The reason of not gaining weight are
* improper diet * indigestion (disease of stomach, loose motions)
* worms in intestine
* thyroid disorder
* diabetes
*addiction (smoking or alcoholism)
dear you can easily gain weight and get a healthy muscular body by following these best things - 1. To gain weight, your digestion should work properly. If you have any disease of digestive system you should treat disease first. For proper digestion - take 10 gms powder of carom seeds (ajwain, 10 gm powder of cumin (jeera), 10 gm of rock salt. Mix them well and take 1/2 teaspoon (3-5 gm) of mixture daily after meal.
2. Take a diet rich with proteins and fats. You should take high calorie food that increase your muscle mass and induce growth in bones.
Follow the diet chart in your daily life to fastly gain weight * in morning, take a glass of milk added with 2 teaspoons of protein powder with breakfast.
* in lunch time, take 1 bowl of curd or 100 gm of cheese, a bowl of vegetable, with chapattis.
* in evening, take 1 glass of milk (300 ml) with 2 bananas.
* in night, take a bowl of careals (daal, a bowl of salad with chapattis.
3. In morning, get up early and take awalk for 10-15 minutes and exercise light weight exercises for at least 30 minutes at home or gym.
4. For proper weight, proper sleep is very important. So you should sleep for 6-8 hours daily.
5. Avoid smoking & alcohol drinkg completely.
6. You can try protein powders like endura mass, milk powder, soya protein. And creatinine for weight gain by taking 1 tea spoon added in milk once a day. I assure you that you definitely gain weight fastly by following them. Be healthy.