Low-carbohydrate diet

Low-carbohydrate diets or low-carb diets are dietary programs that restrict carbohydrate consumption usually for weight control or for the treatment of obesity. Foods high in digestible carbohydrates (e.g. bread, pasta) are limited or replaced with foods containing a higher percentage of proteins and fats (e.g., meat, soy products) and often other foods low in carbohydrates (e.g., green leafy vegetables).

The American Academy of Family Physicians provides the following definition of low-carbohydrate diets.

Low-carbohydrate diets restrict caloric intake by reducing the consumption of carbohydrates to 20 to 60 g per day (typically less than 20 percent of the daily caloric intake). The consumption of protein and fat is increased to compensate for part of the calories that formerly came from carbohydrates.

This definition is typical of most sources although no universally recognized definition has been established. Such diets are generally ketogenic (i.e. they restrict carbohydrate intake sufficiently to cause ketosis) for example, the induction phase of the Atkins diet. Some sources, though, 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, other than for intractable epilepsy in children, these treatments still remain controversial and lack widespread support.

History

Beginnings

Some anthropologists believe that early humans were hunter-gatherers consuming diets high in both protein and fat and mostly low in nutritive carbohydrates (although their diets would have been high in fiber). Indeed some isolated societies exist still today which continue to consume these types of diets. The advent of agriculture brought about the rise of civilization and the gradual rise of carbohydrate levels in human diets. The modern age has seen a particularly steep rise in refined carbohydrate levels in so-called Western societies.

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 1960s 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. Later that decade, Walter Voegtlin and Dr. Herman Tarnower published books advocating the Stone age diet and Scarsdale diet, respectively, each meeting with moderate success.

The concept of the glycemic index was invented in 1981 by Dr. David Jenkins. This concept evaluates foods according to their effect on blood sugar levels -- with fast digesting simple carbohydrates causing a sharper increase and slower digesting complex carbohydrates such as whole grains a shallower one.

Low-carb diets since the 1990s

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.

After 2004 the popularity of this diet trend began to wane significantly although it still remains quite popular. In spite of the decline in popularity 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, the Go Lower Diet and the South Beach Diet). Therefore, there is no widely accepted definition of what precisely constitutes 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 this theory remains, at best, controversial.

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, the U.S. Institute of Medicine recommends a minimum intake of 130 grams of carbohydrate per day (the FAO and WHO similarly recommend that the majority of dietary energy come from carbohydrates). 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.

Although low-carbohydrate diets are most commonly discussed as a weight-loss approach some experts have proposed using low-carbohydrate diets to mitigate or prevent diseases ranging from diabetes to cancer to epilepsy. Indeed, it has been argued by some low-carbohydrate proponents and others that it is the rise in carbohydrate consumption, especially refined carbohydrates, that has caused the epidemic levels of many diseases in modern society.

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).

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) from fats and glucose (gluconeogenesis) from proteins, respectively. Some 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 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 maintaining a metabolic state where glucose is the primary energy source in the body 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 it is argued that, 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.

Scientific research

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. Researchers and other experts have published articles and studies that run the gamut from promoting the safety and efficacy of these diets to questioning their long-term validity to outright condemning them as dangerous. Until recently a significant criticism of the diet trend was that there were no studies that evaluated the effects of the diets beyond a few months. However, studies are emerging which evaluate these diets over much longer periods, controlled studies as long as two years and survey studies as long as two decades.

A meta-analysis of randomized controlled trials by the Cochrane Collaboration in 2002 concluded that fat-restricted diets are no better than calorie restricted diets in achieving long term weight loss in overweight or obese people. A 2003 meta-analysis that included a randomized controlled trials published after the Cochrane review found that "low-carbohydrate, non-energy-restricted diets appear to be at least as effective as low-fat, energy-restricted diets in inducing weight loss for up to 1 year.

Potential favorable changes in triglyceride and high-density lipoprotein cholesterol values should be weighed against potential unfavorable changes in low-density lipoprotein cholesterol values when low-carbohydrate diets to induce weight loss are considered. A 2008 systematic review of randomized controlled studies that compared low-carbohydrate diets to low-fat/low-calorie diets and found that measurements of weight, HDL cholesterol, triglyceride levels and systolic blood pressure were significantly better in groups that followed low-carbohydrate diets. The authors of this review also found a higher rate of attrition in groups with low-fat diets, and concluded that "evidence from this systematic review demonstrates that low-carbohydrate/high-protein diets are more effective at 6 months and are as effective, if not more, as low-fat diets in reducing weight and cardiovascular disease risk up to 1 year," but they also called for more long-term studies.

In addition to research on the efficacy of the diets some research has directly addressed other areas of health affected by low-carbohydrate diets. For example, 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. Modern effective anticonvulsant drugs mean that it is now used only for children with difficult-to-control epilepsy, and there may be cause for concern over issues such as stunted growth for these children.

Major governmental and medical organizations

Although opinions regarding low-carbohydrate diets vary greatly through the medical and nutritional science communities, major government bodies as well as major medical and nutritional associations have generally opposed this nutritional regimen. In recent years, however, some of these same organizations have gradually begun to relax their opposition to the point that some have even voiced cautious support for low-carbohydrate diets. The following are official statements from some of these organizations.

American Academy of Family Physicians

The AAFP released a discussion paper on the Atkins Diet specifically in 2006. Although the paper expresses reservations about the Atkins plan they acknowledge it as a legitimate weight loss approach.

