Overlooked nutrient seen responsible for positive effect on babies born to mothers who drink milk
by Craig Weatherby
Low birth weight is a chronic public health problem in Third World countries, and is epidemic even in low-income areas of some First World cities. But growing numbers of women are cutting back on vitamin-D-fortified milk—the leading dietary source for most women, albeit not the richest possible one—during pregnancy.
Apparently, many expectant mothers cut back on milk for three reasons, none of which are backed by research: 1) to minimize weight gain, 2) to treat self-diagnosed lactose intolerance, or 3) to prevent their children from developing allergies.
And while restricting milk intake during pregnancy is not proven to accomplish any of those goals, the results of a new study from Canada (Mannion CA 2006) show that mothers who eliminate milk without taking vitamin D supplements or seeking alternative food sources of vitamin D—such as fish—are likely doing their babies a disservice.
According to lead author Cynthia Mannion of the University of Calgary, “Restricting fortified milk or vitamin D intake during pregnancy lowered infant birth weight in otherwise healthy, non-smoking, well-educated mothers.”
The study's authors described the challenge mothers—especially those living in northern latitudes—face in ensuring adequate vitamin D intake:
“…vitamin D is found in few commonly consumed foods except for milk fortified with the vitamin. Dermal conversion [i.e., in the skin] of the [vitamin D] precursor cholecalciferol to activated vitamin D is a seasonally unreliable source, particularly when the exposure of skin to sunlight is limited, and may not compensate for low nutritional intake.
“Under optimal conditions, 15 minutes of daily exposure of the hands and face to sun is required to produce adequate vitamin D, but this conversion is reduced by the prolonged winter season at more polar latitudes, by use of sunscreens with a sun protection factor (SPF) above 8, and by ozone air pollution. People whose skin pigmentation is dark are more vulnerable than others to insufficient vitamin D conversion.”
As we reported last week in our briefing on the highlights of Dr. Andrew Weil's 3rd Annual Nutrition & Health Conference, public health experts say that the calcium in milk is not the bone-boon it's cracked up to be in the US Dairy Council's ubiquitous “Got Milk?” ads (See “Harvard Expert Calls Vitamin D, not Calcium, Key to Bone Health” in last week's issue).
And while vitamin D is a more powerful bone-booster than calcium, you'd have to drink a full quart of milk to get the current RDA for vitamin D (400 IU): an amount exceeded in just one 3.5 ounce serving of sockeye salmon (687 IU).
Vitamin D intake recommendations called inadequate
The Canadian researchers noted that experts in the field consider current US government intake recommendations for vitamin D much too low, and that the resulting inadequacy of American mothers' vitamin D intake may be detrimental to their babies:
“Vitamin D may be an important regulator of fetal growth.… In the past few years, it has been suggested that currently recommended vitamin D intakes may be too low to prevent insufficiency in the general population. Some authors have urged higher intake recommendations for vitamin D in North America for those living at or above latitudes ranging from 42° to 52° N, similar to practices in European countries.
They went on to say, “Our study results suggest that increased recommendations for vitamin D intake should extend to pregnant women.… Practitioners should query pregnant women about their consumption of fortified milk and multivitamins, specifically those containing vitamin D, to avoid the risk of lowered birth weight as a result of insufficient intake, particularly when and where sun exposure is limited.”
And in a lengthy comment accompanying the Canadian study, pediatricians Bruce Hollis, M.D. and Carol Wagner, M.D. of the Medical University of South Carolina echoed these concerns about the inadequacy of current intake recommendations:
“The reality is that we do not know what the actual vitamin D requirement during pregnancy is. For that matter, we do not know the requirement for the general population, either.
“What we are taught and told is that for a pregnant woman, the adequate intake for vitamin D (we do not have a dietary recommended intake [DRI] for the vitamin) is 200 IU per day. What we now know is that this recommended level, which was largely arbitrarily set, will do nothing to improve the nutritional vitamin D status of pregnant women.”
What the study showed
The Canadian team recruited 307 women who were attending prenatal programs in Calgary, Alberta, and who indicated that they restricted milk intake to one cup (250 mL) of milk or less per day.
Using repeated dietary recall surveys, they compared these women and their offspring with women whose daily milk consumption exceeded one cup per day. The babies' birth weights, length and head circumferences were obtained from birth records.
The researchers found that mothers who drink an extra cup of milk every day could increase the weight of their baby by 41 grams (about 1.5 ounces), thanks entirely to its vitamin D content.
This was a refreshingly frank admission, given that funding sources for the study included the Dairy Farmers of Canada, who surely would have preferred the authors highlight milk, rather than the vitamin D added to it, as the source of birth-weight benefit.
As Dr. Mannion reported, “For each one microgram increase of dietary vitamin D (food plus supplements), birth weight increased by 11 grams.”
Vitamin D seen to boost babies' bone strength
Confirming the findings of earlier research (Dennison E 2005), the authors of a study from Britain's University of Southampton (Javaid MK 2006) reported recently that higher intake of vitamin D during late-stage pregnancy produced stronger bones in children:
“Vitamin D supplementation of pregnant women, especially during winter months, could lead to long-lasting reductions in the risk of osteoporotic fracture in their offspring.”
Together with the new birth-weight findings from Canada, these results indicate that expectant mothers should seek out good sources of vitamin D, including sunlight (15-20 minutes per day), fish—especially salmon, tuna, sardines—vitamin-D-fortified milk, and supplements.
And as we've reported in the past, higher-than-average intake of vitamin D also appears to produce preventive benefits in adults: primarily, reduced risks of fractures and certain cancers.
Note: Recent research results suggest that of the two available supplemental forms (D-2 and D-3), vitamin D-3 is the better choice, as it produces a longer-lasting boost in the body's vitamin D levels.