You have good insight - nuts, seeds, and avocados are great foods for adding more calories to your diet. What is your calorie goal per day? Are you adding in extra calories to fuel your physical activities?
“Specific recommendations for a healthy diet include: eating more fruit, vegetables, legumes, nuts and grains; cutting down on salt, sugar and fats. It is also advisable to choose unsaturated fats, instead of saturated fats and towards the elimination of trans-fatty acids."
- United Nations' World Health Organization