Per 68-gram serving of beef liver (approximately one 2.5-inch by 2.5-inch slice)
Calories: 130
Fat: 4g
Protein: 20g
One of the most common organs, liver is a great source of high quality protein and is one of the most concentrated sources of vitamin A, along with copper, folic acid and iron. It also contains Coenzyme Q10 (CoQ10), which is important for cardiovascular functions. Athletes love liver because it improves the oxygen-carrying capacity of blood cells, increasing endurance and strength and fighting fatigue, while its B vitamins aid people with Alzheimer's and other forms of dementia.

Liver is commonly enjoyed in cuisines all over the world: in the South, they deep-fry chicken livers; in Germany, they feast on liverwurst; and in Japan, they use raw fish liver to make sashimi. Opt for beef, veal, goat, lamb, bison, buffalo, chicken, geese or duck liver, preferably from a young animal (it might appear more pale), which is most tender. The only thing off limits is polar bear liver, which, because of its high density of vitamin A, can cause drowsiness, irritability, bone pain, vomiting and even peeling skin.

Liver can retain toxins from drugs and other chemicals, so buy grass-fed meat without added antibiotics or hormones. Soaking in lemon juice or milk for several hours before cooking reduces liver's strong metallic taste. Sear it until it’s light pink and top with caramelized onions. Cooking too long can make liver tough or rubbery, and releases many of the nutrients and digestive enzymes.