Loss of Muscle

• Muscle is composed of mostly water, which means that low-carbohydrate diets usually cause a significant loss of muscle long before the fat in the body is affected. Glycogen, the substance that makes it possible for the body (including muscles) to work effectively, needs a constant supply of carbohydrates to be produced. In low-carbohydrate diets, not only is glycogen no longer produced effectively but the amount already present in muscles (where glycogen is normally stored) is broken down and used by the body to produce energy. This energy is meant to replace the one that carbohydrates are no longer supplying. Loss of muscle slows down metabolism and causes a flabby appearance.
  
  

Low Energy

• Because the energy of the body comes from glycogen, people on low-carbohydrate diets tend to have less overall energy. This leads to quicker muscle fatigue and makes exercise more difficult. This is especially troublesome for athletes, but even those with a standard exercise routine will see themselves cutting down on the intensity and amount of exercise performed, leading to lower calorie expenditure and a slower metabolism. People who are on low-carbohydrate diets for long periods of time may also suffer from slight memory loss and concentration problems, as both are directly connected with the use of glycogen by the brain.
  
  

Weight Loss That Is Only Temporary

• Much of the weight loss during a low-carbohydrate diet is water, which comes mainly from muscle. Because water weight is regained as soon as liquids are ingested, the lower number on the scale doesn't necessarily mean that real weight (fat weight) has been lost. Also, low-carbohydrate diets are difficult to maintain for long periods of time, which means most people eventually return to eating all food groups. When this happens, the weight is usually regained quickly. Finally, because of the muscle mass lost during the dieting period, metabolism slows down, making it more difficult to lose weight as time goes by.
  
  

High Cholesterol

• Most low-carbohydrate diets advocate eating lots of protein. This is for a variety of reasons. First, proteins are more satisfying than carbs and they provide a feeling of satiety that is meant to starve off the cravings that come from not consuming sugars of any kind. Also, proteins provide a somewhat constant level of energy, while carbs are meant for quicker use. Unfortunately, most high-protein foods are also high in fat and cholesterol. Even lean meats, eggs and low-fat dairy products are still high in cholesterol and can lead to heart disease, kidney stones, certain cancers, stroke and other health issues if consumed in excess for long periods of time. Low-carb diets are difficult to follow if you're a vegetarian, as the only complete protein in the vegetable world is soy.
  
  

Digestive Problems

• Low carbohydrate diets are usually low in fiber, since fiber is mostly present in fruits, vegetables and whole grains, which are drastically reduced (and in some cases eliminated) in a low-carb diet. Lack of fiber in the diet can lead to constipation, hemorrhoids and an increased risk of colon cancer and heart disease.