BMI meter - Version 1.0 Someone with a BMI of below 18.5 is considered underweight. Whilst some people are naturally slim, being underweight from poor nutrition, or as a result of other disease, can have serious health risks. Illness associated with being underweight ranges from simple tiredness due to inadequate energy intake, through to reduced immunity to infections, anaemia, vitamin deficiencies, thinning of the bones, infertility and heart rhythm irregularities. If your BMI is under 18.5 you should discuss it with your doctor. They will want to make sure you don't have any illness causing the weight loss and then will advise you about how to safely gain weight.Great tool for interval training progress check. http://www.intervalstraining.net