Anjum Khanna – The study estimates that 8.8 percent of men and 9.7 percent of women are underweight, defined by a BMI of under 18.5. Sometimes Being underweight is not good for your health. Find out what you can do if you’re concerned about yourself or someone else. Also there are some people who eat alot but can’t gain weight. Being underweight can be just as bad for your health as being obese and being skinny may lead to suffering from many health related issues.