Does My Child Drink Too Much Milk?
A couple of nights ago, I had the pleasure of going out for drinks with a couple of other moms, sans kids. Predictably, we ended up talking about our kids, among other things. One of my friends mentioned that her 2-year-old daughter is a big fan of cow’s milk, drinking about 3 cups per day. This reminded me of a study I’d gotten wind of earlier in the week that recommended kids drink only 2 cups of milk per day. Should my friend cut back on her daughter’s beverage of choice? I needed to look at the study more closely.
The study in question (The Relationship Between Cow’s Milk and Stores of Vitamin D and Iron in Early Childhood (1) – paywall) was conducted by a Toronto research group led by Jonathon Maguire and published on Monday in the journal Pediatrics.
The headlines about this study have made it sound like we now have the final word on just the right amount of cow’s milk for kids. The Atlantic posted a story with the headline “Kids Should Drink Exactly Two Cups of Milk per Day,” and others published similar statements. I will argue that it’s just not this simple, and prescriptive nutrition messages like these just end up confusing parents.
What is neat about this study is that it looked at two important nutrients for young kids: vitamin D and iron. Deficiency of both of these nutrients is common and cause for concern.
With enough UVB sun exposure, we can make our own vitamin D in our skin, but for many of us, that sun exposure is in short supply. Vitamin D deficiency is especially common in people who live at northern latitudes (north of a line between San Francisco and Philadelphia), where exposure to solar UVB radiation is limited. We also want to protect our children and ourselves from skin cancer, so we smartly cover up and use sunscreen to block those rays. In addition, those with darker skin absorb less UVB and are at higher risk for vitamin D deficiency. In the winter in Oregon, I assume that my family needs to get all of our vitamin D from our diet or supplements. There’s a nice list of vitamin D-rich foods here.
The most obvious sign of vitamin D deficiency is rickets, causing weakening of the bones and teeth, skeletal and dental deformities, and slowed growth. There has also been an explosion of new data over the last few decades linking subtler vitamin D deficiency to increased risk of infection, autoimmune diseases, cancer, and type 2 diabetes in adults. Several studies have found that children given vitamin D supplements in infancy and early childhood may be at lower risk for type 1 diabetes (2,3).
Iron is essential to muscle and brain development (4). Iron deficiency early in life can result in delays in motor skill development, as well as learning and behavioral problems that have been shown to last at least until adolescence. Iron is also a critical part of hemoglobin, which transports oxygen in the blood, and a person with severe iron deficiency will also have low hemoglobin and be anemic. See here for a list of dietary sources of iron.
Needless to say, you want your kid to get enough vitamin D and iron. So where does milk fit into this story? Milk is fortified with vitamin D and is one of the best sources of the nutrient in our food supply. Milk is also naturally high in calcium, and this combination is great for growing bones. The problem is that cow’s milk inhibits the absorption of non-heme iron (iron from plant foods). Plus, milk is low in iron, and there is a concern that kids that drink lots of milk will fill up on it and get fewer calories from iron-rich foods.
So, milk is good for vitamin D but bad for iron. This recent study asked an important question: How much milk is enough to provide adequate vitamin D without risking iron deficiency?
The study included children between the ages of 2 and 5, all residing in the Toronto area. This was an observational study, meaning that it simply looked at the relationships between several variables in kids living their normal lives, at one point in time. Parents were asked how much cow’s milk their kids drank, as well as whether they used vitamin D or iron supplements and how much time they spent outside per day. Blood samples were collected from a total of 1311 children. These samples were analyzed for 25-hydroxyvitamin D, an indicator of vitamin D status, and serum ferritin, indicating iron stores.
The data from this study are rather beautiful.
They show that as milk consumption went up, so did vitamin D status, but iron status decreased. In other words, there appeared to be a trade-off between vitamin D and iron nutrition. This is not surprising given what we just discussed above.
The authors of this study then looked at how much milk was associated with adequate serum 25-hydroxyvitamin D, which has been estimated in earlier studies to be 75 nmol/L. Their data show that for the average kid, consuming 2 cups of milk per day was sufficient, with a little wiggle room to spare.
