Liver is considered the most nutrient-dense food available. Liver contains heme iron. Non-heme iron, which makes up the majority of iron in plants, is less well absorbed. Over 95% of the functional iron in the human body is in the form of heme. Heme should be considered an essential nutrient for humans. Vitamin D is essential for iron absorption in the body. An iron deficiency may indicate low vitamin D levels. Liver is rich in vitamin D. People with liver disease, cirrhosis, or small intestinal malabsorption (Crohn's disease, irritable bowel syndrome, colitis) may develop a vitamin K deficiency because the body stores very little of it. Vitamin B12, copper, riboflavin, and large amounts of vitamin A, which is important for vision, the immune system, reproduction, growth, and development, are also important.