Low-carbohydrate diet
Low-carbohydrate diets or low-carb diets are nutritional programs that advocate restricted carbohydrate consumption, based on research that ties consumption of certain carbohydrates with increased blood insulin levels, and overexposure to insulin with metabolic syndrome (the most recognized symptom of which is obesity). Under these dietary programs, foods high in digestible carbohydrates (sugars and starches) are limited or replaced with foods containing a higher percentage of proteins, fats, and/or fiber.

# History
Many anthropologists and biologists claim that the Paleolithic diet of early humans was heavily skewed toward meat and fat, although that viewpoint is disputed by some. The shift towards greater meat consumption allowed humans to obtain calories more reliably and to develop larger brains. The species continued to eat plants as its ancestors had but as evolution progressed they couldn't derive continuous nutrition from plants that may be sporadically available in the natural environment. Eating meat allowed individuals to live long enough to reproduce.
The Neolithic Revolution marks the invention of agriculture, which allowed the developing of crops that could provide energy to feed larger populations (mostly in the form of grains), and developing cooking techniques to make these crops more digestible. This innovation gradually converted human societies from highly carnivorous to highly plant-based diets. On an evolutionary timescale, the technological capability to sustain life with a vegetarian diet is very recent. As a result of the Agricultural Revolution, most plants that humans eat today have been sufficiently developed - by the combined effects of cultivation and breeding - to feed a growing population of people. Some researchers, like Loren Cordain, have attributed the shift to agriculture to various environmental pressures. For example, the extinction of large mammals coincides with the start of Agriculture.
The beginning of the modern history of low-carbohydrate diets is popularly attributed to William Banting and Dr. William Harvey (before this, though, anecdotal and holistic prescriptions, containing passages about limiting certain foods, including foods of mostly carbohydrates, have appeared throughout history). Banting was an overweight undertaker who developed hearing difficulties. He sought the help of Dr. Harvey who diagnosed his hearing difficulties as being directly related to his weight problem (his fat was pressing against his inner ear). He prescribed a diet that was very much like the low carbohydrate diets of today (which indicates this diet was, at least informally, known to be effective even at that time). Banting lost weight and his health problems disappeared. This led to Banting's publication of the book Letter on Corpulence in 1869, the first modern low-carbohydrate diet book. The mainstream thinking, though, formalized by the invention of the concept of the calorie in the late 19th century, was still that weight control was primarily a matter of controlling the amount of food consumed.
In the 1920s, Johns Hopkins Medical Center developed the ultrahigh-fat ketogenic diet for the treatment of epilepsy.  The program is calculated to provide 90% of the day's energy from fat and almost none from carbohydrates.  As drug therapies to treat epilepsy were developed, this treatment gradually fell out of favor, though Johns Hopkins continues to use it with strict medical supervision.).
In 1926, Dr. Clarence Lieb published a case study on anthropologist and explorer Vilhjalmur Stefansson who lived for years with the Inuit consuming a diet that was almost entirely meat and fat. Despite expecting to find serious health problems Lieb had found Stefansson (like the Inuit) to be in perfect health showing no adverse effects from his diet. Later, during World War II the medical department of E. I. DuPont hired Dr. Alfred Pennington to help address weight problems with many of the employees. After some study and experimentation Dr. Pennington determined (partly inspired by previous research at the Russel Sage Institute) that the key to weight loss was not restricting consumption overall but reducing consumption of carbohydrates and increasing consumption of proteins and fats. The diet he developed came to be known as the Dupont Diet. During the 1950s studies such as Kekwick and Pawan, 1956 and Mackarness et al., 1958 continued to demonstrate the effectiveness of carbohydrate restriction and the ineffectiveness of energy restriction.
Mainstream science still favored the idea of energy restriction. In addition, research by Ancel Keys starting in the 1950s led ultimately to the publication of Seven countries: a multivariate analysis of death and coronary heart disease in 1980 which linked consumption of cholesterol and saturated fats to heart disease. This research led to the contemporary low-fat diet trend and discouraged research into low-carbohydrate diets. It should be noted that Keys' theory was not universally accepted when published originally, Dr. George Mann being a noted detractor. It is claimed that Keys selectively chose societies that supported his theories and that globally there is little or no correlation between fat intake and heart disease.
Despite this low-carbohydrate diets such as the Air Force Diet (referred to by some as the first real "low carb" diet) and the Drinking Mans Diet had brief periods of popularity in the 1960s. Austrian physician Dr Wolfgang Lutz published his book 'Leben Ohne Brot' (Life Without Bread) in 1967. However it was hardly  noticed in the English speaking world. Unlike Atkins, Lutz doesn't concentrate on weight loss but rather on good health. The Lutz book is generally more rigorous and factual than the work of Atkins. In 1972, Dr. Robert Atkins published Dr. Atkins Diet Revolution which advocated a low-carbohydrate diet he had successfully used in treating thousands of patients in the 1960s. Like its predecessors the book met with some success but, because of research at that time demonstrating risk factors associated with excess fat and protein, it was very widely criticized by the mainstream medical community as being dangerous and misleading, thereby limiting its appeal at the time. Among other things critics pointed out that Dr. Atkins had done little real research into his theories and based them mostly on anecdotal evidence. Dr. Atkins nevertheless continued to develop his theories and gain followers. During the 1980s and 1990s the obesity epidemic in the United States blossomed in spite of the popularity of low-fat diets thereby leading many doctors to question the efficacy of this approach. The concept of the glycemic index was invented in 1981 by Dr. David Jenkins. This and subsequent research demonstrated that many complex carbohydrates can be as harmful as sugars. In the 1990s Dr. Atkins published Dr. Atkins New Diet Revolution and other doctors began to publish books based on the same principles. This can be said to be the beginning of the "low carb craze."
During the late 1990s and early 2000s low-carbohydrate diets became some of the most popular diets in the U.S. (by some accounts as much as 18% of the population was using a low-carbohydrate diet at its peak) and spread to many countries. These were, in fact, noted by many food manufacturers and restaurant chains as substantially affecting their businesses (notably Krispy Kreme). This was in spite of the fact that the mainstream medical community continued to vehemently denounce low-carbohydrate diets as being a dangerous trend. It is, however, valuable to note that many of these same doctors and institutions at the same time quietly began altering their own advice to be closer to the low-carbohydrate recommendations (e.g. eating more protein, eating less starch, reducing consumption of juices by children). The low-carbohydrate advocates did some adjustments of their own increasingly advocating controlling fat and eliminating trans fat. It is also valuable to note that most of major medical groups have acknowledged at least that the low-carbohydrate diet is effective in the short-term. Many of the diet guides and gurus that appeared at this time intentionally distanced themselves from Atkins and the term low carb (because of the controversies) even though their recommendations were based on largely the same principles (e.g. the Zone diet). As such it is often a matter of debate which diets are really low-carbohydrate and which are not. The 1990s and 2000s also saw the publication of an increased number of clinical studies regarding the effectiveness and safety (pro and con) of low-carbohydrate diets (notably a 2006 NEJM paper by Halton et al. describing a 20-year study). After 2004 the popularity of this diet trend began to wane significantly although it still remains quite popular.

