I am at the age of 27 year old but i still only weight 40kg, i have tried everything to improve and gain weight, but nothing has work. please help ?

Diet and muscle mass: First, we need to know how tall you are--if you are seriously underweight you should see a doctor first. In general, to gain weight you need to consume more calories than you burn. Do this in a healthy way. Make sure you get all of your essential nutrients in calorie-rich food. With weight training, you can increase your muscle mass. A doctor and nutritionist can help you with a specific plan.

Not sure of height?: See a doctor for particularly low BMI. Eat healthy foods high in calories. Eat 6 meals /day. Work on strength training to build large muscle groups. Work out w heavier weights w fewer repetitions. Sleep at least 8 - 9 hrs/night. Hydrate well w water. Foods to help w weight gain: almonds, cashews, pecans, sunflower seeds, flax seeds, walnuts, hummus, avocados, granola, trail mix & dried fruits.