Obesity is a well-known problem that leads to chronic health problems. Being underweight, however, is also detrimental to your health. If your weight isn't in the correct range for your frame, gender and age, you might get sick frequently, suffer a loss of energy, lose hair or have problems with fertility. If you suspect that you don't weigh enough, speak with your doctor right away.
When you're below the correct weight for your frame, your immune system is probably not as strong as it should be. You'll be more likely to catch the flu, a seasonal cold or any other communicable disease. So, if you feel unwell often, this could be a sign that you're underweight. Additionally, when you're underweight and you do get sick, it would probably take longer to recover than for someone at the correct weight.
Feeling Cold or Tired
If you're underweight, it means you're not eating enough and you might not be getting adequate amounts of iron and the B vitamins, which could lead to anemia. When you are deficient in your B vitamins, your body may not have enough red blood cells -- or the red blood cells could be improperly formed -- making it difficult for your body to transport oxygen. Additionally, if you're deficient in iron, your body will have a difficult time delivering oxygen to your cells. When oxygen transportation slows down, you will probably feel chilly all the time. Lack of energy and the inability to focus are also signs of anemia.
It's normal for your body to shed a little hair each day. On average, adults lose approximately 100 hairs each day, according to MedlinePlus. If your hair is falling out in fistfuls or if your eyelashes, eyebrow hairs and other body hairs also begin to fall out, this could be an indication that you're not maintaining an ideal body weight. This is especially likely to happen if you are on a crash diet, since it's likely your body isn't getting enough protein and other nutrients to promote hair health and growth.
If a woman's weight isn't where it should be, she could have hormonal problems. When you don't have enough body fat, your body has difficulty producing hormones, including sex hormones such as estrogen. As your hormones become imbalanced, you could miss one or more menstrual periods, which is a condition known as amenorrhea. Abnormalities in your hormonal level and your menstrual cycle can affect your ability to get pregnant later.