Low-carbohydrate diets or low-carb diets are dietary programs that restrict carbohydrate consumption usually for weight control. Foods high in digestible carbohydrates are limited or replaced with foods containing a higher percentage of proteins and fats.
The precise definition of low-carbohydrate diets varies greatly. The term is most commonly used to refer to ketogenic diets, i.e. diets that restrict carbohydrate intake sufficiently to cause ketosis like the Atkins diet, but some sources consider less restrictive variants to be low-carbohydrate as well.
Apart from obesity low-carbohydrate diets are often discussed as treatments for some other conditions, most notably diabetes and epilepsy, although these treatments still remain controversial and lack widespread support.History
In 1863 William Banting, an obese English undertaker and coffin maker, published "Letter on Corpulence Addressed to the Public" in which he described a diet for weight control giving up bread, butter, milk, sugar, beer and potatoes. His booklet was widely read, so much so that some people used the term "Banting" for the activity usually called "dieting.".
In 1967, Dr. Irwin Stillman published The Doctor's Quick Weight Loss Diet. The "Stillman Diet" is a high-protein, low-carbohydrate and low-fat diet. It is regarded as one of the first low-carbohydrate diets to become popular in the US. Other low-carbohydrate diets in the 1960's included Air Force Diet and the Drinking Man’s Diet. 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.
In 1972, Dr. Robert Atkins published Dr. Atkins Diet Revolution which advocated a low-carbohydrate diet he had successfully used in treating patients in the 1960s (having himself developed the diet from an unspecified article published in JAMA). The book met with some success but, because of research at that time suggesting risk factors associated with excess fat and protein, it was 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.
The concept of the glycemic index was invented in 1981 by Dr. David Jenkins. This concept evaluates foods according to their insulin demand -- with fast digesting simple carbohydrates having a high insulin demand and slower digesting complex carbohydrates such as grains having a lower insulin demand.
Low carb craze
In the 1990s Dr. Atkins published Dr. Atkins New Diet Revolution and other doctors began to publish books based on the same principles. This has been 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 some 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 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 more fiber/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. 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. In spite of the decline in popuarlity this diet trend has continued to quietly garner attention in the medical and nutritional science communities.
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). Therefore, there is no widely accepted definition of what precisely consistutes a low-carbohydrate diet. It is important to note that the level of carbohydrate consumption defined as low-carbohydrate by medical researchers may be different than the level of carbohydrate defined by diet advisors. 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 avoids ketosis, thus causing 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. Some researchers suggest that this causes excess dietary energy and body fat to be eliminated from the body. Although these diets remain controversial, clinical studies show that "Participants on a low-carbohydrate diet had more favorable overall outcomes at 1 year than did those on a conventional diet."
Low-carbohydrate diet advocates in general recommend reducing nutritive carbohydrates (commonly referred to as "net carbs," i.e. grams of 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 typically contains 15 grams of carbohydrate, almost entirely starch). By contrast, more standard nutrition guides typically recommend consumption levels in the neighborhood of 225-325 grams of carbohydrate per day (based on a 2000 calorie a day diet). Low-carbohydrate 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, "low-GI"/"low-GL" diets differ from "low-carb" diets in the following ways. First, low-carbohydrate diets treat all nutritive carbohydrates as having the same effect on metabolism and generally assume that their effect is independent of other nutrients in food. Low-GI/low-GL diets base their recommendations on the actual measured metabolic (glycemic) effects of the foods eaten. Second, as a practical matter, low-GI/low-GL diets generally do not recommend diets with glycemic loads low enough to minimize insulin production and induce ketosis, whereas low-carbohydrate diets generally do.
Another related diet type, the low-insulin-index diet, is very similar except that it is based on measurements of direct insulemic responses (i.e. the amount of insulin in the bloodstream) to food rather than glycemic response (the amount of glucose in the bloodstream). Although the diet recommendations mostly involve lowering nutritive carbohydrates, there are some low-carbohydrate foods that are discouraged as well (e.g. beef).
In contrast to these diets, based on evidence for risk of heart disease and obesity, the Institute of Medicine recommends that American and Canadian adults get between 40-65% of dietary energy from carbohydrates. The Food and Agriculture Organization and World Health Organization jointly recommend that national dietary guidelines set a goal of 55-75% of total energy from carbohydrates, but only 10% should be from Free sugars (their definition of simple carbohydrates).
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, respectively. 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 effect 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 and other health problems. 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.
This debate is on-going and no consensus currently exists.
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. Contrary to popular belief that low-carbohydrate diets damage the heart, one study found that women eating low-carbohydrate, high-fat/protein diets had the same or slightly less risk of coronary heart disease, compared to women eating high-carbohydrate, low-fat diets. Other studies have found possible benefits to individuals with diabetes, cancer, and autism. The ketogenic diet, with 90% of energy from fat and much of the remaining from protein, has been used since the 1920s to treat epilepsy. The introduction of modern anticonvulsant drugs, however, substantially restricted its use. Interestingly, there has very recently been renewed interest in use of the diet, especially in children.
A study conducted in 1965 at the Oakland (California) Naval Hospital used a diet of 1000 calories per day, high in fat and limiting carbohydrates to 10 grams (40 calories) 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 subsequent studies have shown similar results. Many advocates of low-carbohydrate diets have termed this the metabolic advantage of such diets although many experts dispute whether this is truly a general phenomenon. Among others, recent studies from Stanford University (2007) and Duke University (2005) comparing various diets seem to favor low-carbohydrate diets for both weight loss and health indicators.
Criticism and controversies
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.
Some critics argue that low-carbohydrate diets can inherently cause weakness or fatigue 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 several days as the body adjusts) and indeed most highly recommend exercise as part of a healthy lifestyle.
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. Some critics have argued or implied that "carbohydrates contain vitamins" and minerals but this suggestion is strictly false (by definition). While it is true that many foods that are rich in carbohydrates are also rich in vitamins and minerals, there are many low-carbohydrate foods that are similarly rich in vitamins and minerals. Also, the important vitamin B12 is only available in significant quantities from animal sources and not from vegetable sources.
It should be noted that, contrary to the recommendations of most diet guides, some individuals may 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).
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.
Some variants of low carbohydrate diets involve substantially lowered intake of dietary fiber which 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). Most advocates today argue that fiber is a "good" carbohydrate and in fact encourage a high-fiber diet.
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. However ketoacidosis, which is often confused with ketosis, is an acute medical condition caused by extreme fasting or as a symptom of untreated diabetes, and is not likely to be induced by a proper low-carbohydrate diet.
One of the occasional side effects of a ketogenic diet is a noticeable smell of ketones in the urine, perspiration, and breath. This is caused by the temporary metabolism of fatty-acid derived acetyl-coa into the ketone form, so that it may be released from the liver into the blood stream. The ketones are then re-assembled when they reach various body tissue's to form acetyl-coa again, which is used as the precursor to energy.
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