Are You Thinking of Doing a Ketogenic Diet?
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The strict nutritional diet consists of approximately 75 percent calories from fat, 20 percent from protein and 5 percent from carbohydrates.
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Introduction to the ketogenic diet
Many readers may not be familiar with the ketogenic diet. So let’s 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 causes ketone bodies to be produced by the liver, shifting the body’s metabolism away from glucose and towards fat utilization. More specifically, a ketogenic diet is one that restricts carbohydrates below a certain level (generally 100 grams per day), inducing a series of adaptations to take place. Protein and fat intake are variable, depending on the goal of the dieter. However, the ultimate determinant of whether a diet is ketogenic or not is the presence (or absence) of carbohydrates.
Fuel metabolism and the ketogenic diet
Under ‘normal’ dietary conditions, the body runs on a mix of carbohydrates, protein and fat. When carbohydrates are removed from the diet, the body’s small stores are quickly depleted. Consequently, the body is forced to find an alternative fuel to provide energy. One of these fuels is free fatty acids (FFA), which can be used by most tissues in the body. However, not all organs can use FFA. For example, the brain and nervous system are unable to use FFA for fuel ; however, they can use ketone bodies. Ketone bodies are a by-product of the incomplete breakdown of FFA in the liver. They serve as a non-carbohydrate, fat-derived fuel for tissues such as the brain. When ketone bodies are produced at accelerated rates, they accumulate in the bloodstream, causing a metabolic state called ketosis to develop. Simultaneously, there is a decrease in glucose utilization and production. Along with this, there is a decrease in the breakdown of protein to be used for energy, referred to as ‘protein sparing’. Many individuals are drawn to ketogenic diets in an attempt to lose body fat while sparing the loss of lean body mass.
Hormones and the ketogenic diet
Ketogenic diets cause the adaptations described above primarily by affecting the levels of two hormones: insulin and glucagon. Insulin is a storage hormone, responsible for 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, stimulating 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 decrease and glucagon levels increase. This causes an increase in FFA release from fat cells, and increased FFA burning in the liver. The accelerated FFA burning in the liver is what ultimately leads to the production of ketone bodies and the metabolic state of ketosis. In addition to insulin and glucagon, a number of 11
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 will improve the success of the ketogenic diet. However, a diet devoid of carbohydrates is unable to sustain 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 integrate carbohydrates without disrupting the effects of ketosis. Two modified ketogenic diets are described in this book which approach this issue from different directions. The targeted ketogenic diet (TKD) allows carbohydrates to be consumed immediately around exercise, to sustain 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 sustain exercise performance.
There are four substances which man can derive calories from: carbohydrate, protein, fats, and alcohol. As stated above, the body will tend to utilize a given fuel for energy in relation to its availability and concentration in the bloodstream. In general, the body can increase or decrease its use of glucose in direct proportion to the amount of dietary carbohydrate being consumed. This is an attempt to maintain body glycogen stores at a certain level . If carbohydrate consumption increases, carbohydrate use will go up and vice versa. Protein is slightly less regulated . When protein intake goes up, protein oxidation will also go up to some degree. By the same token, if protein intake drops, the body will use less protein for fuel. This is an attempt to maintain body protein stores at constant levels. In contrast, the amount of dietary fat being eaten does not significantly increase the amount of fat used for fuel by the body. Rather fat oxidation is determined indirectly: by alcohol and carbohydrate consumption . The consumption of alcohol will almost completely impair the body’s use of fat for fuel. Similarly the consumption of carbohydrate affects the amount of fat used by the body for fuel. A high carbohydrate diet decreases the use of fat for fuel and vice versa (15). Thus, the greatest rates of fat oxidation will occur under conditions when carbohydrates are restricted. As well, the level of muscle glycogen regulates how much fat is used by the muscle (20,21), a topic discussed in chapter 18. Using exercise and/or carbohydrate restriction to lower muscle and liver glycogen levels increases fat utilization .
What are ketone bodies?
The three ketone bodies are acetoacetate (AcAc), beta-hydroxybutyrate (BHB) and acetone. AcAc and BHB are produced from the condensation of acetyl-CoA, a product of incomplete breakdown of free fatty acids (FFA) in the liver. While ketones can technically be made from certain amino acids, this is not thought to contribute significantly to ketosis . Roughly one-third of AcAc is converted to acetone, which is excreted in the breath and urine. This gives some individuals on a ketogenic diet a ‘fruity’ smelling breath. As a side note, urinary and breath excretion of acetone is negligible in terms of caloric loss, amounting to a maximum of 100 calories per day . The fact that ketones are excreted through this pathway has led some authors to argue that fat loss is being accomplished through urination and breathing. While this may be very loosely true, in that ketones are produced from the breakdown of fat and energy is being lost through these routes, the number of calories lost per day will have a minimal effect on fat loss.
Functions of ketones in the body
Ketones serve a number of functions in the body. The primary role, and arguably the most important to ketogenic dieters, is to replace glucose as a fat-derived fuel for the brain . A commonly held misconception is that the brain can only use glucose for fuel. Quite to the contrary, in situations where glucose availability is limited, the brain can derive up to 75% of its total energy requirements from ketone bodies . Ketones also decrease the production of glucose in the liver (5-7) and some researchers have suggested that ketones act as a ‘signal’ to bodily tissues to shift fuel use away from glucose and towards fat . These effects should be seen as a survival mechanism to spare what little glucose is available to the body. The importance of ketones as a brain fuel are discussed in more detail in the next chapter. A second function of ketones is as a fuel for most other tissues in the body. By shifting the entire body’s metabolism from glucose to fat, what glucose is available is conserved for use by the brain. While many tissues of the body (especially muscle) use a large amount of ketones for fuel during the first few weeks of a ketogenic diet, most of these same tissues will decrease their use of ketones as the length of time in ketosis increases . At this time, these tissues rely primarily on the breakdown of free fatty acids (FFA). 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. A potential effect of ketones is to inhibit protein breakdown during starvation through several possible mechanisms, discussed in detail in the next chapter. The only other known function of ketones is as a precursor for lipid synthesis in the brain of neonates .
Does Ketosis Have Any Negative Effects?
The ketosis produced by fasting or limiting carbohydrate intake does not have negative effects for most people once the body has adapted to that state.
The confusion on this point is mainly due to the fact that people who lack insulin, primarily Type 1 diabetics or insulin-dependent Type 2 diabetics, can get into a dangerous state called diabetic ketoacidosis. In ketoacidosis, ketones levels are higher than in the ketosis produced by diet.
The ketosis caused by diet has been referred to as dietary ketosis, physiological ketosis, benign dietary ketosis (Atkins), and, most recently, nutritional ketosis (Phinney and Volek), in an attempt to clear up possible confusion with ketoacidosis.
The second source of confusion is that there is a transition period while the body is adapting to using fats and ketones instead of glucose as its main fuel. There can be negative symptoms during this period (fatigue, weakness, light-headedness, headaches, mild irritability), but they usually can be eased fairly easily. Most are over by the first week of a ketogenic diet, though some may extend to two weeks.
Athletes who closely track their performance may notice more subtle effects up to 6-8 weeks from the start of the diet, and there is some evidence that it may take even longer, up to 12 weeks, for 100% adaptation.
