1 Foods That Contain Vitamin B3 Niacin is found in foods with a high protein content, such as meat, eggs, and peanuts. 2  It can also be found in milk, mushrooms, and greens.3  Niacin is added to enriched breads and cereals. However, taking niacin supplements in high doses can be dangerous to your health. 2  Foods That Contain Vitamin B3 Niacin is found in foods with a high protein content, such as meat, eggs, and peanuts. 3  It can also be found in milk, mushrooms, and greens. 4  Niacin is added to enriched breads and cereals.