Malnutrition is a term which covers problems of both under nutrition and over nutrition.
The body may adapt to a short period of under nutrition. Some nutrients such as fat-soluble vitamins are stored in the body and stores can be used up if the diet does not provide enough. There is evidence that people become more efficient at absorbing and using some nutrients if they have low intakes. However, after a while signs of a poor diet may start to appear, eg the person may become more prone to infections, and eventually may develop a specific deficiency disease. Energy Weight loss is an obvious sign of a diet too low in energy.