# Practices and theories
The term low-carbohydrate diet today is most strongly associated with the Atkins Diet. However, there is an array of other diets that share to varying degrees the same principles (e.g. the Zone Diet, the Protein Power Lifeplan, and the South Beach Diet). As mentioned above there have been diet recommendations that follow the same principles in existence since before the twentieth century. As such it is difficult to summarize all of these diets and draw a sharp distinction between these and other diets. There is, therefore, no widely accepted definition of what precisely consistutes a low-carbohydrate diet. For the purposes of this discussion, we focus on diets that reduce (nutritive) carbohydrate intake sufficiently to dramatically reduce or eliminate insulin production in the body and to encourage ketosis (production of ketones to be used as energy in place of glucose).
Although originally low-carbohydrate diets were created based on anecdotal evidence of their effectiveness, today there is a much greater theoretical basis on which these diets rest. The key scientific principle which forms the basis for these diets is the relationship between consumption of carbohydrates and their effects on blood sugar (i.e. blood glucose) and hormone production. Blood sugar levels in the human body must be maintained in a fairly narrow range to maintain health. The two primary hormones related to regulating blood sugar levels, produced in the pancreas, are insulin, which lowers blood sugar levels, and glucagon, which raises blood sugar levels. In general, most western diets (and many others) are sufficiently high in nutritive carbohydrates that virtually every meal causes substantial insulin production and shuts down ketosis which causes excess energy in the diet to be stored as fat (discussed in the next section). By contrast, low-carbohydrate diets, or more properly, diets that are very low in nutritive carbohydrates, discourage insulin production and tend to cause ketosis which, according to some, can actually cause excess dietary energy as well as excess body fat to be eliminated from the body. Although these diets remain controversial there are clinical studies related to their effectiveness.
Low-carbohydrate diet advocates in general recommend reducing nutritive carbohydrates (commonly referred to as "net carbs," i.e. total carbohydrates reduced by the non-nutritive carbohydrates) to very low levels. This means sharply reducing consumption of desserts, breads, pastas, potatoes, rice, and other sweet or starchy foods. Some recommend levels as low as 20-30 grams of "net carbs" per day, at least in the early stages of dieting (for comparison, a single slice of white bread may contain 15-25 grams of carbohydrate, almost entirely starch). The diets often differ in the specific amount of carbohydrates allowed, whether certain types of foods are preferred, whether occasional exceptions are allowed, etc. Generally they all agree that processed sugar should be eliminated, or at the very least greatly reduced, and similarly generally discourage heavily processed grains (white bread, etc.). They vary greatly in their recommendations as to the amount of fat allowed in the diet although the most popular versions today (including Atkins) generally recommend at most a moderate fat intake.
As a related note, there is a set of diets known as low-glycemic-index diets (low-GI diets) or low-glycemic-load diets (low-GL diets), in particular the Low GI Diet by Brand-Miller et al.. In reality, low-carbohydrate diets are, literally speaking, low-GL diets (and vice versa) in that they specifically limit what contributes to the glycemic load in foods. In practice, though, the diets that call themselves low-GI/low-GL diets differ from those calling themselves low-carbohydrate diets in the following ways.
Another related diet type, the low-insulin-index diet, is very similar except that it is based on measurements of direct insulemic responses to food rather than glycemic response. Although the diet recommendations mostly involve lowering nutritive carbohydrates, there are some low-carbohydrate foods that are discouraged as well (e.g. beef).

