The ketogenic diet is a high fat, adequate protein, low carbohydrate diet, primarily used in the treatment of difficult-to-control epilepsy in children. The diet mimics aspects of starvation by forcing the body to use fat rather than carbohydrate as an energy source. The body produces excess ketone bodies, a state known as ketosis. The "classic" ketogenic diet contains a 4:1 ratio of fat to combined protein and carbohydrate. To achieve this, a number of foods are effectively eliminated (for example, starchy fruits and vegetables, bread, pasta, grains and sugar). A variant known as the MCT diet uses a form of coconut oil that is very high in medium-chain triglycerides (MCT; most dietry fat contains long chain triglycerides). This oil has a strong ketogenic effect, which allows some relaxation of the regime.

Developed in the 1920s, its popularity waned with the introduction of effective anticonvulsant drugs. In the mid 1990s the Hollywood producer Jim Abrahams, whose son's severe epilepsy was effectively controlled by the diet, created the Charlie Foundation to promote it. Publicity included an appearance on NBC's Dateline programme and ...First Do No Harm (1997), a TV movie starring Meryl Streep. The foundation funded a multicentre study that was published in 1996, which marked the beginning of renewed scientific interest in the diet.

A number of studies demonstrate the effectiveness of this diet in treating refractory epilepsy in children. For this patient group, the diet is more likely to succeed than attempting another anticonvulsant drug. However, the diet has not been tested in a large, prospective, randomised, double-blinded controlled trial. A long-term blinded control trial is regarded as practically impossible. There is some evidence that adults may benefit too, and that a less strict diet (such as a modified Atkins) might also be effective.