Low-carbohydrate diets restrict carbohydrate consumption relative to the average diet. Foods high in carbohydrates (e.g., sugar, bread, pasta) are limited, and replaced with foods containing a higher percentage of fat and protein (e.g., meat, poultry, fish, shellfish, eggs, cheese, nuts, and seeds), as well as low carbohydrate foods (e.g. spinach, kale, chard, collards, and other fibrous vegetables).

There is a lack of standardization of how much carbohydrate low-carbohydrate diets must have, and this has complicated research. One definition, from the American Academy of Family Physicians, specifies low-carbohydrate diets as having less than 20% carbohydrate content.

There is no good evidence that low-carbohydrate dieting confers any particular health benefits apart from weight loss, where low-carbohydrate diets achieve outcomes similar to other diets, as weight loss is mainly determined by calorie restriction and adherence.

An extreme form of low-carbohydrate diet called the ketogenic diet was first established as a medical diet for treating epilepsy. It became a popular fad diet for weight loss through celebrity endorsement, but there is no evidence of any distinctive benefit for this purpose and the diet carries a risk of adverse effects, with the British Dietetic Association naming it one of the "top five worst celeb diets to avoid" in 2018.