Hi , I think the best way to gain healthy weight without fat is to work on gaining muscle (which weighs more than fat anyway): only way to do that is to lift heavy, lift hard. Lift to failure, short reps, high weight and only 2or 3 days a week. Do squats, deadlifts. And you must rest after or you won't grow. Do cardio for health, but keep it moderate. And alternate it increasing the intensity level as well to build muscle. REST. Eat lots of fruit for energy, vegetables next, then moderate fat, keeping it(fat) at about 15% only. Stay away from oils and stick to the whole food: nuts, seeds, avocados, olives, etc., getting a variety. Taking flax should be done everyday regardless of your diet goals because it is necessary for health. Eat as much raw as you can get in and stay away from processed foods, such as breads, desserts, packaged foods, and definitely sugar. EAT up!