What is Undereating and What are its Side effects? Many people under-eat for a while to reduce weight. Undereating is also known as dieting. Under-eating is less damaging for people with a healthy weight but for some people who are already underweight, it could be very harmful. Undereating or overeating both are injurious to health. No matter, for what reason you are eating less food. You will face adverse effects. Here are some signs that prove you are not eating enough food. ● If you are feeling tired all the time or without any reason. ● Get ill often. ● Skin problems ● Hair Loss.