Healthywe is Weight Gain Diet & Practices in Bangalore, Being underweight is defined as having a body mass index below 18.5. This is estimated to be less than the body mass needed to sustain optimal health. For some people, gaining weight or adding muscle can be just as difficult as losing weight for others. However, simply adding certain foods to your diet can make your weight gain efforts both healthy and more effective. Milk provides a good balance of proteins, carbs, and fats and is a good source of calcium, as well as other vitamins and minerals.