Mackerel is a highly nutritious oily fish that is an excellent source of protein and one of the richest sources of omega-3 fatty acids, which support heart health, brain function, and reduce inflammation. It also provides plenty of vitamin D for strong bones and immunity, as well as vitamin B12, selenium, magnesium, and potassium for energy, blood health, and muscle function. Because it is higher in healthy fats, mackerel is more calorie-dense than lean fish, but its nutritional benefits make it one of the healthiest seafood choices when eaten in moderation.