Nuts are chock-full of important minerals (copper, manganese, magnesium, selenium, zinc, potassium, and even calcium) and vitamin E, plus they're easily portable, making them a filling, on-the-go snack. Even though they're high in fat, it's mainly the good-for-you kind. So in a nutshell, go nuts with nuts (a little if you're gaining quickly, liberally if you're gaining slowly).
Tip: Nuts are a versatile superfood — toss them into just about any dish: salads, pasta, meat dishes, baked goods and more.