Short answer: Cow's milk has little iron. When kids drink a lot of milk, they don't eat iron-containing foods in sufficient volumes. Cow's milk also has big proteins that can cause microscopic bleeding in the gut. The more milk consumed, the more bleeding (though usually still not seen in the stools).
More milk = more blood loss from the gut
but less blood produced because less iron in the diet= anemia
Iron is used to build healthy red blood cells that carry oxygen throughout our bodies. Too few red blood cells in the body is called anemia. Red blood cells are made in our bone marrow and they live for about 3 months. It is important for the body to continually make new red blood cells as it breaks down and removes old ones.
In general anemia can be caused from several factors:
- too little blood produced (iron deficiency being a major risk for this)
- increased blood loss (ie excessive bleeding)
- increased destruction of blood cells in the body (typically from abnormal blood cells or infection)
- poor iron absorption due to disease (some studies show milk inhibits iron absorption)
- poor iron in the diet (the most common cause)
- long term slow blood losses (such as heavy monthly periods or GI bleeding)
- increased iron need (such as a growth spurt or pregnancy)
Why does preventing iron deficiency anemia matter? Because the most common symptom of anemia is no symptoms. It can go unnoticed for quite a while in some kids, yet cause long term problems with growth and development. Symptoms develop when the anemia becomes more severe and include tiredness, looking pale, irritability, decreased appetite, slowed development, weakness, immune dysfunction, and pica (eating non food substances- such as dirt).
Newborns are designed to drink their mother's milk. Humans have learned to make formulas that can nourish babies if they aren't able to drink their mother's milk for whatever reason. Cow's milk, soy milk, and goat's milk are not acceptable for infants due to the nutritional voids they have (not just iron). After about 1 year of age babies tend to wean from mother's milk and/or formula onto whole milk. (Newer recommendations allow weaning onto low fat milk--another topic entirely.) Unfortified non-human milks contain very little iron. The iron in human milk is better absorbed and iron is supplemented into formula. If toddlers and children drink too much milk, they fill up on it and don't eat a variety of other food groups that include iron and other important nutrients missing in their milk.
Foods that are good sources of iron:
- meats and poultry (especially organ meats, such as liver)
- lentils, peas, and dried beans
- oysters, clams, and fish
- peanut butter
- pumpkin or sesame seeds
- fruits such as prunes, apricots, and raisins
- vegetables such as spinach, kale, broccoli, and other greens
- whole grain fortified breads and cereals
Vitamin C increases iron absorption, so eat foods with iron and Vitamin C at the same meal!