# 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 summarized as follows. Nutritive carbohydrates (starches and sugars) in the diet tend to break down very easily into glucose in the bloodstream (blood sugar) when consumed. Glucose in the blood is used by the cells in the body for energy for their basic function. Excessive amounts of glucose in the blood are toxic to the human body (the reason diabetes causes such serious health problems). In general, unless a meal is very low in starches and sugars the level of glucose will tend to rise to potentially dangerous levels. When this occurs, the pancreas automatically produces insulin to cause the liver to convert glucose into glycogen (glycogenesis) and triglycerides (which can become body fat), thus reducing the blood sugars to safe levels. Diets with a high starch/sugar content, therefore, cause sharp spikes in insulin production. As such the blood sugar levels are highly variable with every meal.
By contrast, if the diet is very low in starches and sugars (low-carbohydrate diets) the blood sugar level can fall so low that there is insufficient glucose to fuel the cells in the body. This state causes the pancreas to produce glucagon. Glucagon causes the conversion of stored glycogen to glucose and, once the glycogen stores are exhausted, causes the liver to synthesize ketones (ketosis) and glucose (gluconeogenesis) from fats and proteins. Most cells in the body can use ketones for energy instead of glucose and, since ketones are easier to produce, only a small amount of glucose is created (in other words, ketosis is the more significant process in this case). Because diets low in starches and sugars do not tend to directly affect blood sugar levels significantly, meals tend to have little direct affect on insulin levels (and so such diets tend to discourage insulin production in general).
The diets of most people in modern, so-called western nations, especially the United States contain significant amounts of starches (and, frequently, significant amounts of sugars). As such, the metabolisms of most westerners tend to operate outside of ketosis and tend to involve significant insulin production. This has been regarded by medical science in the last century as being "normal." Ketosis has generally been regarded as a dangerous (potentially life-threatening) state which unnecessarily stresses the liver and causes destruction of muscle tissues. The view that has been developed is that getting energy more from protein than carbohydrates causes liver damage and that getting energy more from fats than carbohydrates causes heart disease. This view is still the view of the majority in the medical and nutritional science communities.
Most advocates of low-carbohydrate diets (specifically those that recommend diets similar to the Atkins Diet) argue that this metabolic state (using primarily blood glucose for energy) is not normal at all and that the human body is, in fact, supposed to function primarily in ketosis. They argue that high insulin levels can, in fact, 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 related but very different process). 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 not a major concern (although most do not advocate unrestricted fat intake and do advocate avoiding trans fat). Further, whereas insulin in the bloodstream causes storage of food energy, when the body is in ketosis, excess ketones (which contain excess energy) are excreted in the urine and the breath. Many argue, on this basis, that the ketosis offers a so-called metabolic advantage in that the body automatically eliminates food energy that it does not need even with a high-energy diet (this argument has not yet been explicitly demonstrated by any clinical studies).
A study by Arizona State University which compared low-carbohydrate diets did not find metabolic advantage for ketogenic diet. "Ketogenic low-carbohydrate diets have no metabolic advantage over nonketogenic low-carbohydrate diets" 
This debate is on-going and no general consensus exists at this time.

