What foods should someone eat to gain weight?

High Protein complex: Carbs and healthy fat (fish and plant based fat) is what i would recommend. If you want to put on "healthy" weight, you need to build more muscle. So protein supplements with added 10 grams of sugar and some fruit right after work-outs will help build muscle. I suggest getting your metabolic rate checked and based on that, go on a high calorie zone type diet which is healthy and high protein.

Foods to help with: Healthy weight gain: almonds, cashews, pecans, sunflower seeds, flax seeds, walnuts, hummus, avocadoes, granola, trail mix, dried fruits, extra virgin olive oil, canola oil, protein shakes, protein bars, lean meats, poultry, fish, smoothies (with added silken tofu or powdered milk), greek yogurt (full –fat), pasta/ bagels ; use of juice rather than soda. Some people add whey proteins.