What You Should Know About Keto-Adaptation

Here's what to expect as your body transitions to using fat for energy

A ketogenic (or "keto") diet is an eating plan that's designed to seriously minimize carbohydrates, your body's favorite fuel source, and dramatically increase fats. The idea is that as carbohydrate levels drop, the body becomes forced to burn stored fat as its primary source of fuel, which can result in often dramatic weight loss. The diet represents a total turnaround from how most people eat: While the typical American diet is about 50 percent carbohydrate, 15 percent protein, and 35 percent fat, the breakdown on most typical keto diets is 5 to 10 percent carbs, 70 to 75 percent fat, and the rest from protein.

Keto-adaptation (also sometimes called fat-adaptation) is the process your body goes through on the diet as it changes from using primarily glucose for energy to using primarily fat.

The "keto" part refers to ketones, which are water-soluble molecules that the liver makes when metabolizing fats, particularly when carbohydrate intake is low. Ketones can be used for energy by most tissues in your body, including the brain, which can't use unrefined fats as fuel.

Your body is always using a mix of fat and glucose for energy, but in a non-keto-adapted state, it reaches for glucose first, since only low amounts of ketones are normally generated during fat metabolism and some tissues of the body—for example, the heart—prefer using ketones when they're available. The brain can't use fat, so it depends on glucose when you're in a non-keto-adapted state.

If glucose is the body's normal go-to source of energy, you may be wondering what happens when it suddenly doesn't have enough to use as its main fuel.

Getting to a Keto-Adaptive State

Once stores of glycogen (the way the body warehouses glucose) become depleted, your brain and other organs begin the process of adapting to using fats and ketones instead of glucose as its main fuel. But reaching ketosis, the state in which fat provides most of the fuel for your body, isn't usually a pleasant experience.

The extreme carb restriction is often accompanied by adverse side effects. Commonly known as the "keto flu," the transition may cause a period of fatigue, weakness, lightheadedness, "brain fog," headaches, irritability, muscle cramps, and nausea.

While the length of time it takes to adapt to a keto diet varies, the process begins after the first few days. Then, after about a week to 10 days, many low-carbers suddenly start to feel the positive effects of keto-adaptation. They report improved mental concentration and focus and more physical energy as well.

By the end of the second week (sometimes up to three weeks), the body has usually accomplished the majority of its work in adapting to using fat for energy. By this point, hunger and food cravings are diminished and stamina and vitality increased.

After this, the body continues to make more subtle changes. For example, it gradually becomes more conserving of protein, so people often crave less protein. Another change that athletes often notice is less lactic acid buildup in their muscles with long training sessions, which translates into less fatigue and soreness. It can take up to 12 weeks for these changes to occur and for you to fully reach ketosis.

Helping Your Body Adapt

There are a number of ways you can get over the hurdle of the first week of carbohydrate withdrawal:

  • Eat lots of fat and fiber. The fuller you feel, the less likely you are to miss your favorite carb-laden foods. Foods made with flaxseeds are high in both fiber and healthy omega-3 fats.
  • Increase salt and water intake. Many of the negative side effects are caused by a loss of fluid and electrolytes like sodium (carbs hold on to water, so you'll probably urinate a lot more once you cut them out). To replenish both, drink a cup of water with a half teaspoon of salt stirred into it or a cup of bouillon several times a day for a few days.
  • Go easy with physical activity. As you adapt to a new fuel source, strenuous workouts can further stress your body, so stick to gentle forms of exercise like walking and stretching for a few weeks.

Other Changes to Expect

Research so far shows that ketogenic diets (and low-carb diets in general) can reverse the signs of metabolic syndrome, type 2 diabetes, and polycystic ovarian syndrome (PCOS). Keto diets are also successfully used to treat some seizure disorders, and studies indicate they may help other neurological disorders, such as Parkinson's disease, though more research is needed.

The more scientists look at the keto diet, the more positive benefits they seem to find. For example, people on these diets have less of the saturated fat in their blood that's linked to insulin resistance, metabolic syndrome, and heart disease. Emerging research also shows that using ketones for energy may decrease oxidative stress and inflammation on the body, and may even be involved in turning on some genes that may be beneficial to health.

Managing Keto-Adaptation

Some people find that their ketosis is pretty stable as long as they eat a low-carb diet under about 50 grams of carbs a day, while others find they need to eat fewer carbs to stay in ketosis. Athletes and heavy exercisers often can eat more than 50 grams of carbs and still stay in ketosis. Other influences, such as hormonal fluctuations and stress, have been known to throw people out of ketosis.

Some people find value in measuring their blood ketones, which can be done at home using a special meter and test strips. But most low-carb diet authors don't recommend bothering with it. If you're getting the benefits you hoped for on a keto diet, worrying about how high your ketones are may just add a level of complication you don't need.

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