.
Glucose doesn’t fare as well in the lab. In one study, participants given a glucose drink performed significantly worse than placebo-fed controls on a series of cognitive tests. (10) In other words, excess glucose slows your brain down.
If you feel a sudden increase in mental clarity after a couple of weeks on a keto diet, odds are you’re fat adapted.
Improved Sleep
Fat adaptation and ketosis, more broadly, have a stabilizing effect on blood sugar levels. (11) By minimizing carbs, you reduce blood glucose spikes and insulin responses. This is particularly beneficial for individuals with insulin resistance or type 2 diabetes.
We’re talking about sleep, though.
Blood sugar levels, insulin sensitivity, and sleep are directly connected. (12) High blood sugar can cause restlessness and disrupt your sleep. Low blood sugar can trigger stress hormones like cortisol. Stable levels support uninterrupted, restorative sleep.
If you find yourself sleeping better on keto, that means you’re getting that steady source of energy throughout the night, unlike a carb-adapted metabolic state. Fat adaptation ensures consistent energy from fat stores and ketones, reducing how often you wake up each night.
Ketones may also have a calming effect on the brain, supporting deeper sleep and reducing inflammation that can mess with your body’s natural sleep-wake cycle. (13) With fewer disruptions and better energy regulation, fat-adapted individuals often experience more restorative sleep.
Increased Fat Loss
The initial phase of a low-carbohydrate diet can be thrilling. Often, several pounds fly off within the first week. This is usually water weight, though, or the result of rapidly emptying glycogen stores. Glycogen (stored sugar) is mostly water.
Real, sustained weight loss — fat loss, that is — kicks in after about 4 weeks of pretty strict keto dieting.
Once you’re fat adapted, your metabolism favors burning adipose tissue, or body fat, for fuel whenever it needs energy. Insulin levels remain low, promoting the breakdown of fat and reducing fat storage. (14)
You’re even using fat for energy at rest times, supporting consistent calorie and fat burning. That’s provided you aren’t overeating, which is hard to do on a keto diet because it’s so satiating.
If you’re losing weight after about a month of low-carb dieting, you’re probably fat adapted.
Fat Adapted Versus Ketosis
You can be in a fat-adapted state without being keto-adapted or in active ketosis. The reserve is true, too.
The average person may enter a mild state of ketosis during an overnight fast. Fasting causes your blood sugar and insulin levels to drop. These metabolic changes signal your liver to turn fatty acids into ketones. (15)
Specifically, your liver cells burn fatty acids derived from saturated fats, monounsaturated fats, and polyunsaturated fats. This process produces ketone bodies, which you then break down into acetyl-CoA, and then into ATP (cellular energy).
The above process produces ketones, and it can happen during a simple fast. But you can also convert fatty acids directly to acetyl-CoA to fuel your energy needs.
When this happens regularly, you’re fat adapted. Your body has fully shifted over to burning fat as its primary fuel source to the point where you have metabolic flexibility. You can eat carbs and fall out of ketosis, and your body will snap right back into keto once it processes the carbs.
Getting into ketosis, even on the keto diet, does not guarantee fat adaptation or increased fat oxidation. You’ll start making ketones in the first week of keto.
The fat adaptation process, however, may take a month or more. That’s why it’s essential to stick to strict keto for at least a few weeks when you’re just starting out if you want all the biggest benefits of ketosis.
Foods to Help Fat Adaptation
Foods that help you maintain ketosis and get you to that ideal fat-adapted state are generally high in healthy fats and low in carbs. Intermittent fasting can also trigger the process, but it can depend on the state of your metabolism when you start making dietary changes.
Here are some of our favorite foods to boost your dietary fat intake and help you achieve fat adaptation:
- Avocados
- Eggs
- Fatty fish (e.g. salmon, mackerel, sardines)
- Nuts and seeds
- Olive oil or olives
- Coconut oil
- Grass-fed meat or poultry
- Full-fat butter or ghee
- Yogurt
Try to avoid starchy vegetables, refined sugars, and foods that can cause you to exceed your daily carb limits. Supplements like MCT oil can be an easy addition to morning smoothies for an added boost of fatty acids.
If you’re not sure about your targets when it comes to the right amount of fats, carbs, and proteins you need to meet your goals, our Keto Calculator can help.
The Bottom Line
Ketosis can happen overnight, but with a month or so of low-carb eating, your cells start to shift over to burning fat more permanently. This is fat adaptation.
Signs you are fat adapted include a boost in energy levels, reduced cravings, better focus, and reduced body fat. You don’t need blood ketone meters or other tools to see the effects of burning fat full-time.
If you’re among the beginners out there curious about how keto could fit into your life, sign up for our Keto Kickstart. It’s a 30-day, step-by-step guide to keto, tracking macros, and meeting your health goals, including tackling fat loss.