Low-carbohydrate diets 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 is a lack of standardization of how much carbohydrate low-carbohydrate diets must have, and this has complicated research. One definition, from the American Academy of Family Physicians, specifies low-carbohydrate diets as having less than 20% carbohydrate content.
There is no good evidence that low-carbohydrate dieting confers any particular health benefits apart from weight loss, where low-carbohydrate diets achieve outcomes similar to other diets, as weight loss is mainly determined by calorie restriction and adherence.
An extreme form of low-carbohydrate diet called the ketogenic diet was first established as a medical diet for treating epilepsy. It became a popular fad diet for weight loss through celebrity endorsement, but there is no evidence of any distinctive benefit for this purpose and the diet carries a risk of adverse effects, with the British Dietetic Association naming it one of the "top five worst celeb diets to avoid" in 2018.
Definition and classification
The macronutrient ratios of low-carbohydrate diets are not standardized. As of 2018 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 (g) 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. The UK National Health Service recommend that "carbohydrates should be the body's main source of energy in a healthy, balanced diet."
Like other leafy vegetables, curly kale is a food that is low in carbohydrates.
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 daily average of 130 g of carbohydrates 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 any such 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.
Low-carbohydrate diet advocates including Gary Taubes and David Ludwig have proposed a "carbohydrate-insulin hypothesis" in which carbohydrates are said to be uniquely fattening because they raise insulin levels and cause 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, fat accumulation, and obesity. The hypothesis predicted that low-carbohydrate dieting would offer a "metabolic advantage" of increased energy expenditure equivalent to 400-600 kcal(kilocalorie)/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 (National Institutes of Health) 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."
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.
In the short and medium term, people taking a low-carbohydrate diet can experience more weight loss than people taking a low-fat diet. Such people 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 stated 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 comparing low-fat vs. 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. A 2018 review said "higher-quality meta-analyses reported little or no difference in weight loss between the two diets." Low-quality meta-analyses have tended to report favourably on the effect of low-carbohydrate diets: a systematic review reported that 8 out of 10 meta-analyses assessed whether weight loss outcomes could have been affected by publication bias, and 7 of them concluded positively. A 2017 review concluded that a variety of diets, including low-carbohydrate diets, achieve similar weight loss outcomes, which are mainly determined by calorie restriction and adherence rather than the type of diet.
Low-carbohydrate dieting tends to raise levels of LDL cholesterol, but it is unclear how this might effect cardiovascular health. 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 (Dietary Approaches to Stop Hypertension) performs best.
There is limited 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 in 2019 consensus report on nutrition therapy for adults with diabetes and prediabetes the American Diabetes Association (ADA) states "Reducing overall carbohydrate intake for individuals with diabetes has demonstrated the most evidence for improving glycemia (blood sugar) and may be applied in a variety of eating patterns that meet individual needs and preferences.", it also states that reducing overall carbohydrate intake with low- or very low- carbohydrate eating plans is a viable approach. While other sources states that 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 "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. The ADA also wrote that "reducing overall carbohydrate intake for individuals with diabetes has demonstrated the most evidence for improving glycemia and may be applied in a variety of eating patterns that meet individual needs and preferences." For individuals with type 2 diabetes who can't meet the glycemic targets or where reducing anti-glycemic medications is a priority, the ADA says that low or very-low carbohydrate diets are a viable approach.
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.
The ketogenic diet is a high-fat, low-carbohydrate diet used to treat drug-resistant childhood epilepsy. The premise of the ketogenic diet for weight loss is that if the body is deprived of glucose obtained from carbohydrate foods, it will produce energy from stored fat.
In the 2010s, the ketogenic diet became a fad diet for people wanting to lose weight. According to dietitian Keri Gans, the "keto diet" was, disappointingly, the most popularly searched for diet on Google in 2018. In 2021, it was ranked by U.S. News & World Report 37th (tie) out of 39 best diets overall, and 4th in best fast weight-loss diets.
Users of the ketogenic diet may not achieve sustainable weight loss, as this requires strict carbohydrate abstinence, and maintaining the diet is difficult. Side effects may include constipation, high cholesterol, growth slowing, acidosis, and kidney stones.
It has been hypothesized that some people have an atypical metabolism, and would therefore benefit metabolically from adopting a ketogenic diet, but as of 2020 there had been no long-term research into this.
High and low-carbohydrate diets that are rich in animal-derived proteins and fats may be associated with increased mortality. On the contrary, with plant-derived proteins and fats, there may be a decrease of mortality.
As of 2018, research has paid insufficient attention to the potential adverse effects of carbohydrate restricted dieting, particularly for micronutrient sufficiency, bone health and cancer risk. One low-quality meta-analysis reported that adverse effects could include "constipation, headache, halitosis, muscle cramps 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.