The ketogenic diet is based on research from the 1920s that found lowering the availability of carbohydrates made the body rely more on using other substances (such as fat) for energy. By metabolising fat to produce glucose or energy, the body generates ketones in the process – hence the term “ketogenic”. Any diet containing less than 20g per day of carbohydrate is deemed to be ketogenic.

The production of ketones by the liver indicates that fat, instead of sugar, is being metabolised and that this fat is close to our entire source of energy. This is thought to correlate to weight loss but really correlates with an altered blood insulin profile. Whether this enhances weight loss compared to other diets is debatable, since withdrawal of carbohydrates results in losses in body water, exaggerating the appearance of weight loss.

But many people report experiencing something called the “keto flu” after changing their diet. People report symptoms such as nausea, constipation, headaches, fatigue and sugar cravings, similar to influenza – apart from the sugar cravings.

These side effects are related to the key concept of the ketogenic diet: carbohydrate withdrawal. Glucose (which is produced from foods containing carbohydrates, such as potatoes or bread) is the primary energy source of the central nervous system, including the brain. A reduced supply of carbohydrates will result in reduced function, leading to headaches. Nausea can be explained through consuming high volumes of fat. This is because fat takes a long time to digest and absorb.

When eating a conventional diet that includes carbohydrates, glucose increases in the blood. This stimulates a rise in the hormone insulin, which regulates blood sugar levels and allows your body to use the glucose for energy. It lowers the presence of fat in the blood, and helps glucose enter the body’s cells. Insulin also suppresses the release of fat particles from fat stores in the body by the same mechanism. The hope is that by eating low (or no) carbs, this mechanism will be reversed, helping to increase the appearance of fat in the blood and its availability to other cells to use for energy and result in fat loss.

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