# Scientific studies
Because of the substantial controversy regarding low-carbohydrate diets and even disagreements in interpreting the results of specific studies it is difficult to objectively summarize the research in a way that reflects scientific consensus.  Although there has been some research done throughout the twentieth century, most directly relevant scientific studies have occurred in the 1990s and early 2000s and, as such, are relatively new. One study found no correlation between a low-carbohydrate, high fat/protein diet and coronary heart disease in women, and a moderate reduction in risk if the fat and protein were primarily from plant rather than animal sources.  Other studies have found possible benefits to individuals with diabetes, renal cancer and autism.  The Johns Hopkins diet, with 90% of energy from fat and much of the remaining from protein, has also been used for more than 80 years to treat epilepsy, though generally it has been superseded by medication.

# Arguments for low-carbohydrate diets

## The evolutionary argument
The Paleolithic diet did not include grains and refined sugar, and the human body has not evolved significantly since the time of the Neolithic Revolution, implying that their consumption should still be avoided today and causes undesired and largely unknown effects. Specifically, they cause the body to produce excess amounts of the hormone insulin, which tells the body to store rather than burn fat, hence causing obesity and its complications (heart disease, cancer, Type 2 diabetes). Humans ate a diet which consisted mainly of meat and that the current "epidemic" of obesity is due to the popular assumption, reinforced by the food industry and the new field of dietary medicine, that the low-fat approach is healthier.
Supporters claim the exclusive focus on reducing fat is oversimplified, and that low-fat diets are not automatically healthy ones. They claim that the western world is not suffering from a collective failure of will to exercise, but has been encouraged to eat more carbohydrates, which in turn stimulate appetite and more eating. Some go so far as to suggest that if the human body were truly as fragile and susceptible to illness due to small variations in diet as many doctors and dietitians have been suggesting, the species could never have survived its more primitive days. They argue that the fact that industrialized nations (notably the U.S.) are showing such an epidemic of health problems indicates that some fundamental and important aspect of the diet among these people is drastically different from early humans (and indeed the intake of sugars and starches certainly qualifies as a big difference).  Conversely, early human lifespans were much shorter, and little is known about the specific diets, overall wellness or quality of life of those early humans.
The recent rise in western obesity rates has coincided with a widespread belief in low-fat, high-carbohydrate as a healthy way of eating. By contrast, traditional high-fat French cooking has led to a much lower incidence of obesity, morbid obesity and chronic heart disease than the high-sugar American diet, despite overall energy intake and exercise levels being the same.

