A ketogenic diet is similar to a strict low-carbohydrate diet with two main differences: restriction of protein intake and the purposeful induction of “nutritional (or physiologic) ketosis.” (The low-carbohydrate diet was discussed in a previous article, and I encourage the reader to refer to “What Is a Low Carb Diet?” for more information.)
Nutritional ketosis is a metabolic state that shifts the body’s main energy source from glucose to ketones. Ketones are derived from the metabolism of fatty acids. So, instead of utilizing carbohydrates for energy, the body burns fat. Nutritional ketosis should not be confused with diabetic ketoacidosis. In nutritional ketosis, ketonemia reaches a maximum level of 7-8 mmol/L with no change in blood pH. In diabetic ketoacidosis, ketonemia can exceed 20 mmol/L with a concomitant lowering of blood pH. Nutritional ketosis is a normal adaptation that can occur in the human body to sustain itself during periods of decreased food availability. In today’s society, the majority of the U.S. population has an abundance of food, most of which is processed grains, starches, and sugars. This food abundance precludes the need for nutritional ketosis.
A strict low-carbohydrate diet limits the amount of dietary carbohydrate intake to less than 20 grams of carbohydrates per day. A ketogenic diet has the same carbohydrate restriction with the additional restriction of protein intake to 1-1.5 grams of protein per kilogram of ideal body weight per day. This “limitation” of protein intake is well above the Recommended Daily Allowance of 0.8 g/kg for adults, and therefore poses no threat for protein malnourishment or deficiency in otherwise healthy adults.
Why the protein restriction? Excess amino acids, from the ingestion of protein beyond that which is needed for muscle growth and repair, can be converted into glucose via gluconeogenesis. Therefore, excessive protein intake can hinder the development of nutritional ketosis and “fat burning.” To a small degree, protein also stimulates insulin secretion, which inhibits lipolysis and stimulates fat storage.
Nutritional ketosis can only occur if there is minimal glucose present for the body to use as energy. If glucose is not readily available, the body is forced to break down fat into ketones to use as an energy source. It is this shift in energy consumption that produces weight loss. In order for a ketogenic diet to be effective, both carbohydrates and protein must be limited, with the remainder of calories being replaced with dietary fat.
A ketogenic diet is a low-carb, moderate-protein, high-fat diet. Generally speaking, a well-formulated ketogenic diet will consist of approximately 5-10% carbohydrates, 15-30% protein, and 60-75% fats. These numbers are guidelines and should be adjusted for the needs and goals of the patient. As with many things in medicine, there is “no one size fits all” treatment for obesity, and a ketogenic diet can be an important tool in the fight against obesity and the medical conditions associated with it.