April 24, 2006 -- Expectant mothers who don't drink milk aren't getting enough vitamin D -- and it's affecting their babies, a Canadian study shows.
Babies born to mothers who do not drink milk weigh a bit less than those born to moms who do, report Kristine G. Koski, PhD, RD, director of the School of Dietetics and Human Nutritionat McGill University in Montreal, and colleagues.
"You can actually reduce an infant's birth weight by reducing the mother's milk intake," Koski tells WebMD.
Koski's team studied 279 healthy pregnant women and their newborns. The difference in weight wasn't large. Kids born to mothers who did not drink milk averaged 7.5 pounds, versus the 7.8-pound babies born to the average milk drinker.
But by not drinking milk, mothers got far too little vitamin D. And data analysis shows that it was vitamin D -- not calcium or any other milk-related nutrient -- that accounted for the infants' lower weight.
A fraction of a pound in body weight difference does not sound like much, but it's a danger sign, says vitamin D expert Bruce W. Hollis, PhD, of the Medical College of South Carolina. Hollis's editorial accompanies the Koski report in the April 25 issue of the Canadian Medical Association Journal.
"The results the Koski team found were surprising, given the extremely low levels of vitamin D in even the milk drinkers," Hollis tells WebMD. "It shows how important vitamin D is."
There are about 2.25 micrograms of vitamin D in a cup of milk. The Koski study, Hollis says, shows that for each daily microgram of vitamin D a mother consumes, her baby's birth weight will increase by 11 grams -- more than a third of an ounce. That's a little less than one extra ounce of birth weight for every daily cup of milk a mother drinks.
"Vitamin D is important in the development of the skeleton. [A mother's vitamin D level] impacts the baby 10 years out from birth," Hollis says. "Whether in the womb or later in life, vitamin D deficiency is dangerous."
Avoiding Milk: Bad Advice for Pregnant Women
When asking pregnant women about their diets, Koski kept hearing the same strange thing, over and over again. Women kept telling her they were avoiding milk and milk products during their pregnancy. Most of the women in the study cited bowel discomfort from milk as a major reason for avoidance. But in this well-educated group, reading material also reinforced this behavior.
"We found that a large number of the books pregnant women read advise them not to drink milk," Koski says.
These books alarmed pregnant women. They told them that by drinking milk, they put themselves -- and their yet-to-be-born children -- at risk of allergies, diabetes, and lactose intolerance.
"None of these reasons is strong enough to make the recommendation that pregnant women should avoid milk, except for the small number of women clearly diagnosed with problems," Koski says.
But is there a good reason for pregnant women to drink milk? Koski led a research team that explored the issue.
Only 72 of 307 women who said they were avoiding milk agreed to be studied. These women drank less than one cup of milk per day. The researchers also studied 207 women who drank a cup of milk or more each day.
Drink More Milk, Have Bigger Babies
The babies born to women who avoided milk averaged 7.5 pounds at birth. Those born to milk-drinking moms gave birth to babies averaging 7.8 pounds.
And it's not that the milk-drinking women were consuming vitamin D by the gallon. Hollis says even these women may have been deficient in vitamin D. There are two reasons for this.
One is that the women lived in western Canada where, most of the year, there is little sunshine. And it's only when sun shines on the skin that a human body makes vitamin D, which is actually a hormone.
"In Canada we have an epidemic of vitamin D deficiency. From October through May, we are not getting enough sun on our skin, and if you don't take in sun you don't get enough vitamin D," Koski says. "In the U.S., you probably are not going to see so much of a problem in the South. But you would see it in Chicago, New York, Boston, and other cities at northern latitudes."
The other reason the women in Koski's study had relatively low vitamin D levels is that even when they take vitamin D supplements, people may not get enough vitamin D. Hollis says that after about 15 minutes of noonday sun, the body makes 20,000 international units (IU) of vitamin D. Yet the recommended daily amount of vitamin D -- based, Hollis says, on very sketchy and outdated evidence -- is only 200-600 IU.
Hollis has grants from the U.S. National Institutes of Health to investigate high-dose vitamin D supplements for pregnant and lactating women. In the pregnancystudy, he's giving women up to 4,000 IU of vitamin D per day. In the lactation study, he's exploring doses up to 6,000 IU per day.
The results won't be in for another 2.5 years. Meanwhile, Hollis advises pregnant women to take up to 2,000 IU of vitamin D per day -- far higher than current recommendations. But those may change. Hollis is a member of an Institute of Medicine panel that in 2007 will review official vitamin D advice.
"In my estimation, the recommendations will change dramatically," Hollis says.