People who don't get enough food often experience hunger, and over the long term this can lead to malnutrition. But someone can become malnourished for reasons that have nothing to do with hunger. Even people who have plenty to eat may be malnourished if they don't eat foods that provide the right nutrients, vitamins, and minerals.

Some diseases and conditions prevent people from digesting or absorbing their food properly. For example:
Someone with celiac disease has intestinal problems that are triggered by a protein called gluten, which is found in wheat, rye, and barley.
Kids with cystic fibrosis have trouble absorbing nutrients because the disease affects the pancreas, an organ that normally produces enzymes necessary for digestion.
Someone who doesn't get enough of one specific nutrient has a nutritional deficiency, a form of malnutrition (although it doesn't necessarily mean the person will become seriously ill). The most common nutritional deficiency in the world is iron deficiency, which can lead to anemia.

The signs and symptoms of malnutrition depend on which nutritional deficiencies a person has, although they can include fatigue (tiredness) and low energy, dizziness, poor immune function (which can harm the body's ability to fight off infections), slowed reaction times and trouble paying attention, underweight, poor growth, muscle weakness, bloated stomach, problems with organ function, problems learning.

If a pregnant woman is malnourished, her child may weigh less at birth and have a lower chance of survival.

Vitamin A deficiency from malnutrition is the chief cause of preventable blindness in the developing world, and kids with severe vitamin A deficiency have a greater chance of getting sick or dying from infections such as diarrhea or measles.

Iodine deficiency can cause mental retardation and delayed development. Iron deficiency in infancy can delay development and make older kids less active and less able to concentrate. Teens who are malnourished often have trouble keeping up in school.