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.

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). While most fruits tend to have a significant concentration of sugar, most vegetables are 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, the exception not the rule. Most low-carbohydrate diet plans accommodate vegetables such as broccoli, spinach, cauliflower, avocado and peppers. 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 in order to minimize carbohydrates. It is more likely that such a diet could be nutritionally deficient and is strongly discouraged. Low-carbohydrate vegetarianism is also 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, is incomplete. Although many dietary carbohydrates do break down into glucose, most of that glucose does not remain in the bloodstream for long. Its presence stimulates the beta cells in the pancreas to release insulin which has the effect of causing about 2/3 of body cells to take in glucose, and to cause fat cells to take in fatty acids and store them. As the blood glucose level falls, the amount of insulin released is reduced; the entire process is completed in non-diabetics in an hour or two after eating. It is generally agreed that 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 require more insulin production and release. There is some suspicion that repeated large boluses of carbohydrates in the diet, and the resulting increased release of insulin may have some connection with the development of Type 2 diabetes. There has been no credible experimental evidence supporting this view, however.

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 an otherwise adequate 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.[118] This is caused by the presence of larger than usual amounts of the three ketone bodies normally produced during fat metabolism. One of the ketone bodies is actually a ketone, acetone, and it is released via the lungs. It has a characteristic smell, often said to resemble fruit or nail polish remover. In most cases, periodic ketosis (as occurs between widely separated meals) does not cause a noticeable odor. When the other two ketone bodies are produced in large quantities, the resulting condition is called ketoacidosis, and can be quite dangerous as even small changes in blood pH are incompatible with life.