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In the first week or two of a low-carbohydrate diet, much of the weight loss comes from eliminating water retained in the body. A ketogenic diet is known to cause dehydration as an early, temporary side-effect.
Advocates of LCDs generally dispute any suggestion that such diets cause weakness or exhaustion (except in the first few weeks as the body adjusts), and indeed most highly recommend exercise as part of a healthy lifestyle. A large body of evidence stretching back to the 1880s shows that physical performance is NOT negatively affected by ketogenic diets once a person has been accustomed to such a diet.
Arctic cultures, such as the Inuit, were found to lead physically demanding lives consuming a diet of about 15–20% of their calories from carbohydrates, largely in the form of glycogen from the raw meat they consumed. The Eskimo paradox of eating high fat, high protein and low CHO diet and low incidence of CAD is well known to Cardiologists.
However, LCDs do deteriorate anaerobic performance such as strength-training or sprint-running because these processes rely on glycogen for fuel.
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 treat the words 'vegetable' and 'carbohydrate' as synonymous). While some fruits may contain relatively high concentrations of sugar, most are largely water and not particularly calorie-dense. Thus, in absolute terms, even sweet fruits and berries do not represent a significant source of carbohydrates in their natural form, and also typically contain a good deal of fiber which attenuates the absorption of sugar in the gut. Lastly, most of the sugar in fruit is fructose, which has a reported negligible effect on insulin levels in obese subjects.
Most vegetables are low- or moderate-carbohydrate foods (in the context of these diets, fiber is excluded because it is not a nutritive carbohydrate). Some vegetables, such as potatoes and carrots, have high concentrations of starch, as do corn and rice. Most low-carbohydrate diet plans accommodate vegetables such as broccoli, spinach, cauliflower, and peppers. The Atkins diet recommends that most dietary carbs come from vegetables. Nevertheless, debate remains as to whether restricting even just high-carbohydrate fruits, vegetables, and grains is truly healthy.
Contrary to the recommendations of most low-carbohydrate diet guides, some individuals may choose to avoid vegetables altogether to minimize carbohydrate intake.
The major LCD guides generally recommend multivitamin and mineral supplements as part of the diet regimen, which may lead some to believe 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 it may require extra vitamins and minerals. This is because the body goes through a temporary "shock" if the diet is changed to low-carbohydrate quickly. This does not indicate that LCD is nutritionally deficient.
The restriction of starchy plants, by definition, severely limits the dietary intake of microbiota accessible carbohydrates(MACs) and may negatively affect the microbiome in ways that contribute to disease. Starchy plants, in particular, are a main source of resistant starch — an important dietary fiber with strong probiotic properties. Resistant starches are not digestible by mammals and are fermented and metabolized by gut flora into short chain fatty acids, which are well known to offer a wide range of health benefits.
Resistant starch consumption has been shown to improve intestinal/colonic health, blood sugar, glucose tolerance, insulin-sensitivity and satiety. "One of the major developments in our understanding of the importance of carbohydrates for health in the past twenty years has been the discovery of resistant starch."