Maria A. A. Oliveira, PhD, and Mônica M. Osório, PhD, Professors in the Nutrition Department at the Universidade Federal de Pernambuco, wrote in their 2005 article "Cow's Milk Consumption and Iron Deficiency Anemia in Children," published in the Journal de Pediatria:

“Foods that are sources of dietary protein can both increase and reduce the absorption of nonheme iron [iron contained within a protein]. Animal tissue (beef, pork, liver, chicken and fish) increases the absorption of this micronutrient. On the other hand, casein and whey proteins from cow’s milk account for the greater proportion of the protein present in the majority of infant formulae and foods and have a negative influence on iron uptake…

Cow’s milk has approximately four times more calcium than human milk, which can contribute to low iron absorption. The practical nutritional implications of the inhibitory effect of calcium on iron uptake were demonstrated by Hallberg et al. when they observed that the consumption of milk or milkshake with hamburgers or the addition of cheese to a pizza reduced nonheme iron absorption by 63, 47 and 61%, respectively. This maximum inhibitory effect is achieved with an approximate intake of 150-200 mg of calcium, which is the equivalent to that contained in one glass of milk or a piece of cheese, for which reason the authors recommend that these should not be consumed regularly at the main iron-providing meals, particularly by those who have increased iron requirements (children, adolescents and women of fertile age).”