My bmi is 19.5 and i feel quite thin so i am trying to gain weight. what foods should i overeat without affecting my health? i.e milk? protein powder?

Why?: Why do you want to increase your weight. A bmi of 19.5 is healthy. Once you start consuming more, even healthy foods, and gain weight, you may end up weighing more than you may have bargained for. Eat a well balanced diet rich in fresh vegetables, fruits and low in meats.

Weight gain: Foods to help w weight gain: almonds, cashews, pecans, sunflower seeds, flax seeds, walnuts, hummus, avocados, granola, trail mix, dried fruits, protein shakes & bars, lean meats, poultry, fish, smoothies (with added silken tofu or powdered milk), extra virgin olive oil, canola oil Greek yogurt (full –fat), pasta/ bagels & use of juice rather than soda. Some people add whey proteins - but don't