American Diabetes Association

The ADA revised their Nutrition Recommendations and Interventions for Diabetes in 2008 to acknowledge low-carbohydrate diets as a legitimate weight-loss plan. The recommendations fall short of endorsing low-carbohydrate diets as a long-term health plan nor do they given any preference to these diets. Nevertheless, this is perhaps the first statement of support--albeit for the short-term--by one of the foremost medical organizations. In its 2009 publication of Clinical Practice Recommendations, The ADA again reaffirmed its acceptance of carbohydrate-controlled diets as an effective treatment for short-term (up to one year) weight loss among obese people suffering from type two diabetes.

American Dietetic Association

As of 2003 in commenting on a study in the Journal of the American Medical Association, a spokesperson for the American Dietetic Association reiterated the association's belief that "there is no magic bullet to safe and healthful weight loss." The Association specifically endorses the high-carbohydrate diet recommended by the National Academy of Sciences.

American Heart Association

The official statement from the AHA regarding these diets states categorically that the association "doesn't recommend high-protein diets." A science advisory from the association further states the associations belief that these diets are "associated with increased risk for coronary heart disease." The AHA has been one of the most adamant opponents of low-carbohydrate diets. Notably, though, Dr. Robert Eckel, past president of the association, was quoted as saying that "a low-carb approach is consistent with heart association guidelines so long as there are limitations on the kinds of saturated fats often consumed by people on the Atkins diet."

Australian Heart Foundation

The position statement by the Heart Foundation regarding low-carbohydrate diets states that "the Heart Foundation does not support the adoption of VLCARB diets for weight loss." Although the statement clearly recommends against use of low-carbohydrate diets it makes clear that their major concern is saturated fats as opposed to carbohydrate restriction and protein. Moreover, other statements suggest that their position might be re-evaluated in the event of more evidence from longer-term studies.

Food Standards Agency (UK)

The consumer advice statements of the FSA regarding low-carbohydrate diets state that "rather than avoiding starchy foods, it's better to try and base your meals on them." They further state concerns regarding fat consumption in low-carbohydrate diets.

Heart & Stroke Foundation (Canada)

The official position statement of the Heart & Stroke Foundation states "Do not follow a low carbohydrate diet for purposes of weight loss." They state concerns regarding numerous health risks particularly those related to high consumption of "saturated and trans fats".

National Board of Health and Welfare (Sweden)

In 2008, the Socialstyrelsen in Sweden altered its standing regarding low-carbohydrate diets. Although formal endorsement of this regimen has not yet appeared, the government has given its formal approval for using carbohydrate-controlled diets for medically supervised weight loss.

U.S. Department of Health and Human Services

The HHS issues consumer guidelines for maintaining heart health which state regarding low-carbohydrate diets that "they're not the route to healthy, long-term weight management." Nevertheless HHS has issued some statements indicating wavering on this position.

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.

Exercise

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.

Vegetables and Fruits

Many critics argue that low-carbohydrate diets inherently require minimizing vegetable and fruit consumption which in turn robs the body of important nutrients. Some critics imply or explicitly argue that vegetables and fruits are inherently all heavily concentrated sources of carbohydrates (so much so that some sources literally treat the words vegetable and carbohydrate as synonymous). This is, in fact, false. It is true that most fruits tend to have a significant concentration of sugar meaning that they can only be eaten in small quantities. But most vegetables are, in fact, low- or moderate-carbohydrate foods (note that in the context of these diets fiber is excluded because it is not a nutritive carbohydrate). The vegetables that the average person tends to enjoy the most, potatoes, rice, maize (corn), and others, are typically those that have high concentrations of starch but these are, in reality, the exception not the rule. Most low-carbohydrate diet plans easily accommodate most vegetables such as broccoli, spinach, cauliflower, avocado, peppers, etc. Nevertheless debate remains as to whether restricting even just high-carbohydrate fruits, vegetables, and grains is truly healthy.

It should be noted that, contrary to the recommendations of most low-carbohydrate 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 and is strongly discouraged. It should also be noted that low-carbohydrate vegetarianism can be and is practiced successfully.

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. 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 (and bacterial) sources and not from vegetable sources.

Glucose availability

A common argument in favor of high-carbohydrate diets is that most carbohydrates break down readily into glucose in the bloodstream and, therefore, the body does not have to work as hard to get its energy in a high-carbohydrate diet as a low-carbohydrate diet. This argument, by itself, omits certain salient details. Although most carbohydrates do break down readily into glucose, most of that glucose does not remain in the bloodstream for long. The excess glucose is toxic to the body and so insulin is produced in the pancreas to cause the body to convert most of the glucose to glycogen and fat for storage. So, in reality, some of the glucose used by the body on a high-carbohydrate diet must be explicitly converted from non-carbohydrate sources just as a low-carbohydrate diet produces both ketones and small amounts of glucose from non-carbohydrate sources. It is generally agreed that the low-carbohydrate diets tax the liver more in these processes than high-carbohydrate diets but there is significant disagreement as to how large the difference is and even more disagreement as to how much of a burden the liver can handle safely. It is also generally agreed that the high-carbohydrate diets tax the pancreas more and that higher insulin production (characteristic of high-carbohydrate diets) can be a risk factor for diabetes although the exact risks and burden on the pancreas are highly debated.

Other controversies

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 tissues to form acetyl-CoA again, which is used as the precursor to energy.