## The Inuit paradox
Vilhjalmur Stefansson, a Canadian Arctic explorer and ethnologist, documented the fact that most Inuit lived on a diet of about 90% meat and fish, often going 6-9 months a year on nothing but meat and fish--essentially, a zero-carbohydrate diet. He found that he and his fellow European-descent explorers were also perfectly healthy on such a diet. When medical authorities questioned him on this, he and a fellow explorer agreed to undertake a study under the auspices of the Journal of the American Medical Association to demonstrate that they could eat a 100% meat diet in a closely-observed laboratory setting for the first several weeks, with paid observers for the rest of an entire year. The results were published in the Journal of the AMA, and both men were perfectly healthy on such a diet, without vitamin supplementation or anything else in their diet except animal product. It is worthy of note that the health of the participants deteriorated in the first few weeks of the trial as the participants were fed only with lean muscle meats, unlike the Inuits who ate at least 50% fat.  Once fat was added, their health was regained. Like the Eskimos, they also ate other parts of the animal, such as the brains, liver, bone marrow, and other offal, providing more nutrition than muscle meat alone. 

## Favourable studies
Advocates point to scientific trials demonstrating the efficacy and safety of low carb diets. Several independent clinical trials have shown that low carb diets can be successfully used to lose weight. These trials found that, in the short term, risk factors for heart disease and Type 2 diabetes  such as blood serum cholesterol and insulin levels  tended to improve in spite of increased consumption of saturated fat and cholesterol. The trials were of short duration, and were not able to assess the long-term health effects of the diet.
A study conducted in 1965 at the Oakland (California) Naval Hospital used a diet of 1000 kilocalories per day, high in fat and limiting carbohydrates to 10 grams (40 kilocalories) daily. Over a ten-day period, subjects on this diet lost more body fat than did a group who fasted completely. (Benoit et. al. 1965).  Some advocates of low-carbohydrate diets have termed this the metabolic advantage of such diets.
The results of studies from Stanford University (2007) and Duke University (2005) favored low-carbohydrate diets for both weight loss and health indicators.

# Arguments against low-carbohydrate diets
In 2004, the Canadian government ruled that foods sold in Canada could not be marketed with reduced or eliminated carbohydrate content as a selling point because reduced carbohydrate content was not determined to be a health benefit, and that existing "low carb" and "no carb" packaging would have to be phased out by 2006. Since the Canadian government pays virtually all of the medical costs associated with lifestyle-induced diseases, they have a direct financial interest in preventing them.

## Harmful Side effects
Increased consumption of unprocessed animal protein results in higher consumption of saturated fat and cholesterol, which some assert is a predictor for Cardiovascular disease.  Others believe that a link between heart disease and saturated fat/cholesterol consumption remains unproven..
The lowered intake of dietary fiber that often accompanies dramatically reduced carbohydrate intake can result in constipation if not supplemented.  For example, this has been a criticism of the Induction stage of the Atkins diet (note that today the Atkins diet is more clear about recommending a fiber supplement during Induction).
It has been hypothesized that a diet related change in blood acidity can lead to bone loss through a process called ketoacidosis, as mentioned earlier in this article.
One of the telltale signs of a ketogenic diet is a noticeable smell of ammonia in the urine, perspiration, and breath. A complaint frequently noted by low-carb dieters and those around them is that they smell (from the ketones being produced).
A diet rich in fruits, vegetables, whole grains, and beans is, by definition, a high-carb diet. Limiting fruits, vegetables, whole grains, and beans means limiting the vitamins and plant phytonutrients that can only be obtained from those foods.
The prehistoric diet of most early humans during the Middle and Upper Paleolithic period was heavily skewed toward animal protein and fat  anthropologists' estimates of the average lifespan of these early humans range from 27 to 38 years. However, their average life-span was skewed by high infant mortality, infection, and injuries, as noted by Loren Cordain.

