Hello! A good way to gain lean body weight & to enhance your overall health & fitness is to start regular gym workout (2 to 3 days a week) along with a well balanced diet - Include a variety of foods in your diet like fruits, vegetables, milk products, whole grains, nuts, dry fruits, seeds (sesame, flax, sunflower etc.) , & if non vegetarian then eggs, fish & chicken.
You need to increase your protein intake to go along with the gym routine. And remember,Weight gain will happen when you consistently take in more calories than you burn. Hope this helps.
Health Tips
Kindly make sure that you follow these guidelines after consulting your physician & or a nutritionist/dietitian