DEAR DOCTOR: My family tries to limit dairy products; it just doesn't make sense to me that, marketing aside, humans should need milk from another species. But now I read that non-cow milk may be linked to shorter kids. Why would that be?
DEAR READER: Your question is a logical one. The milk from breastfeeding is necessary for infant growth across all mammalian species, and humans are the only species of mammal that not only raises their young using the milk of another animal, but also consumes this milk in later life. So while it doesn't make biological sense to drink cow milk, eat cheese, or have creams and yogurt well beyond the time of breastfeeding, does it have a positive or negative effect on our bodies? And, if cow milk has an effect, what are the effects of cow milk substitutes like soy milk, almond milk and goat milk?
A 2017 article in the American Journal of Clinical Nutrition, which received widespread news coverage, attempted to answer these questions. The authors asked 5,034 Canadian parents of children ages 2 to 6 about their children's daily consumption of cow milk and non-cow milk products. Based on the parents' answers, 4,632 children were identified as cow milk drinkers, and 643 were identified as non-cow milk drinkers. (Note that some non-cow milk drinkers also drank cow milk.) There were no appreciable differences between the two groups of children, except for height.
Each daily cup of non-cow milk was linked to a height difference of 0.4 centimeter. That is, children who drank three cups of non-cow milk a day were 1.2 centimeters (approximately 1/2 inch) shorter than children who drank no non-cow milk. Similarly, children who drank three cups of cow milk a day were 1.5 centimeters taller than those who drank 3 cups of non-cow milk.
Here's a possible explanation: The cow milk proteins, casein and whey, have been linked to an increase in size. Also, cow milk contains insulin-like growth factor-1 (IGF-1), which leads to the formation of more bone and cartilage and thus greater height. IGF-1 is also found in goat milk. Note that this study lumped goat milk with soy, almond and other non-mammalian milk, so goat milk may not actually have the height correlation seen with these non-animal milks.
Another possible explanation is that plant-based milks do not contain as much protein and fat as mammalian milk. Thus, children who drink these types of milks don't get the calories or protein needed for greater growth.
Lastly, children who have food allergies are less likely to drink cow milk. Because food allergies have been associated with decreased height and weight, the allergies themselves may be a factor.
In summary, there could be myriad reasons why children who drink cow milk are taller than those who drink plant-based milks, but the fact remains that this seems to be the case. If you're worried that avoiding dairy products will interfere with your child's height, but you're opposed to dairy products for personal or health reasons, I'd recommend you ensure that your child consumes plenty of protein in other ways -- and that your pediatrician keeps a close eye on your child's development.
(Robert Ashley, M.D., is an internist and assistant professor of medicine at the University of California, Los Angeles.)
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