primarily as an energy source. Under normal dietary conditions, most tissues in the body use carbohydrates, in the form of glucose, for fuel. There are exceptions such as the heart which rely primarily on fatty acids for fuel.
Carbohydrates are generally subdivided into complex and simple carbohydrates. All carbohydrates are formed from the simple sugars glucose, fructose and galactose, called monosaccharides. Monosaccharides combine into chains of two, called disaccharides. As monosaccharides form into longer chains, they are called polysaccharides or simply starch, which is a chain of hundreds or thousands of glucose molecules attached to each other.
The term 'complex carbohydrates' refers to starches such as breads, pasta, potatoes, rice, and all grains. Simple carbohydrates refer to sugars such as table sugar (sucrose), fructose and fruit. Carbohydrates can be further delineated based on the Glycemic Index (GI) which is discussed below.
Despite dietary differences between carbohydrates, almost all ingested carbohydrate will enter the bloodstream as glucose, raising blood glucose levels. Each gram of dietary carbohydrate appears in the bloodstream as 1 gram of glucose. A very small amount of ingested carbohydrate (approximately one percent) will enter the bloodstream as fructose (fruit sugar).
Once in the bloodstream, glucose has a variety of fates. It can be burned immediately for energy by most tissues of the body or stored as glycogen (a long chain of glucose molecules attached to one another) in the muscle or in the liver for later use. If an excess of carbohydrates is consumed, glucose can be converted to fat in the liver (a process called de novo lipogenesis) or pushed directly into the fat cell as alpha-glycerophosphate.
How much carbohydrate can be eaten per day?
Chapter 5 established that the body can survive indefinitely on a diet completely devoid of dietary carbohydrate (assuming protein and vitamin/mineral intake is sufficient). However, from a practical standpoint, it is nearly impossible to avoid all sources of carbohydrate in the diet. Additionally, a diet completely devoid of carbohydrate foods may rapidly become monotonous. The question to be answered is how many grams of carbohydrate can be consumed without interrupting ketosis.
Although up to 100 grams of carbohydrate will allow ketosis to develop, it would be rare to see ketones excreted in the urine at this level of intake. Since the only measure of ketosis available to ketogenic dieters are Ketostix (tm) carbohydrates must be restricted below this level if ketosis is to be measured. As a general rule of thumb, dietary carbohydrates should be below 30 grams per day for ketosis to be rapidly established and for ketones to be lost in the urine.
However, this value varies from person to person and depends on other factors such as protein intake and activity, which allows individuals to consume relatively more carbohydrate without disrupting ketosis.
Assuming a non-excessive protein intake (see next section), a carbohydrate intake of 30 grams per day or less is advised during the first weeks of a SKD to allow for adaptations to take place. After adaptation to the diet, it appears that individuals can tolerate relatively greater carbohydrate intakes without disrupting ketosis. Although not completely accurate, Ketostix (tm) can provide a rough measure of how many carbohydrates can be consumed while still maintaining ketosis. As long as trace ketosis is maintained, carbohydrates can be gradually added to the diet. See chapter 15 for details on using Ketostix (tm).
An extremely low carbohydrate intake is relatively more important for those individuals following the CKD, who only have five or six days to establish ketosis. In this case, carbohydrate intake should be minimized as much as possible (meaning that protein intake must be adequate) during the first few days of each cycle so that ketosis will occur as quickly as possible. Individuals on a TKD follow a separate set of rules for daily carbohydrate intake which is discussed in chapter 11.
Carbohydrates are generally differentiated into complex and simple carbohydrates. This is a crude measure of the quality of carbohydrates. A more accurate measure of carbohydrate quality is the Glycemic Index (GI) which is a measure of how much insulin a given carbohydrate food will cause to be released (see appendix 1 for a partial GI).
The GI of a food is defined relative to white bread, which is arbitrarily given a value of 100. A food with a GI of 60 will cause glucose levels to rise in the blood 60% as quickly as white bread, causing the release of 60% as much insulin. Similarly, a food with a GI of 130 will raise blood glucose 30% more quickly than white bread, causing the body to release 30% more insulin. In general, starches and complex carbohydrates tend to have lower GI values than simple sugars like glucose and sucrose.
Since our wish is to minimize insulin release during a standard ketogenic diet, any dietary carbohydrates which are consumed on a SKD should come from low GI sources. This means that the majority of carbohydrates consumed will come from vegetable sources, as most starches have a GI that is too high.
One thing to note is that the GI of carbohydrates is affected by the ingestion of protein, fats and fiber at the same meal. The ingestion of other nutrients slows digestion of carbohydrates, lowering the effective GI (less insulin response) than eating that same carbohydrate by itself.
Although there is little data on ideal timing of carbohydrates on a SKD, we can create a few guidelines. While the amount of insulin released from the ingestion of dietary carbohydrates is related to their quality (GI), it is also related somewhat to the quantity of carbs ingested. Ingestion of 30 grams of broccoli will cause a greater insulin release than the ingestion of 10 grams of broccoli although the GI is identical.
One approach is to spread carbohydrate intake throughout the day in small amounts. While this may minimize insulin response, it should be noted that 5-6 grams of carbohydrate per meal is not much carbohydrate. Some individuals may wish to have all of their daily carbohydrate at one main meal, such as a large salad with dinner or lunch. Although this will cause a slightly greater insulin release than spreading out the same amount of carbohydrate throughout the day, the low GI of vegetables coupled with the digestion slowing effect of protein, fat and fiber should prevent an excessive insulin response. Even if a large enough insulin response occurred to disrupt ketosis, it should be transient and ketosis should resume soon thereafter.
Although carbohydrate intake must be severely restricted on a SKD, a diet completely devoid of carbohydrate is impossible to achieve in practice and would be monotonous in any case. Depending on factors such as protein intake, a carbohydrate intake of 30 grams per day or less will generally allow the induction of ketosis although this varies from person to person. As a general rule, low GI carbohydrates such as vegetables are the best source as they have the least effect on insulin release. Fruits and starches should generally be avoided on a SKD. The daily carbohydrate amount can either be spread throughout the day or eaten all at once.
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