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Low-carbohydrate diets or low-carb diets are dietary programs that restrict carbohydrate consumption, often for the treatment of obesity or diabetes. Foods high in easily digestible carbohydrates (e.g. sugar, bread, pasta) are limited or replaced with foods containing a higher percentage of fats and moderate protein (e.g., meat, poultry, fish, shellfish, eggs, cheese, nuts and seeds) and other foods low in carbohydrates (e.g., most salad vegetables such as spinach, kale, chard and collards), although other vegetables and fruits (especially berries) are often allowed. The amount of carbohydrate allowed varies with different low-carbohydrate diets.
Such diets are sometimes 'ketogenic' (i.e., they restrict carbohydrate intake sufficiently to cause ketosis). The induction phase of the Atkins diet is ketogenic.
The term "low-carbohydrate diet" is generally applied to diets that restrict carbohydrates to less than 20% of caloric intake, but can also refer to diets that simply restrict or limit carbohydrates to less than recommended proportions (generally less than 45% of total energy coming from carbohydrates).
Low-carbohydrate diets are used to treat or prevent some chronic diseases and conditions, including cardiovascular disease, metabolic syndrome, auto-brewery syndrome, high blood pressure and diabetes.
Low-carbohydrate diets 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 less than 20 g of "net carbs" per day, at least in the early stages of dieting (for comparison, a single slice of white bread typically contains 15 g of carbohydrate, almost entirely starch). By contrast, the U.S. Institute 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.
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, including diabetes, metabolic disease and epilepsy. Some low-carbohydrate proponents and others argue that the rise in carbohydrate consumption, especially refined carbohydrates, caused the epidemic levels of many diseases in modern society, including metabolic disease and type 2 diabetes.
A category of diets is known as low-glycemic-index diets (low-GI diets) or low-glycemic-load diets (low-GL diets), in particular the Low GI Diet. In reality, low-carbohydrate diets can also be low-GL diets (and vice versa) depending on the carbohydrates in a particular diet. 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 their effect is predictable. Low-GI/low-GL diets are based on the measured change in blood glucose levels in various carbohydrates, these vary markedly in laboratory studies. The differences are due to poorly understood digestive differences between foods. However, as foods influence digestion in complex ways (e.g. both protein and fat delay absorption of glucose from carbohydrates eaten at the same time) it is difficult to even approximate the glycemic effect (e.g. over time or even in total in some cases) of a particular meal.
The low-insulin-index diet, is similar, except 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 such diet recommendations mostly involve lowering nutritive carbohydrates, some low-carbohydrate foods are discouraged, as well (e.g. beef). Insulin secretion is stimulated (though less strongly) by other dietary intake. Like glycemic-index diets, predicting the insulin secretion from any particular meal is difficult, due to assorted digestive interactions and so differing effects on insulin release.
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 some research has been done throughout the 20th century, most directly relevant scientific studies have occurred in the 1990s and early 2000s. 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. A significant criticism of the diet trend was that no studies evaluated the effects of the diets beyond a few months. However, studies emerged which evaluate these diets over much longer periods, controlled studies as long as two years and survey studies as long as two decades.
A systematic review published in 2014 included 19 trials with a total of 3,209 overweight and obese participants, some with diabetes. The review included both extreme low carbohydrate diets high in both protein and fat, as well as less extreme low carbohydrate diets that are high in protein but with recommended intakes of fat. The authors found that when the amount of energy (kilojoules/calories) consumed by people following the low carbohydrate and balanced diets (45 to 65% of total energy from carbohydrates, 25 to 35% from fat and 10 to 20% from protein) was similar, there was no difference in weight loss after 3 to 6 months and after 1 to 2 years in those with and without diabetes. For blood pressure, cholesterol levels and diabetes markers there was also no difference detected between the low carbohydrate and the balanced diets. The follow-up of these trials was no longer than two years, which is too short to provide an adequate picture of the long term risk of following a low carbohydrate diet.
A 2003 meta-analysis that included randomized controlled trials 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 one year" A 2007 JAMA study comparing the effectiveness of the Atkins low-carb diet to several other popular diets concluded, "In this study, premenopausal overweight and obese women assigned to follow the Atkins diet, which had the lowest carbohydrate intake, lost more weight and experienced more favourable overall metabolic effects at 12 months than women assigned to follow the Zone, Ornish or LEARN diets" A July 2009 study of existing dietary habits associated a low-carbohydrate diet with obesity, although the study drew no explicit conclusion regarding the cause: whether the diet resulted in the obesity or the obesity motivated people to adopt the diet. A 2013 meta-analysis that included only randomized controlled trials with one year or more of follow-up found, "Individuals assigned to a very low carbohydrate ketogenic diet achieve a greater weight loss than those assigned to a low fat diet in the long term" In 2013, after reviewing 16,000 studies, Sweden's Council on Health Technology Assessment concluded low-carbohydrate diets are more effective as a means to reduce weight than low-fat diets, over a short period of time (six months or less). However, the agency also concluded, over a longer span (12–24 months), no differences occur in effects on weight between strict or moderate low-carb diets, low-fat diets, diets high in protein, Mediterranean diet or diets aiming at low glycemic indices.
In one theory, one of the reasons people lose weight on low-carbohydrate diets is related to the phenomenon of spontaneous reduction in food intake. Carbohydrate restriction may help prevent obesity and type 2 diabetes, as well as atherosclerosis.