The ketogenic diet is a nutritional strategy that focuses on high fat intakes, moderate protein intakes, and a low carbohydrate intake. The low intake of carbohydrates simulates a metabolic state that resembles fasting, leading to a higher use of fat as a fuel source. This process produces ketones and gives the diet its name. Most frequently, the ketogenic diet approach aims for 50 grams of carbohydrates per day (or less) from vegetable sources. Some people might require less than 50 grams to cause ketosis, and other people might be able to consume slightly higher amounts of carbohydrates and still maintain a ketotic state.^[1][2]

The ketogenic diet is most frequently used as a strategy for weight loss. It can be a useful tool to help some people adhere to a calorie deficit when trying to lose weight. It has been shown to assist with weight loss, decrease body mass index, and decrease waist circumference in people with overweight and obesity in the short term.^[5][6][7][8] However, long term studies (more than 6 months) found that the results were similar for people who followed other diets that produced weight loss using a caloric deficit.^[9][2][10]

In chronic conditions, the ketogenic diet might be a useful adjunct to regular medical treatments. The ketogenic diet can help to improve glycemic control in people with diabetes.^{[2][11][6][7]} This positive effect on HbA1c, fasting blood glucose levels, and insulin tapers off after about 6 months, similarly to other diets that result in weight loss.^[10] For high blood pressure (hypertension), the ketogenic diet has not been shown to be beneficial.^[1][6][12] In people with high cholesterol, a ketogenic diet may assist with improving lipid profiles, particularly in increasing the concentration of HDL cholesterol and decreasing triglyceride levels. Caution should be taken in people with very high cholesterol and other risk factors for cardiovascular disease, because a ketogenic diet can increase total cholesterol, LDL cholesterol, and lipoprotein(a) in some people.^{[1][2][13][11][6][7][10]}

The ketogenic diet has also been shown to decrease seizure frequency in people with epilepsy, specifically in childhood epilepsy that does not respond well to standard medical treatments. However, this should always be done in consultation with a doctor and is not a replacement for medical treatments.^{[14][15][16][17]}

Also of interest is the use of the ketogenic diet to prevent, delay, or even treat neurological disorders such as Alzheimer’s disease, Parkinson’s disease, and stroke. At present, not enough evidence exists to recommend the ketogenic diet as a treatment for these complex disorders.^[18]

There are other reported benefits of the ketogenic diet (for example, its use in the treatment of depression and anxiety^[19] or its use to increase testosterone in hypogonadism^[20]) that are not yet supported by robust evidence.

While a ketogenic diet is considered to be safe for most people, it does have some potential drawbacks.

Some people experience mild adverse effects when starting a ketogenic diet, and this is sometimes referred to as “keto flu”.^[21] The symptoms include nausea, abdominal discomfort, vomiting, fatigue, and headache. These symptoms are likely to resolve within a few weeks.^[9] Due to a very low carbohydrate intake leading to a low fiber intake, constipation is also a possibility.^[10]

Another potential drawback is the effect that a diet high in fat can have on lipid profiles. This effect varies between individuals, but in some people the ketogenic diet can cause a significant increase in total cholesterol and LDL cholesterol. This could have a serious effect on the risk of cardiovascular disease, and care should be taken in recommending this diet to people with high cholesterol and other cardiovascular risk factors.^{[1][2][13][11][6][7][10]}

The high fat and low fiber intake may increase the risk of developing kidney stones, which are extremely painful and can have serious consequences.^[21] The restrictive nature of the ketogenic diet could lead to vitamin and mineral deficiencies in some people.^[21]

Another significant drawback to the ketogenic diet is that it can be very difficult to adhere to such strict dietary limitations. A decrease in adherence over time may contribute to the decrease in effectiveness seen in the long term after initial improvements in the short term.^[10]

In people who are prone to hypoglycemia (low blood sugar) and people who use insulin, the ketogenic diet should be started with caution. Close monitoring of glucose levels is important, because a sudden drop in blood glucose can be dangerous. Furthermore, the ketogenic diet is not safe for people with certain medical conditions, including porphyria, pancreatitis, liver failure, and disorders of fat metabolism.^[21][22]

When the body is deprived of external sources of glucose, the liver tries to release its glucose stores or to generate glucose through a process called gluconeogenesis. (Glucose can be stored in the form of glycogen in the liver and muscles.) These glucose sources, however, are only sufficient for a few days, and the body will eventually move to a new source of energy: fatty acids. Fatty acids are metabolized into ketone bodies (namely, acetone, beta-hydroxybutyrate, and acetylacetone), which are then used as an alternative energy source.^[21] This is how ketosis develops during the ketogenic diet, even in the absence of a caloric deficit. When ketosis is used as a tool to lose body fat or body weight, however, a calorie deficit is necessary.^[10][21]