if your BMI is over 25 you are most likely overweight (unless you are muscular). being overweight puts you at a greater risk for heart disease, high blood pressure, breathing problems, type 2 diabetes, and much more
the same of for a BMI of under 18 meaning you are underweight. people who are underweight are at risk for bone loss, weak immune system, infertility, cardiac (heart) problems, and much more