## Environmental impact
Since changes in habit of 6 billion people over a long term totals up to an enormous amount of food, it should be noted that the growth in global population over the last few centuries was only possible because of grain crops. Poor quality pasture has traditionally been fit only for raising livestock and that it has been turned to grain production only through massive government subsidies, fertilizers and pesticides.
Raising livestock instead of eating vegetable food has poor energy efficiency - around 3% of the intake of energy consumed by livestock can be employed from animal-based food.However, meat and dairy products are enriched on proteins and other nutrients. It is often also more economical to raise cattle instead of attempting to harness the area on grain production, especially when the climate conditions strongly disfavour grain and human consumable vegetablesMoreover, pasture needs less fertilization, pesticides and cultivation than grain fields and their environmental impact is likely to be less than the same area of grain fields.
It must be pointed out too that parts of the above argument imply that a low-carbohydrate diet must be high in meat consumption, especially ruminants. Although certainly this is common among many practitioners of the low-carbohydrate diets few if any mainstream guides for this
diet say that this is a requirement (e.g. a diet which concentrates on soy and/or fish can be quite low in carbohydrates, and fat for that matter,
without requiring raising more land animals).

# Misconceptions
The low-carbohydrate diet trend has generated a great deal of controversy which has spawned many myths and misconceptions.

## Water-related weight loss
In the first week or two of a low-carbohydrate diet a great deal of the weight loss comes from eliminating water retained in the body (many doctors say that the presence of high levels of insulin in the blood causes unnecessary water retention in the body). However, this is a short-term effect and is entirely separate from the general weight loss that these diets can produce through eliminating excess body fat.

## Exercise
Low-carbohydrate diets could inherently cause weakness or fatigue by giving rise to the occasional assumption that low-carbohydrate dieting cannot involve an exercise regimen. Advocates of low-carbohydrate diets generally dispute any suggestion that such diets cause weakness or exhaustion (except in the first few days) and indeed most highly recommend exercise as part of a healthy lifestyle.

## Carbohydrate intake today and in the past
The human diet has changed significantly through history and, as such, assertions that the diets that most humans eat today are representative of the diets humans have always eaten are, at best, exaggerations. It is well established that just in the twentieth century, the consumption of sugar per capita in the U.S. and the U.K. has steadily and dramatically increased. Starch consumption has increased as well. Moreover it is well known that early humans ate diets that were heavily meat-based and that the shift toward high levels of starch and sugar consumption occurred much later. The current trend toward very high-carbohydrate, low-fat diets in the West is, in reality, a quite recent trend owing in large part to the research of Ancel Keys.

## Micronutrients and vitamins
The major low-carbohydrate diet guides generally recommend multi-vitamin and mineral supplements as part of the diet regimen which may lead some to believe that these diets are nutritionally deficient. The primary reason for this recommendation is that if the switch from a high-carbohydrate to a low-carbohydrate, ketogenic diet is rapid, the body can temporarily go through a period of adjustment during which the body may require extra vitamins and minerals (the reasons have to do with the body's releasing excess fluids that were stored during high-carbohydrate eating). In other words, the body goes through a temporary "shock" if the diet is changed to low-carbohydrate dieting quickly just as it would changing to a high-carbohydrate diet quickly. This does not, in and of itself, indicate that either type of diet is nutritionally deficient.
It should be noted that, contrary to the recommendations of most diet guides, some individuals choose to avoid vegetables altogether in order to minimize carbohydrates. It is more likely that such a diet could be nutritionally deficient (some would dispute this based on cases like Vilhjalmur Stefansson).