If you're underweight, it can compromise your immune system, energy and quality of life. People with a body mass index of less than 20 are considered underweight, while a measurement of below 18.5 indicates a severe underweight status. An underweight status can be a sign that you're purposefully, or inadvertently, eating a diet that doesn't contain all the nutrients you need for good health and the calories necessary to maintain a healthy weight 2. The lack of these nutrients can make you feel weak and tired, and prevent your body from adequately fighting off infection. Being too thin can compromise your immune system, your sports performance, your self-esteem, and, if you're a woman, your fertility.Book Your Slot!