The ketogenic diet is a low-carbohydrate, high-fat eating pattern that changes how the body produces energy. By sharply reducing carbohydrate intake, the diet encourages the body to use more fat and produce ketones, which can serve as an alternative fuel.
The approach has been studied for weight management and some medical conditions, but it is not suitable for everyone and its longer-term effects remain an area of research. Mayo Clinic advises people to consult a health professional before starting a weight-loss diet, particularly those with conditions such as diabetes, heart disease or kidney disease.
How ketosis works
Normally, the body relies heavily on glucose derived from carbohydrates for energy. When carbohydrate availability falls substantially, the liver increases the production of ketones from fat.
This metabolic state is known as nutritional ketosis. It should not be confused with diabetic ketoacidosis, a potentially life-threatening medical emergency in which ketones build up alongside serious metabolic disturbances, most commonly in people with diabetes.
The amount of carbohydrate needed to reach ketosis varies from person to person. There is no single carbohydrate intake that guarantees ketosis for everyone, so claims that a person must consume a particular amount of carbohydrate or fat each day should be treated cautiously.
What does the evidence show?
Ketogenic and other low-carbohydrate diets can produce weight loss, particularly in the short term. Mayo Clinic notes that very low-carbohydrate diets may result in greater short-term weight loss than low-fat diets, although the difference tends to become smaller over longer periods.
Research has also examined ketogenic diets in relation to blood sugar and other metabolic measures. A 2026 meta-analysis of 24 studies involving more than 47,000 participants found that ketogenic diets were associated with improvements in body weight, blood glucose measures and blood pressure. The same analysis also found increases in LDL, or “bad”, cholesterol and total cholesterol, highlighting the importance of the type of fat consumed and individual health risks.
The evidence does not support treating keto as a universally superior diet. Results vary according to the person’s health, the foods included in the diet and how long the diet is followed.
Possible side effects and risks
A sudden and substantial reduction in carbohydrates can cause short-term problems including headaches, constipation, muscle cramps, tiredness and weakness. Very restrictive low-carbohydrate diets can also make it harder to obtain adequate amounts of some nutrients and fibre.
The amount and type of fat in the diet also matter. Diets that rely heavily on saturated fats can increase cholesterol levels and may raise cardiovascular risk, while research into the longer-term cardiovascular effects of ketogenic diets remains mixed. A recent review found both metabolic benefits and increases in LDL cholesterol, supporting an individualised approach rather than a blanket recommendation.
People with diabetes need particular medical guidance because changes in carbohydrate intake can affect blood sugar and medication requirements. Diabetic ketoacidosis is different from nutritional ketosis and requires urgent medical treatment when it occurs.
What about athletic performance?
The claim that ketogenic diets automatically improve athletic performance is not supported by the available evidence.
A 2023 systematic review and meta-analysis of 18 studies found that ketogenic diets could help reduce total and fat mass, but were not suitable for enhancing strength or high-intensity cyclic performance.
More recent research has produced a mixed picture. A 2026 systematic review and meta-analysis found that low-carbohydrate and ketogenic diets can increase the body’s use of fat during exercise, but their effects on aerobic performance vary depending on the type and duration of exercise.
Another 2026 review found that repeated high-intensity exercise could be more vulnerable to impairment when carbohydrate intake is restricted, although some forms of short-duration performance were maintained. The researchers said the findings should be interpreted cautiously because of differences among the studies.
Who should seek medical advice?
A ketogenic diet can substantially change carbohydrate intake, energy sources and, for some people, blood glucose and cholesterol levels. Anyone considering a major change in eating habits should discuss it with a qualified health professional, particularly if they have diabetes, kidney disease, heart disease or take medication affected by changes in diet.
The evidence suggests that ketogenic diets can have some benefits for weight and metabolic measures, but they also carry potential drawbacks. Rather than treating ketosis itself as a guarantee of better health, researchers continue to examine which people may benefit, which foods should make up the diet and what the long-term effects may be.
