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Low-carbohydrate diets (also low carb, LC, or carbohydrate-restricted diets, CRD) are diets that restrict carbohydrate consumption relative to the average diet. Foods high in carbohydrates (e.g., sugar, bread, pasta) are limited, and replaced with foods containing a higher percentage of fat and protein (e.g., meat, poultry, fish, shellfish, eggs, cheese, nuts, and seeds), as well as low carbohydrate foods (e.g. spinach, kale, chard, collards, and other fibrous vegetables).

There are many definitions of low carb, which complicates research. Some definitions define the diet based on percent of calories from carbohydrates, whereas others are based on absolute weight of carbohydrates, frequently measured in grams.

Low-carbohydrate diets are controversial, with major organizations issuing contradictory recommendations. The lack of standardized definitions in research make it difficult to interpret long term safety and effect. In the short term, reduced carbohydrate diets are more effective than reduced fat diets. Both low and high carbohydrate diets have been associated with increased mortality, and reduced carbohydrate diets has been argued may mean missing out on health benefits from high-quality carbohydrate in legumes and fruit and vegetables. Claimed disadvantages of the diet might include halitosis, headache and constipation, and in general the potential adverse effects of the diet are under-researched, particularly for more serious possible risks such as for bone health and cancer incidence. Benefits include relative ease of sticking to the diet, and little confusion as to what is permitted or not in the diet.

Carbohydrate-restricted diets are more effective than low-fat diet s in helping achieve weight loss in the short term. . The hypothesis proposed by diet advocates that carbohydrate causes undue fat accumulation via the medium of insulin, and that low-carbohydrate diets have a "metabolic advantage", has been falsified by experiment.

It is not clear how low-carbohydrate dieting affects cardiovascular health; any benefit from HDL cholesterol might be offset by raised LDL cholesterol, which risks causing clogged arteries in the long term.

Carbohydrate-restricted diets are no more effective than a conventional healthy diet in preventing the onset of type 2 diabetes, but for people with type 2 diabetes they are a viable option for losing weight or helping with glycemic control. There is little evidence that low-carbohydrate dieting is helpful in managing type 1 diabetes. The American Diabetes Association recommends that people with diabetes should adopt a generally healthy diet, rather than a diet focused on carbohydrate or other macronutrients.

An extreme form of low-carbohydrate diet – the ketogenic diet – is established as a medical diet for treating epilepsy. Through celebrity endorsement it has become a popular weight-loss fad diet, but there is no evidence of any distinctive benefit for this purpose, and it may have a number of initial side effects. The British Dietetic Association named it one of the "top 5 worst celeb diets to avoid in 2018".

Macronutrient ratios
The macronutrient ratios of low-carbohydrate diets are not standardized. the conflicting definitions of "low-carbohydrate" diets have complicated research into the subject.

The American Academy of Family Physicians defines low-carbohydrate diets as diets that restrict carbohydrate intake to 20 to 60 grams per day, typically less than 20% of caloric intake. A 2016 review of low-carbohydrate diets classified diets with 50g of carbohydrate per day (less than 10% of total calories) as "very low" and diets with 40% of calories from carbohydrates as "mild" low-carbohydrate diets.

Carbohydrate intake
Low-carbohydrate diets can also be measured by the daily intake of carbohydrate, often as measured in grams.

Other definitions are based on the amount of daily consumed carbohydrates, such as Diet Doctor defining: 20–50 grams of consumed carbohydrate (~4–10% of consumed energy) as "moderate low carb" and 50–100 grams per day (10–20% of consumed energy) as "liberal low carb".

Foodstuffs
There is evidence that the quality, rather than the quantity, of carbohydrate in a diet is important for health, and that high-fiber slow-digesting carbohydrate-rich foods are healthful while highly-refined and sugary foods are less so. People choosing diet for health conditions should have their diet tailored to their individual requirements. For people with metabolic conditions, a diet with approximately 40-50% carbohydrate is recommended.

Most vegetables are low- or moderate-carbohydrate foods (in some low-carbohydrate diets, fiber is excluded because it is not a nutritive carbohydrate). Some vegetables, such as potatoes, carrots, maize (corn) and rice are high in starch. Most low-carbohydrate diet plans accommodate vegetables such as broccoli, spinach, kale, lettuce, cucumbers, cauliflower, peppers and most green-leafy vegetables.

Adoption and advocacy
The National Academy of Medicine recommends a minimum intake of 130 g of carbohydrate per day. The FAO and WHO similarly recommend that the majority of dietary energy come from carbohydrates. Low-carbohydrate diets are not an option recommended in the 2015-2020 edition of Dietary Guidelines for Americans, which instead recommends a low fat diet.

