Ketosis and insulin synthesis: what is normal?

At the heart of the debate about most low carbohydrate diets are fundamental questions about what is a normal diet and how the human body is supposed to operate. These questions can be outlined as follows:

The diets of most people in modern Western nations, especially the United States, contain large amounts of starches and often substantial amounts of sugars, including fructose. Most westerners seldom exhaust stored glycogen supplies and hence rarely go into ketosis. This has been regarded by the majority of the medical community in the last century as normal for humans.[citation needed] Ketosis had been confused with ketoacidosis, a dangerous and extreme ketotic condition associated with diabetes, and had been regarded by the medical community as harmful and potentially life-threatening, who believe it unnecessarily stresses the liver and causes destruction of muscle tissues.[citation needed] A perception developed that getting energy chiefly from dietary protein rather than carbohydrates causes liver damage and that getting energy chiefly from dietary fats rather than carbohydrates causes heart disease and other health problems. This view is still held by the majority of those in the medical and nutritional science communities.[60][61][62] However, it is now widely recognized that periodic ketosis is in fact normal, and that ketosis provides a number of surprising benefits, including neuroprotection against diverse types of cellular injury.[63]

People who eschew low carbohydrate diets cite hypoglycemia and ketoacidosis as a risk factor. While mild acidosis may be a side effect when beginning a ketogenic diet,[64][65] it is benign and should not be confused with diabetic ketoacidosis, which can be life threatening.

A diet very low in starches and sugars induces several adaptive responses. Low blood glucose causes the pancreas to produce glucagon,[66] which stimulates the liver to convert stored glycogen into glucose and release it into the blood. When liver glycogen stores are exhausted, the body starts utilizing fatty acids instead of glucose. The brain cannot use fatty acids for energy, and instead uses ketones produced from fatty acids by the liver. By using fatty acids and ketones as energy sources, supplemented by conversion of proteins to glucose (gluconeogenesis), the body can maintain normal levels of blood glucose without dietary carbohydrates.

Most advocates of low-carbohydrate diets, such as the Atkins Diet, argue that the human body is adapted to function primarily in ketosis.[67][68] They argue that high insulin levels can cause many health problems, most significantly fat storage and weight gain. They argue that the purported dangers of ketosis are unsubstantiated (some of the arguments against ketosis result from confusion between ketosis and ketoacidosis which is a mostly diabetic condition unrelated to dieting or low-carbohydrate intake).[69] They also argue that fat in the diet only contributes to heart disease in the presence of high insulin levels and that if the diet is instead adjusted to induce ketosis, fat and cholesterol in the diet are beneficial. Most low carb diets plans discourage consumption of trans fat.

On a high-carbohydrate diet, glucose is used by cells in the body for the energy needed for their basic functions, and about 2/3 of body cells require insulin in order to use glucose. Excessive amounts of blood glucose are thought to be a primary cause of the complications of diabetes; when glucose reacts with body proteins (resulting in glycosolated proteins) and change their behavior. Perhaps for this reason, the amount of glucose tightly maintained in the blood is quite low. Unless a meal is very low in starches and sugars, blood glucose will rise for a period of an hour or two after a meal. When this occurs, beta cells in the pancreas release insulin to cause uptake of glucose into cells. In liver and muscle cells, more glucose is taken in than is needed and stored as glycogen (once called 'animal starch').[70] Diets with a high starch/sugar content, therefore, cause release of more insulin and so more cell absorption. In diabetics, glucose levels vary in time with meals and vary a little more as a result of high carbohydrate content meals. In non-diabetics, blood sugar levels are restored to normal levels within an hour or two, regardless of the content of a meal.

While there are essential fatty acids (EFA) and essential amino acids (EAA) and while a diet devoid of EFA or EAA will result in eventual death, a diet completely without carbohydrates can be maintained indefinitely because fatty acids include a carbohydrate backbone (glycerol).[71] There are essential fatty acids and amino acids for structural building blocks, not energy. EFA and EAA will be converted into intermediates for the carbohydrate metabolism, even if it depletes their essential stocks. However, a very low carbohydrate diet (less than 20g per day) may negatively affect certain biomarkers[72] and produce detrimental effects in certain types of individuals (for instance, those with kidney problems). The opposite is also true – for instance, clinical experience suggests very low carbohydrate diet for patients with metabolic syndrome.[73]

Более того, в то время как инсулин в крови вызывает сохранение калорий, если организм находится в состоянии кетоза, излишние кетоны (которые содержат излишние калории) выделяются с мочой и дыханием. Многие утверждают на этом основании, что кетоз предлагает так называемое метаболическое преимущество, заключающееся в том, что тело автоматически устраняет калории, которые ему не нужны, даже при высококалорийной диете (этот аргумент еще не был подтвержден клиническими исследованиями).