How to gain weight safely and build muscle fast RD Jaime Gnau explains. April 21, 2022 by Share:Click to share on Twitter (Opens in new window)Click to share on Facebook (Opens in new window)While being overweight is unhealthy, being underweight — a BMI of 18.5 or lower — can lead to health complications like a weakened immune system or fragile bones. To gain weight the right way, it’s important to get proper nutrition. Registered dietitian Jaime Gnau offers some helpful tips to Insider. She’s also a clinical instructor of biomedical sciences at Missouri State University.