Malnutrition is a general term to describe deficiencies in nutrition, generally of several nutrients at one time. It is common among the poor and those with eating disorders such as Anorexia nervosa, Bulimia , OSFED and EDNOS. It is a major health problem in infants and children, even in developed countries.
Lack of sufficient calories or macronutrients is usually the major component of malnutrition, but this alone will usually not result in major health problems. Often, people without enough food will also not be receiving one or more micronutrients (vitamins and minerals). Malnutrition is not uncommon in people of high body mass, and is the result of a diet that is often too high in fats and sugars and too low in vegetables, protien, and fruits.
Some of the resulting conditions from malnutrition are merely those related to specific vitamin deficiencies. However, prolonged malnutrition can have permanent effects and is particularly dangerous in the young.
Those who have been severely malnourished for a period of more than one week are at serious risk of refeeding syndrome, a condition of which a shift in electrolytes caused by sufficient or increased intake, leads the body to shift out of ketosis, causing hypophosphatimia, hypomagnesemia, thiamine deficiency, hypokalemia. These condtions cause water retention, confusion, weakness, fatigue, and in more serious cases, tachycardia, seizures, heart arrhythmias, coma and heart failure.
Moreover, a very vitamin deficient individual may not react immediately to a better diet the way a person who is suffering from a single vitamin deficiency will. For example, an individual with scurvy will recover almost immediately when given vitamin C. A severely malnourished person may recover slowly, if at all.