Carbohydrate has been wrongly accused of being a uniquely "fattening" macronutrient, misleading many dieters into compromising the nutritiousness of their diet by eliminating carbohydrate-rich food. Low-carbohydrate diet proponents emphasize research saying that low-carbohydrate diets can initially cause slightly greater weight loss than a balanced diet, but some studies suggest that such an advantage does not persist. In the long-term successful weight maintenance is determined by calorie intake, and not by macronutrient ratios.

The public has become confused by the way in which some diets, such as the Zone diet and the South Beach diet are promoted as "low-carbohydrate" when in fact they would more properly be termed "medium" carbohydrate diets.

Carbohydrate-insulin hypothesis
Low-carbohydrate diet advocates including Gary Taubes and David Ludwig have proposed a "carbohydrate-insulin hypothesis" in which carbohydrate is said to be uniquely fattening because it raises insulin levels and so causes fat to accumulate unduly. The hypothesis appears to run counter to known human biology whereby there is no good evidence of any such association between the actions of insulin and fat accumulation and obesity. The hypothesis predicted that low-carbohydrate dieting would offer a "metabolic advantage" of increased energy expenditure equivalent to 400-600&thinsp;kcal/day, in accord with the promise of the Atkin's diet: a "high calorie way to stay thin forever".

With funding from the Laura and John Arnold Foundation, in 2012 Taubes co-founded the Nutrition Science Initiative (NuSI), with the aim of raising over $200 million to undertake a "Manhattan Project for nutrition" and validate the hypothesis. Intermediate results, published in the American Journal of Clinical Nutrition did not provide convincing evidence of any advantage to a low-carbohydrate diet as compared to diets of other composition – ultimately a very low-calorie, ketogenic diet (of 5% carbohydrate) "was not associated with significant loss of fat mass" compared to a non-specialized diet with the same calories; there was no useful "metabolic advantage". In 2017 Kevin Hall, a NIH researcher hired to assist with the project, wrote that the carbohydrate-insulin hypothesis had been falsified by experiment. Hall wrote "the rise in obesity prevalence may be primarily due to increased consumption of refined carbohydrates, but the mechanisms are likely to be quite different from those proposed by the carbohydrate–insulin model".

Adherence
It has been repeatedly found that in the long-term, all diets with the same calorific value perform the same for weight loss, except for the one differentiating factor of how well people can faithfully follow the dietary programme. A study comparing groups taking low-fat, low-carbohydrate and Mediterranean diets found at six months the low-carbohydrate diet still had most people adhering to it, but thereafter the situation reversed: at two years the low-carbohydrate group had the highest incidence of lapses and dropouts. This may be due to the comparatively limited food choice of low-carbohydrate diets.

Body weight
Studies have shown that people losing weight with a low-carbohydrate diet, compared to a low-fat diet, have very slightly more weight loss initially, equivalent to approximately 100kcal/day, but that the advantage diminishes over time and is ultimately insignificant. The Endocrine Society state that "when calorie intake is held constant [...] body-fat accumulation does not appear to be affected by even very pronounced changes in the amount of fat vs carbohydrate in the diet."

Much of the research into low-carbohydrate dieting has been of poor quality and studies which reported large effects have garnered disproportionate attention in comparison to those which are methodologically sound. Higher quality studies tend to find no meaningful difference in outcome between low-fat and low-carbohydrate dieting. Low-quality meta-analyses have tended to report favourably on the effect of low-carbohydrate diets: a systematic review found that 9 out of 10 meta-analyses with positive conclusions were affected by publication bias.

Cardiovascular health
it was unclear whether low-carbohydrate dieting had any beneficial effect on cardiovascular health, though such diets can cause high LDL cholesterol levels, which carry a risk of atherosclerosis in the long term. Potential favorable changes in triglyceride and HDL cholesterol values should be weighed against potential unfavorable changes in LDL and total cholesterol values.

Some randomized control trials have shown that low-carbohydrate diets, especially very low-carbohydrate diets, perform better than low-fat diets in improving cardiometabolic risk factors in the long term, suggesting that low-carbohydrate diets are a viable option alongside low-fat diets for people at risk of cardiovascular disease.

There is only poor-quality evidence of the effect of different diets on reducing or preventing high blood pressure, but it suggests the low-carbohydrate diet is among the better-performing ones, while the DASH diet performs best.

Diabetes
There is little evidence for the effectiveness of low-carbohydrate diets for people with type 1 diabetes. For certain individuals, it may be feasible to follow a low-carbohydrate regime combined with carefully-managed insulin dosing, this can be hard to maintain and there are concerns about potential adverse health effects caused by the diet. In general, people with type 1 diabetes are advised to follow an individualized eating plan.

