Having a low body mass index (BMI) can be associated with certain health risks and problems like osteoporosis, anemia related to iron deficiency, loss of menstruation and infertility, explains WebMD. A person with a BMI below 18.5 is considered underweight. Causes for being underweight can include being sick, a medical problem, stress, being malnourished or too much dieting.
A person's BMI is a measure of body fat that is related to his weight and height. A low BMI means that a person is underweight, according to the National Heart, Lung and Blood Institute. When it is due to being malnourished, a person may not be getting important vitamins and mineral like iron and calcium, which are necessary for the body. For example, calcium is needed to maintain one's bones healthy, while iron is essential to produce hemoglobin. Low levels of calcium in the body can lead to bone loss and osteoporosis asa person ages.