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Many readers may be unfamiliar with the ketogenic diet. This introduction discusses some general ideas about ketogenic diets, as well as defining terms that may be helpful. In the most general terms, a ketogenic diet is any diet that makes the liver produce ketone bodies, shifting the body’s metabolism away from glucose and towards fat utilization. More specifically, a ketogenic diet is one that limits carbohydrates below a certain level (generally 100 grams per day), inducing a series of adaptations to take place. Protein and fat intake are changeable, depending on the goal of the dieter. However, the decisive criteria of whether a diet is ketogenic or not is the presence (or absence) of carbohydrates.

Fuel metabolism and the ketogenic diet

Normally, the body uses a combination of carbohydrates, protein and fat for energy. When carbohydrates are removed from the diet, the body’s small stores are quickly depleted. As a result, the body is compelled to find an alternative fuel to provide energy. One of these fuels is free fatty acids (FFA), which can be utilized by most tissues in the body. However, not all organs can use FFA. For instance, the brain and nervous system are unable to utilize FFA for fuel ; however, they can use ketone bodies.

Ketone bodies are a by-product of the partial breakdown of FFA in the liver. They are used as as a non-carbohydrate, fat-derived energy source for tissues such as the brain. When ketone bodies are formed at fast rates, they mount up in the bloodstream, leading to a metabolic state called ketosis. At the same time, there is a decline in glucose utilization and production. Along with this, there is a decline in the breakdown of protein to be used for energy, referred to as ‘protein sparing’. Many individuals are attracted towards ketogenic diets in an attempt to lose bodyfat while reducing the loss of lean body mass.

Hormones and the ketogenic diet

Ketogenic diets cause the effects described above primarily by changing levels of two hormones: insulin and glucagon. Insulin is a storage hormone, in charge of moving nutrients out of the bloodstream and into target tissues. For example, insulin causes glucose to be stored in muscle as glycogen, and FFA to be stored in adipose tissue as triglycerides. Glucagon is a fuel-mobilizing hormone, causing the body to break down stored glycogen, especially in the liver, to provide glucose for the body.

When carbohydrates are removed from the diet, insulin levels fall and glucagon levels rise. This causes a rise in FFA release from fat cells, and higher FFA burning in the liver. The quick burning of FFA in the liver is what eventually leads to the production of ketone bodies and the metabolic state of ketosis. Other than insulin and glucagon, a number of other hormones are also affected, all of which help to shift fuel use away from carbohydrates and towards fat.

Exercise and the ketogenic diet

As with any fat-loss diet, exercise goes well with the ketogenic diet. However, a diet without carbohydrates is unable to support high-intensity exercise performance, although low-intensity exercise may be performed. For this reason, individuals who wish to use a ketogenic diet and perform high-intensity exercise must add carbohydrates to the diet without disrupting the effects of ketosis. Two modified ketogenic diets are:

The targeted ketogenic diet (TKD) permits carbohydrates to be consumed immediately around exercise, to support performance without affecting ketosis.
The cyclical ketogenic diet (CKD) alternates periods of ketogenic dieting with periods of high-carbohydrate consumption. The period of high-carbohydrate eating refills muscle glycogen to support exercise performance.

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