There are not people who are not getting nor have the right amount of calories for their body is called underweight. Those who are not taking the right amount of vitamins and minerals from food for their body are suffering so called malnutrition.
There are problems that can be encounter if you are underweight. Anemia is deficiency with folate, iron and vitamin B12 that will result to fatigue, headache and dizziness. Weak bones or osteoporosis is lack of calcium and vitamin D. Weak immune system happen because you do not store or have enough nutrients to be able to fight illness. Inhibited development and growth is concerning to children and teenager because to be able to stay healthy and grow they both need a lot of nutrients.
There are different causes of underweight not only because of not eating proper food or balanced diet. Aside from this other causes is genetics, medicines, illness, high physical activity and psychological issues. Genetics because there are some that are born with fast metabolism and it sometimes in it in your blood line or maybe the person have natural small appetite. There are some medicines that can cause weight loss and nausea such as those people undergoing chemotherapy. Illness can affect a person appetite. High physical activities are for those people who work out a lot such as athlete where in they burn a lot of calories. Psychological issues such as depression and stress can be a factor for not eating properly.
Increase the intake of your calorie in every meal like by adding cheese, healthy side dishes and seed or nuts on it and make sure the food is rich in nutrients.