Malnutrition occurs when an individual doesn't have the nutrients they need to function, which includes the amount of protein and calories they need to give their body energy, though it also means micronutrients like vitamins and minerals. When someone's diet is unbalanced, they might get enough calories without getting enough nutrients. Overnutrition is also a form of malnutrition and occurs when an individual takes in too many nutrients, causing their body's metabolism and functioning to become unbalanced. Different types of malnutrition present with different symptoms. Certain vitamin deficiencies have hallmark symptoms that might present even if an affected individual is taking in enough calories.
A reduced appetite is one of the most common signs of undernutrition. When the body isn't getting the calories it needs, the individual's metabolism will slow down. This means they will expend less energy and require fewer calories to maintain their body. Though this protects against starvation, long-term shifts in metabolism like this will make individuals more likely to gain weight in the long run, which is why crash dieting and highly restrictive diets are unlikely to result in long-term weight loss.
Affected individuals might lose interest in food and drink. It might become difficult to eat large portions because the stomach has reduced the amount of matter it can hold at one time. Trying to eat the same amount as the average person may cause intestinal distress. Children who have a reduced appetite due to malnutrition are at a higher risk of stunted growth. Children need to take in a certain number of calories for their bodies to develop healthily. An unexplained reduction in appetite can also be a sign of an underlying health problem.