From the ArcaMax Publishing, The Culinary World, with Chef James Newsletter:
http://www.arcamax.com/news/chefjames/s-361648-622696
Iron and calcium in plant foods are not highly absorbed by the body.
Spinach contains a chemical called oxalic acid, which binds with iron
and calcium and reduces the absorption of these minerals. To improve
iron absorption, spinach should be eaten with vitamin C-rich foods
such as orange juice, tomatoes, or citrus fruit.
Courtesy of FoodReference.com.