Iron has many important roles in animal biology, specifically when considering oxygen transport in the bloodstream, oxygen storage in muscles, and enzymes. Known isotope variations are shown in the figure below. Iron isotopes could be useful tracers of the iron biochemical pathways in animals, and also be indicative of trophic levels in a food chain.
Iron isotope variations in humans reflects a number of processes. Specifically, iron in the blood stream reflects dietary iron, which is isotopically lighter than iron in the geosphere. Iron isotopes are distributed heterogeneously throughout the body, primarily to red blood cells, the liver, muscle, skin, enzymes, nails, and hair. Iron losses in the body (intestinal bleeding, bile, sweat, etc.) favor the loss of isotopically heavy iron, with mean losses averaging a  δFe of +10‰. Iron absorption in the intestine favors lighter iron isotopes. This is largely due to the fact that iron is carried by transport proteins and transferrin, both of which are kinetic processes, resulting in the preferential uptake of isotopically light iron.
The observed iron isotopic variations in humans and animals are particularly important as tracers. Iron isotopic signatures are utilized to determine the geographic origin of food. Additionally, anthropologists and paleontologists use iron isotope data in order to track the transfer of iron between the geosphere and the biosphere, specifically between plant foods and animals. This allows for the reconstruction of ancient dietary habits based on the variations in iron isotopes in food.