How to gain healthy fat with fast metabolism?

Eat healthy fats: Make sure you're not hyperthyroid 1st. There's no such thing as "healthy fat" on your body. If you want to add some weight, eat more healthy carbs and unsaturated fats like avocado and olive oil. Enjoy being slim and start doing physical activity 300 minutes for life combined with muscle strengthening. Keep your metabolism naturally high for a long lifespan and healthspan!