What is ketosis?

Achieving nutritional ketosis

Nutritional ketosis is a metabolic state achieved when your body burns fat as its primary source of fuel rather than carbohydrates. In order to reach ketosis, you must reduce your intake of carbohydrates to around 5 percent of your total calorie intake, or roughly 20 grams of carbohydrates per day.

For most of us, our bodies are accustomed to using glucose, or sugar, for energy. When you reduce your carbohydrate intake, your body has to search for other sources of energy. It first turns to glycogen, which is excess glucose that has been stored in the liver. Each gram of glycogen is bound to 3 to 4 grams of water, so as the glycogen is released, the water goes with it. This is why it's common to lose water weight at the beginning of a low-carb or ketogenic diet.

Once liver stores of glycogen are depleted, your body will begin burning fat. When this happens, the liver produces molecules called ketones, which are then used as fuel. Dr. Richard Veech, one of the world's experts on ketosis, has shown that the heart and brain work 25 percent more efficiently when fueled by ketones rather than blood sugar, which may account for the increased mental clarity that is a hallmark of the ketogenic diet. Some parts of the brain do require glucose, but the body can produce glucose from protein through a process known as gluconeogenesis.

Ketosis should not be confused with ketoacidosis, which is a life-threatening condition marked by a high concentration of ketone bodies that can occur in people with diabetes. Ketoacidosis is very rare in people who do not have diabetes.

Keto flu

During the the first few days of a ketogenic diet, as the body adapts to burning fat instead of glucose, it's common to experience unpleasant side effects, which may include headaches, nausea, fatigue, irritability, and brain fog. Commonly referred to as keto flu or induction flu, this is a result of your body withdrawing from carbs and learning to create the enzymes that it needs in order to burn fat. Don’t worry — keto flu is normal and it doesn’t mean that a low-carb / ketogenic diet is wrong for you! It just means that your body is adjusting. Keto flu typically only lasts a couple of days.

You can reduce symptoms of induction flu by taking several steps:

  • If your diet is very high in refined carbohydrates, wean yourself off of them slowly and begin adding more complex carbs and fats into your diet.
  • Drink lots and lots of water. It’s important to stay properly hydrated. You will also likely find yourself peeing more than usual as your body releases glycogen stores. Drink more water than you think you need.
  • Replenish your electrolytes. With all of that water getting flushed out, your body will also be shedding essential electrolytes (sodium, potassium, and magnesium). To keep your electrolytes at necessary levels, eat lots of leafy greens, drink broth, take magnesium and potassium supplements, and don’t be afraid to use lots of salt on your food.

Being in ketosis vs. being keto adapted

There is a difference between being in ketosis and being “keto adapted,” or “fat adapted.” Ketosis can occur rather quickly. You may even shift into ketosis while you're sleeping without realizing it if you have a healthy metabolism. Most people who adopt a low-carb diet can achieve ketosis in a couple of days. You can check for ketones in your urine using ketone strips. If the color on the end of the ketone strip changes — even just a little bit — it means your body is producing ketones.

Being keto adapted, however, means that your body recognizes fat as its preferred source of fuel and is burning fat efficiently. This transition, often referred to as keto adaptation or keto induction, typically requires maintaining ketosis for several weeks.

Once you’re fat adapted, the body can convert fats into fuel more efficiently, and can switch between using carbohydrates and fat as needed.