Now let’s look at the iron side of the story. As I mentioned, the data showed that as milk consumption increased, iron stores (measured by serum ferritin) decreased. And since the authors found that 2 cups of milk were enough to meet the vitamin D needs of the average child, they simply assumed that drinking any more milk could be to the detriment of iron nutrition.
The authors’ conclusion is this: “Two cups of cow’s milk per day appears sufficient to maintain healthy vitamin D and iron stores for most children.” That’s fair enough, but making the recommendation that children drink exactly 2 cups – as I’ve seen in the headlines – is quite a different story. There are a few reasons why I don’t think this study tells us that:
1. It is an observational study based on parents’ guesstimates of how much milk their kids drink. That’s problematic. For example, parents may intentionally or unintentionally misreport milk consumption – sort of rounding up or down – based on what they think their pediatrician wants to hear. They may simply not have a good mental picture of what 250 mL of milk looks like. (For the study, parents were asked, “How many 250-mL cups of cow’s milk does your child drink in a typical day?”) They may also not know exactly how much milk their kid drinks. If your child is in childcare or school, do you know how much milk he drinks during the day? (I don’t.) And a 5-year-old can help himself to milk from the fridge without being served by a watchful parent. All of these factors can introduce a lot of error to the variable of milk intake. I would hesitate to set a global nutrition recommendation based on an observational study.
2. The study doesn’t adequately address other sources of vitamin D. The data were adjusted for vitamin D supplementation, which was a factor for about 60% of the kids, though the paper doesn’t report how much supplemental vitamin D they were taking. There were also no data collected on other dietary sources of vitamin D in these kids. This seems particularly important since there appears to be a disconnect between the 2 cups of milk per day recommendation from this study, which would provide about 250 IU of vitamin D, and the AAP’s recommendation that children and adolescents get 400 IU of vitamin D per day (Wagner et al. 2008). Obviously these kids are probably getting some additional vitamin D from eggs, fish, or other fortified foods, but since that will vary from child to child, I hesitate to spread the word that 2 cups of milk provides enough vitamin D for kids. Maybe 2 cups of milk plus a tuna sandwich, but you have to consider other food sources of vitamin D.
3. There are many non-dietary factors that impact vitamin D status. The Maguire study did a nice job of investigating one of these: skin pigmentation. They found that for kids with dark skin who were not taking a vitamin D supplement, only those drinking 4 cups of milk per day appeared to have adequate vitamin D in the winter months. Other important factors include latitude, time spent in the sun, and obesity. Two cups of milk might be just right for the average kid in Toronto, but a child in Alaska would likely need more, and a child in Arizona would likely need less. The study also found that heavier kids had lower vitamin D status, so for these children, greater milk intake or vitamin D supplements might be recommended.
4. The conclusion that drinking more than 2 cups of milk per day would hurt iron status was arbitrary. Average serum ferritin in this group of kids was 31 μg/L. A child is considered iron deficient if serum ferritin is <12 μg/L. Among these kids, 4% fell into this range. These iron-deficient kids drank an average of half a cup more milk than kids that weren’t iron-deficient. But according to the data in this paper, drinking an extra half a cup of milk per day would account for only a 0.5 μg/L drop in ferritin. And the data show that for the average kid drinking 4-5 cups of milk per day, serum ferritin was about 26 μg/L, nowhere close to iron deficient. I have no doubt that milk is one factor influencing iron nutrition, but these data suggest that it plays a relatively small role.
What is likely most important to iron nutrition is what is going on in the rest of the diet. Does the child eat good sources of dietary iron? How are they combined with milk and other foods in meals? Milk doesn’t completely block iron absorption; it lowers it, probably by about half (5,6). Other foods, including egg, soy, and whole grains, also lower iron absorption. Specifically, these foods lower the absorption of non-heme iron, which is found mainly in plant foods. The absorption of heme iron, found in meat, is very efficient and unaffected by other foods, including milk, in the meal. In addition, the presence of heme iron increases the absorption of non-heme iron in the same meal. Vitamin C also increases the absorption of non-heme iron. If you pair milk with meat, it won’t affect iron absorption. Or if your kid has a snack of a glass of milk with an apple, which has very little iron to begin with, that milk isn’t likely to impact your child’s overall iron status.
