Mortality
By persistently consuming fewer calories than the body needs to maintain itself, an individual can become underweight. One study has shown that having a BMI lower than 18, for women, is associated with significantly increased mortality from noncancer, non−cardiovascular disease causes. The results were the same when not accounting for those who were underweight because they might have been already sick or were smokers. However, the study focused solely on BMI and did not look specifically at diet.
Starvation
Severe calorie restriction may result in starvation, unless metabolism is also slowed by some means. The concept of a reduced calorie diet should not be confused with anorexia nervosa or other eating disorders. If such a pattern is repeated for prolonged periods of severe caloric restriction, the body may burn lean tissue (including but not limited to muscle and collagen) along with its remaining fat reserves. The combination of starvation and the associated lethargy and decreased physical activity can result in muscular atrophy which leads to lower quality of life.
Beyond using lean tissue as energy source, the presence of catabolic hormones, such as cortisol, and lack of anabolic ones, such as insulin, disrupts protein synthesis, amino acid uptake and weakens the immune system.
Lack of essential nutrients
When reducing calorie intake, intake of essential nutrients may also be reduced, especially fat-soluble vitamins, which require fat for proper absorption and other nutrients generally associated with high calorie foods, such as oils, nuts, meat and dairy products.
Abnormal hair growth
When undernourished, the body slows the growth rate of hair and nails.
Neuroglycopenia
Hypoglycemia can lead to neuroglycopenia.
Binge eating behavior
A calorie restriction diet can cause extreme hunger that may lead to binge eating behaviour.