Fish/Seafood
Fish or seafood is another nutritious and high-calorie food that can cause healthy weight gain.
One serving (3ounce) of Atlantic mackerel contains up to 223 calories, 3 ounces of tuna gives you 156 calories, and one cup of sardine gives you 310 calories. Most fishes are rich in protein and heart-healthy fats (unsaturated). They are excellent sources of omega-3 fatty acids, which reduce LDL (bad) cholesterol levels and increases HDL (good) cholesterol levels.
Salmon, herring, and lake trout are other seafood that has high calories and is highly nutritious.
As healthy as fish/seafood is, it contains mercury, a chemical that can cause serious health conditions over time.