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Everything You Need to Know About How Ketosis Works

Ketosis has made its way into the mainstream as a weight loss tool. Before we understand the diet, we need to learn about what ketosis is, how it works, and how it alters the metabolism.

Ketosis has made its way into the mainstream as a weight loss tool. But before we understand the diet, we need to learn about what ketosis is, how it works, and how it alters the metabolism.

Ketosis is a metabolic process that occurs when the body is in a state of low carbohydrate intake. Carbohydrates break down into glucose, the body’s preferred source of fuel, down during digestion. When glucose is not provided through food for a period of time (like during a time of fasting), the body has a backup mechanism in place to avoid starvation and malnutrition: ketosis. Without carbohydrates as a source of fuel, the body’s metabolism changes and enters into ketosis, where stored fat is used for energy. The molecules providing energy are ketone bodies (mostly acetoacetate and beta-hydroxybutyrate) instead of glucose. Most of the body’s cells (including the brain, which is the most important) can use ketone bodies as fuel if there is no glucose.

The ketogenic diet (KD) is a high-fat, low-carbohydrate, and moderate protein diet. By not providing much glucose (by not eating a lot of carbohydrates), this essentially mimics a state of starvation, forcing the body into ketosis. But by eating high fat and moderate protein at the same time, it prevents malnutrition associated with starvation. By following the diet, you can provide your body with enough calories to survive, but without glucose so that the body produces ketone bodies, and enters into a state of ketosis. In this state, your body breaks down fat stores for energy, which can facilitate weight loss.

The dietary breakdown is typically about 75 percent fat, 20 percent protein and only 5 percent carbohydrate. It can also be calculated as a ratio of fat to carbohydrates plus protein. The classic KD can be as high as 4:1. There are many modified versions of the diet that vary in the macronutrient breakdown (see table). The exact dietary breakdown or ratio for an individual may vary; the only way to know if you are in ketosis (and know which dietary breakdown works for you) is to test for the presence of ketone bodies. To be in a state of ketosis, the blood concentration of ketone bodies should be between 0.5mM-3.0mM. It’s also possible to test in the urine, and while this is less invasive, it’s also less accurate. You can buy strips over the counter that test for acetoacetate in the urine and conduct the testing at home. Ketosis can take about 3 days to occur, meaning you need to follow the diet for a few days while your body transitions into ketosis before you see any ketone bodies in the blood or urine.

J Acad Nutr Diet. 2017;117:1279-1292.

As you can imagine, it’s a very difficult diet to adhere to, because it requires strict attention to nutrition labels, weighing food, and calculating all your food and beverage intake to make sure you are eating the right amount of each macronutrient. Further, it is difficult to take in very high levels of fat in a healthy way. Very low carbohydrate intake is equally challenging and needs to be followed carefully to stay in ketosis. Because it can take a few days to change the metabolism and enter into ketosis, one slip-up of a high-carbohydrate food may take you out of ketosis and essentially “start the clock” again to get back into ketosis. You cannot have “cheat days” on this type of diet, making it difficult to go to restaurants or buy convenience foods. Followers of this diet may stay in ketosis for a few months to years and slowly transition back to a more typical diet.

Interestingly, for over 100 years doctors have used this specialized diet as a treatment for children and adults who have seizures not controlled by medication. The exact mechanism for this is unknown but still practiced today. For this population, the ketogenic diet is a great therapy.

If you're interested in trying the ketogenic diet, consult with your physician and a Registered Dietitian to make sure you are healthy enough, especially if you have a chronic disease like diabetes or heart disease. While it’s rare for a healthy person on the KD, it’s possible for the number of ketone bodies in the blood to get too high, which can throw off the body’s acid/base balance and create ketoacidosis, a fatal condition in extreme cases. Your doctor may recommend blood tests to ensure the high-fat intake is not negatively affecting blood fats (cholesterol, triglycerides, etc). Often, a multivitamin will be recommended, along with additional supplements if you found to be deficient (e.g., vitamin D or iron).

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