Vitamin D and Iron Preserved With 2 Cups of Milk per Day

Emma Hitt, PhD

December 17, 2012

Two cups of cow's milk daily appears to be the amount needed to provide adequate vitamin D and still maintain iron stores for most children, according to a new report.

Jonathon L. Maguire, MD, FRCPC, from the University of Toronto in Ontario, Canada, and colleagues report their findings in an article published online December 17 in Pediatrics.

According to the researchers, cow's milk consumption is known to increase vitamin D levels and decrease iron levels in children; however, "the amount of cow's milk intake required for sufficient stores of vitamin D and iron is poorly understood, and existing guidelines on consumption are unclear."

The current study sought to examine the association between cow's milk intake and vitamin D and iron stores in 1311 urban preschoolers who fit inclusion and exclusion criteria through the TARGet Kids! primary care practice–based research network in Toronto.

Parents of healthy children aged 2 to 5 years reported cow's milk consumption by responding to a standardized survey during primary care physicians' visits with their children between December 2008 and December 2010. Serum 25-hydroxyvitamin D and ferritin measurements were used as markers of vitamin D and iron stores, respectively. Complete sets of data were available for 76% of the children included.

"Children were excluded from this TARGet Kids! study if they had any chronic illnesses (other than asthma) or acute inflammation (C-reactive protein [CRP] >10 mg/L), [or] were on medications known to alter vitamin D or iron metabolism (ie, antiseizure medications)," the authors write.

Mean cow's milk intake was 460 mL each day. Mean 25-hydroxyvitamin D level was 88 nmol/L (95% confidence interval [CI], 87 - 89 nmol/L); 35% (95% CI, 33% - 38%) of the participants had 25-hydroxyvitamin D levels less than 75 nmol/L, and 6% (95% CI, 5% - 7%) of the participants had 25-hydroxyvitamin D levels less than 50 nmol/L. Mean ferritin was 31 mg/L; 4% (95% CI, 3% - 5%) of the participants had ferritin levels lower than 12 mg/ L.

Increasing cow's milk consumption was associated with decreasing serum ferritin (P < .0001) and increasing 25-hydroxyvitamin D (P ≤ .0001). Each cup (250 mL) of cow’s milk increased 25-hydroxyvitamin D levels an average of 6.5% and decreased serum ferritin levels, on average, by 3.6%.

According to the researchers, 2 cups, equivalent to 500 mL, of cow's milk per day seemed to be the amount needed to maintain 25-hydroxyvitamin D at levels greater than 75 nmol/L without resulting in low serum ferritin for most children.

Also according to the researchers, children with darker skin pigmentation not receiving vitamin D supplementation during the winter required 3 to 4 cups of cow's milk per day to maintain 25- hydroxyvitamin D at the same levels. They also found that "[a]mong children using a bottle, cow’s milk consumption did not increase median 25-hydroxyvitamin D and resulted in more dramatic decreases in median serum ferritin."

Study limitations include possible recall bias from the parent surveys, lack of generalizability because of the use of an urban setting for patient recruitment, and the inability because of the cross-sectional design to draw conclusions about causality.

"Our findings suggest that there is a trade-off between increasing 25-hydroxyvitamin D and decreasing serum ferritin with increasing milk intake," Dr. Maguire and colleagues conclude. Moreover, "bottle use, gender, season, vitamin D supplementation, skin pigmentation, and [body mass index] were significant modifiers of this trade-off." Vitamin D supplementation, current use of a bottle, summertime, light skin, and low body mass index were each associated with higher levels of 25-hydroxyvitamin D; the winter season was associated with increased ferritin levels.

According to the researchers, the findings are consistent with American Academy of Pediatrics recommendations of milk intake, depending on the clinical scenario.

They add that the findings highlight "the importance of vitamin D supplementation during the winter among children with darker skin pigmentation to maintain vitamin D stores."

Support for the TARGet Kids! program was provided by the Canadian Institutes of Health Research Institute of Human Development, Child and Youth Health; the Institute of Nutrition, Metabolism and Diabetes; and the St. Michael’s Hospital Foundation. This study was supported in part by the Canadian Institutes of Health Research. The Paediatric Outcomes Research Team is supported by a grant from the Hospital for Sick Children Foundation. The authors have disclosed no relevant financial relationships.

Pediatrics. Published online December 17, 2012. Abstract