The Ketogenic Diet - Lyle McDonald (continued, part 2)

3 Fuel

• The body has three storage depots of fuel which it can tap during periods of caloric deficiency: protein, which can be converted to glucose in the liver and used for energy ; carbohydrate, which is stored primarily as glycogen in the muscle and liver ; and fat , which is stored primarily as body fat.
• It appears that there are least 4 distinct fuels which the body can use:
  glucose, protein, free fatty acids (FFA), and ketones. However when we look at the relationships between these four fuels, we see that only glucose and FFA need to be considered.
• When present in sufficient quantities, glucose is the preferred fuel for most tissues in the body. The major exception to this is the heart, which uses a mix of glucose, FFA and ketones. The major source of glucose in the body is from dietary carbohydrate.
  
• While it is true that a high carbohydrate intake can be protein sparing, it is often ignored that this same high carbohydrate also decreases the use of fat for fuel.
• If glucose requirements are high but glucose availability is low, as in the initial days of fasting, the body will break down its own protein stores to produce glucose. This is probably the origin of the concept that low carbohydrate diets are muscle wasting. However, an adequate protein intake during the first weeks of a ketogenic diet will prevent muscle
  loss by supplying the amino acids for gluconeogenesis that would otherwise come from body proteins.
  
• The consumption of alcohol will almost completely impair the body’s use of fat for fuel.
• The greatest rates of fat oxidation will occur under conditions when carbohydrates are restricted.

Hormones

• Insulin is a peptide (protein based) hormone released from the pancreas, primarily in response to increases in blood glucose. When blood glucose increases, insulin levels increase as well, causing glucose in the bloodstream to be stored as glycogen in the muscle or liver.
• When insulin levels are increased, protein synthesis is stimulated and free amino acids (the building blocks of proteins) are be moved into muscle cells and incorporated into larger proteins.
• FFA release from fat cells is inhibited by even small amounts of insulin.
• Glucagon is also a peptide hormone released from the pancreas and its primary role is also to maintain blood glucose levels. It acts by raising blood glucose when it drops below normal.
• Glucagon release is stimulated by a variety of stimuli including a drop in blood glucose/insulin, exercise, and the consumption of a protein meal.
• High GH levels along with high insulin levels will raise IGF-1 levels as well as increasing anabolic reactions in the body. To the contrary, high GH levels with low levels of insulin, as seen in fasting or carbohydrate restriction, will not cause an increase in IGF-1 levels. This is one of the reasons that ketogenic diets are not ideal for situations requiring tissue synthesis, such as muscle growth or recovery from certain injuries: the lack of insulin may compromise IGF-1 levels as well as affecting protein synthesis.
• When liver glycogen is full, blood glucose is maintained and the body is generally anabolic, which means that incoming glucose, amino acids and free fatty acids are stored as glycogen, proteins, and triglycerides respectively. This is sometimes called the ‘fed’ state.
  
• When liver glycogen becomes depleted, via intensive exercise or the absence of dietary carbohydrates, the liver shifts roles and becomes catabolic. Glycogen is broken into glucose, proteins are broken down into amino acids, and triglycerides are broken down to free fatty acids. This is sometimes called the ‘fasted’ state.
• If liver glycogen is depleted sufficiently, blood glucose drops and the shift in insulin and glucagon occurs. This induces ketone body formation, called ketogenesis.

4 Ketone Physiology

• The primary role of ketones is to replace glucose as a fat-derived fuel for the brain.
• In practical terms, after three weeks of a ketogenic diet, the use of ketones by tissues other than the brain is negligible and can be ignored.
• When the proper signal reaches the fat cell, stored triglyceride (TG) is broken down into glycerol and three free fatty acid (FFA) chains. FFA travels through the bloodstream, bound to a protein called albumin. Once in the bloodstream, FFA can be used for energy production by most tissues of the body, with the exception of the brain and a few others.
• Under normal dietary conditions, ketone concentrations are simply too low to be of any physiological consequence.
• Ketosis is the end result of a shift in the insulin/glucagon ratio and indicates an overall shift from a glucose based metabolism to a fat based metabolism.
• Ketoacidosis, as it occurs in Type I diabetics and alcoholics and which is potentially fatal, will not occur in nondiabetic individuals due to built in feedback loops whereby excess ketones stimulate the release of insulin, slowing ketone body formation.

5 Adaptations to Ketosis

• In one sense, the ketogenic diet is identical to starvation, except that food is being consumed. The primary difference is that the protein and fat intake of a ketogenic diet will replace some of the protein and fat which would otherwise be used for fuel during starvation.
  
• Starvation Phases:
1. First 8 hours: the body is still absorbing fuel from previous meals. Within 10 hours after the last carbohydrate containing meal, roughly 50% of the body’s total energy requirements are being met by free fatty acids (FFA).
2. First day or two: the body will rely on FFA and the breakdown of liver glycogen for its energy requirements.
3. First week: the body will drastically increase the production of glucose from protein and other fuels such as lactate, pyruvate and glycerol. During this phase, protein breakdown increases greatly.
• In a non-ketotic state, the brain utilizes roughly 100 grams of glucose per day.  This means that any diet which contains less than 100 grams of carbohydrate per day will induce ketosis.