Many popular versions of this type of diet have evolved over the years, but the Atkins diet is probably the most well known. Low-carbohydrate diets can provide as little as 5 to 10 percent of calories from carbohydrates, although some can provide up to 40 percent of calories from carbohydrates. (The Dietary Guidelines for Americans 2005 recommends 45 to 65 percent total calories from carbohydrates.)
Low-carbohydrate diets are typically high in protein and fat. Breads, cereals, grains, starchy vegetables (like potatoes), and most fruits -- all of which contain carbohydrates -- are restricted, so there are fewer foods from which to choose on this diet. When cutting back on these carbohydrates, the proportions of protein and fat in the diet tend to change as a result.
Ultimately, a person on a low-carbohydrate diet must take care to reduce overall calories; otherwise, it won't result in weight loss. It is possible that a high-protein diet may lower calories overall because protein, in particular, decreases the feeling of hunger, which means you're likely to eat less.
Carbohydrates are the primary fuel for the brain, nerves, and lungs. On low-carbohydrate diets, the body compensates for the lack of fuel by producing acids called ketones that are made from body fat. Besides being used for energy, ketones are also excreted in the urine. Proponents of low-carbohydrate diets claim that using ketones for energy and excreting them in urine burns calories and, thus, facilitates weight loss. In reality, ketones are only used and excreted in small quantities, at the rate of 100 or fewer calories per day.
Initially, large weight losses can occur rapidly on a low-carbohydrate diet; within the first couple weeks, an individual may lose up to 10 pounds. This kind of weight loss can make the diet appealing; however, only a small part of the weight lost during this time is body fat. Muscle protein, which is more dense and heavier than fat, is broken down as fuel for the brain, nerves, and lungs.
The carbohydrate deficiency causes large amounts of salt and water to be lost from the body. In addition, high levels of ketones in the blood can act as an appetite suppressant. Together, these result in a temporary weight loss. When normal eating patterns are resumed, the water and muscle will be regained.
In a comparison of low-fat and low-carbohydrate diets, weight loss at three and six months was about twice as much in the low-carbohydrate group as in the low-fat group. However, at the end of one year there was no difference in weight loss between the two groups, suggesting that long-term adherence to a low-carbohydrate diet may be difficult.
Research indicates that low-carbohydrate diets result in a greater improvement in triglycerides and HDL cholesterol. However, low-fat diets are typically better for lowering LDL cholesterol.
Because low-carbohydrate diets are generally high in protein from animal sources, which are also high in saturated fat and cholesterol, and because they restrict carbohydrates, which limit the only sources of dietary fiber that play a role in lowering blood cholesterol, low-carbohydrate diets are not designed to lower LDL cholesterol. Further research is necessary to determine the long-term effects of a low-carbohydrate diet on heart disease.
A low-carbohydrate diet, and the high levels of ketones it can produce, is not safe for everyone. Many of the vitamins and minerals that carbohydrates provide are essential for pregnant women and their developing babies. For some, especially diabetics, a low-carbohydrate diet can cause blood-sugar levels to fall very low, known as hypoglycemia, which can induce dizziness, fatigue, weakness, irritability, and fainting spells. Cutting down on carbohydrates should be done under your doctor's guidance, especially if you are on any medications, to avoid the potentially dangerous consequences of low blood sugar.
The low glycemic index diet separates carbs into “good” and “bad” forms. The next page explains the low glycemic index diet, including the as-yet inconclusive research on it.