This might seem complicated, and it is. That’s why I think that telling parents that 2 cups of milk is just the right amount is silly. The data from this study are just not strong enough to add limiting milk to your list of nutrition worries.
There’s another reason why I object to limiting milk to 2 cups of milk per day. I think that we Americans are too preoccupied with restricting our diets in one way or another. It takes the joy out of eating, and it rubs off on our kids. And since the advice on what to restrict seems to change all the time, we end up confused and frustrated. If your child likes milk, I would hesitate to restrict it. It is, after all, a nice source of protein and fat and one of our best sources of calcium and vitamin D, as well as providing many other vitamins and minerals.
All of that said, it is your job as a parent to put balanced meals and snacks in front of your child. If milk is getting in the way of that, then you may want to rethink how you do things. Here’s a little advice:
- Serve milk at the table in an open cup. (We only make an exception to this rule if we’re traveling or at a restaurant.) Serving milk this way slows the pace of drinking a bit, allowing your child to taste the milk and to feel satiety signals from it and the other food in the meal. Toddlers tend to drink more via sippy cups or bottles, especially if they’re allowed to carry them around between meals. In fact, I’d be willing to bet that most of the kids who have a problem with too much milk are getting it from a bottle, not a cup.
- If you’re worried that your child is drinking too much milk, ask yourself these questions: Do you think milk is limiting how much food your child is eating? Does your child eat very few good sources of dietary iron? Is your child constipated often? If the answer to any of these questions is yes, then you may want to consider cutting back on milk. I would encourage you to talk to your pediatrician about your concerns and have your child tested for iron deficiency.
- If you do limit milk, serve it at predictable times. Decide that you’ll just have it at say breakfast and dinner, and let your child know that’s the plan. And don’t be afraid to break your milk rule for something like an oven-fresh batch of chocolate chip cookies!
- Keep offering good sources of iron at meals and snacks, including both plant and animal-based sources, and add in some vitamin C-rich foods while you’re at it. Iron-rich foods can be challenging for toddlers, but don’t get discouraged. Cee is very hesitant about meat (except bacon) these days, but we keep offering it, while also trying to serve good sources of iron like legumes and fortified cereals.
- If your child doesn’t drink much milk, be sure that he’s getting enough vitamin D in his diet or from a supplement. If you live north of Philadelphia, have dark skin, or just don’t get much sun exposure, I think that a vitamin D supplement is good for everyone in the family, especially this time of year.
How much milk does your child drink? Are you concerned that it’s too much or too little?
(1) Maguire, J. L., G. Lebovic, et al. (2012). The Relationship Between Cow’s Milk and Stores of Vitamin D and Iron in Early Childhood. Pediatrics.
(2) Hypponen, E., E. Laara, et al. (2001). Intake of vitamin D and risk of type 1 diabetes: a birth-cohort study. Lancet 358(9292): 1500-1503.
(3) The EURODIAB Substudy 2 Study Group (1999). Vitamin D supplement in early childhood and risk for Type I (insulin-dependent) diabetes mellitus. The EURODIAB Substudy 2 Study Group. Diabetologia 42(1): 51-54.
(4) Baker, R. D. and F. R. Greer (2010). Diagnosis and prevention of iron deficiency and iron-deficiency anemia in infants and young children (0-3 years of age). Pediatrics 126(5): 1040-1050.
(5) Abernathy, R. P., J. Miller, et al. (1965). Metabolic Patterns in Preadolescent Children. Xii. Effect of Amount and Source of Dietary Protein on Absorption of Iron. J Nutr 85: 265-270.
(6) Hurrell, R. F., S. R. Lynch, et al. (1989). Iron absorption in humans as influenced by bovine milk proteins. Am J Clin Nutr 49(3): 546-552.