A deficiency describes a lack of a particular nutrient and can be a sign of malnutrition caused by not eating the correct balance of food. Examples of nutrient deficiencies are anaemia (iron, folate or vitamin B12 deficiency) and rickets (caused by lack of vitamin D and calcium). Usually a blood test can identify if the body is lacking in a particular nutrient, but signs such as fatigue and changes in mood and body weight can also be signs.