Hai, calculate your Body mass index if it says you are underweight. start the below mentioned advise to gain weight in natural way. If are active and healthy no need for any supplementation. protein supplements Endura advised for sports personnel for sustain release of energy during their exercise. Take proper nourished food. Take foods like Almond,cashew,channa, soybean,peanut in your daily food intake. Add jaggery,black gram,and dal prepared with ghee. Take diary product like milk,yogurt,cheese,butter etc. Add more fruits vegetables. As you got open heart surgery (cause don t know.). if it is coronary heart disease because of your life style. Avoid junk,fried,oily ,caffeinated drinks. do exercise or walk 3-4 kilometers. Hope i answered your query. Thank you