The proportion of carbohydrate in a diet is not linked to the risk of type 2 diabetes, although there is some evidence that diets containing certain high-carbohydrate items – such as sugar-sweetened drinks or white rice – are associated with an increased risk. Some evidence indicates that consuming fewer carbohydrate foods may reduce biomarkers of type 2 diabetes. A 2018 report on type 2 diabetes by the American Diabetes Association (ADA) and the European Association for the Study of Diabetes (EASD) found that a low-carbohydrate diet may not be as good as a Mediterranean diet for improving glycemic control, and that although having a healthy body weight is important, "there is no single ratio of carbohydrate, proteins, and fat intake that is optimal for every person with type 2 diabetes". There is no good evidence that low-carbohydrate diets are better than a conventional healthy diet, in which carbohydrates typically account for more than 40% of calories consumed. Low-carbohydrate dieting has no effect on the kidney function of people who have type 2 diabetes.

Limiting carbohydrate consumption generally results in improved glucose control, although without long-term weight loss. Low-carbohydrate diets can be useful to help people with type 2 diabetes lose weight, but that "no single approach has been proven to be consistently superior". According to the ADA, people with diabetes should be "developing healthy eating patterns rather than focusing on individual macronutrients, micronutrients, or single foods". They recommended that the carbohydrates in a diet should come from "vegetables, legumes, fruits, dairy (milk and yogurt), and whole grains", while highly-refined foods and sugary drinks should be avoided.

Exercise and fatigue
A low-carbohydrate diet has been found to reduce endurance capacity for intense exercise efforts, and depleted muscle glycogen following such efforts is only slowly replenished if a low-carbohydrate diet is taken. Inadequate carbohydrate intake during athletic training causes metabolic acidosis, which may be responsible for the impaired performance which has been observed.

Ketogenic diet
The ketogenic diet is a high-fat, low-carbohydrate diet used to treat drug-resistant childhood epilepsy. In the 2010s, it became a fad diet for people wanting to lose weight. Users of the ketogenic diet may not achieve sustainable weight loss, as this requires strict carbohydrate abstinence, and maintaining the diet is difficult. Possible risks of using the ketogenic diet over the long term may include kidney stones, osteoporosis, or increased levels of uric acid, a risk factor for gout.

Safety
Low-carbohydrate diets can be associated with increased mortality, as can high-carbohydrate diets.

research has paid insufficient attention to the potential adverse effects of carbohydrate restricted dieting, particularly for micronutrient sufficiency, bone health and cancer risk. One meta-analysis reported that adverse effects could include "constipation, headache, halitosis, muscle cramp and general weakness".

Ketosis induced by a low-carbohydrate diet has led to reported cases of ketoacidosis, a life-threatening condition. This has led to the suggestion that ketoacidosis should be considered a potential hazard of low-carbohydrate dieting.

In a comprehensive systematic review of 2018, Churuangsuk and colleagues reported that other case reports give rise to concerns of other potential risks of low-carbohydrate dieting including hyperosmolar coma, Wernicke's encephalopathy, optic neuropathy from thiamine deficiency, acute coronary syndrome and anxiety disorder.

Significantly restricting the proportion of carbohydrate in diet risks causing malnutrition, and can make it difficult to get enough dietary fiber to stay healthy.

As of 2014 it appeared that with respect to the risk of death for people with cardiovascular disease, the kind of carbohydrates consumed are important; diets relatively higher in fiber and whole grains lead to reduced risk of death from cardiovascular disease compared to diets high in refined-grains.

First descriptions
In 1797, John Rollo reported on the results of treating two diabetic Army officers with a low-carbohydrate diet and medications. A very low-carbohydrate, ketogenic diet was the standard treatment for diabetes throughout the nineteenth century.

In 1863, William Banting, a formerly 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 now called "dieting".

In the early 1900s Frederick Madison Allen developed a highly restrictive short term regime which was described by Walter R. Steiner at the 1916 annual convention of the Connecticut State Medical Society as The Starvation Treatment of Diabetes Mellitus. This diet was often administered in a hospital in order to better ensure compliance and safety.

Modern low-carbohydrate diets
Other low-carbohydrate diets in the 1960s included the Air Force diet and the Drinking Man's Diet. In 1972, Robert Atkins published Dr. Atkins Diet Revolution, which advocated the low-carbohydrate diet he had successfully used in treating people in the 1960s. The book was a publishing success, but was widely criticized by the mainstream medical community as being dangerous and misleading, thereby limiting its appeal at the time.

The concept of the glycemic index was developed in 1981 by David Jenkins to account for variances in speed of digestion of different types of carbohydrates. This concept classifies foods according to the rapidity of 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 slower one.

In 1992, Atkins published an update from his 1972 book, Dr. Atkins New Diet Revolution, and other doctors began to publish books based on the same principles. During the late 1990s and early 2000s, low-carbohydrate diets became some of the most popular diets in the US. By some accounts, up to 18% of the population was using one type of low-carbohydrate diet or another at the peak of their popularity. Food manufacturers and restaurant chains noted the trend, as it affected their businesses. Parts of the mainstream medical community have denounced low-carbohydrate diets as being dangerous to health, such as the AHA in 2001, and the American Kidney Fund in 2002.