Commentary to Article Entitled “Vitamin D Deficiency Is Not Associated With Growth or the Incidence of Common Morbidities Among Tanzanian Infants”

See “Vitamin D Deficiency Is Not Associated With Growth or the Incidence of Common Morbidities Among Tanzanian Infants” by Sudfeld et al on page 467. The recent article by Sudfeld et al (1) examining the incidence of vitamin D deficiency in non–HIV-infected Tanzanian infants provides ongoing evidence that vitamin D insufficiency continues to be a global problem in exclusively breast-fed infants who do not receive routine vitamin D supplementation. Interestingly, no associations between suboptimal vitamin D status and post-partum growth and incidence of common comorbidities (upper/lower respiratory infections, clinical malarial symptoms) were found at 6 weeks and 6 months, even though infants with serum 25(OH) vitamin D levels higher than 30 ng/mL were shown to have an increased relative risk for experiencing lower respiratory symptoms at 6 months. Unfortunately, the authors did not provide information on maternal vitamin D status/intake. Maternal vitamin D status has been well documented to be a determining factor influencing maternal breast milk vitamin D content (2). Most studies indicate that only small amounts of vitamin D (10–20 IU/L) are present in human milk (2,3). Maternal vitamin D supplementation in the order of 4000 to 6000 IU/D is needed to increase vitamin D breast milk content to levels sufficiently high to prevent deficiency in exclusively breast-fed infants with limited sunlight exposure (4,5). In contrast, commercial infant formulas typically contain in the range of 400 IU/L (2). Infants younger than 6 months and exclusively, however, fed commercial infant formulas rarely meet the recommended daily allowance of 400 IU/D due to the inability to consume volumes that are required to meet the recommended daily allowance (2). This could explain the overall high prevalence of suboptimal vitamin D status (<20 ng/mL) at 6 weeks (76.4%) observed in this study for both the exclusively breast-fed and formula-fed infants. A major challenge in interpreting study findings for vitamin D status at 6 months was lack of information regarding the influence of sunlight exposure, exclusivity of breast-feeding and/or the use of commercial formulations to meet the needs of the infants. The authors noted that approximately 4.1% of infants were either receiving breast milk and/or formulas with the remaining 95% of infants being largely unreported. Hence, it is difficult to determine the extent to which breast milk and/or commercial formulas were contributing to overall vitamin D status at this time point. Presumably, complementary table foods were being fed to infants studied at 6 months, but their relative contribution to overall vitamin D intake is unclear, even though this may have played a role given the lower prevalence of suboptimal vitamin D status in the infants (21.2%) at 6 months. Complementary infant foods in this region as noted by authors typically include maize based porridges, which may be supplemented with local foods including milk, meat, and/or legumes. All of these contain relatively low amounts of vitamin D (6). Foods rich in vitamin D include egg yolks, fatty fish, and some fortified margarines/cow's milk; foods that may not be routinely available or consumed by younger infants in this population (7). Although authors note that the insufficient power study that may have precluded the ability to determine associations between vitamin D status and outcomes of interest (growth, respiratory infection), other factors also contributed. This included random allocation to either zinc or micronutrient supplementation in the larger randomized clinical trial that infants were also enrolled, which may have independently or synergistically influenced growth and risk for infection. Deficiency and/or micronutrient repletion of other nutrients must be evaluated before any conclusions can be drawn as to whether optimizing vitamin D status in the general population of Tanzanian infants is needed to promote improved growth and minimize infection risk. Finally, a closer evaluation of the cutoffs defining sufficiency/insufficiency for serum 25(OH) vitamin D in healthy infants to promote overall health would be beneficial. Although slightly different cutoffs for vitamin D sufficiency/insufficiency have been defined in the range of 10 to 20 ng/mL in infants in the first year of life, the Institute of Medicine suggests that the midrange of 15 ng/mL can be used to define for level of vitamin D deficiency for infants up to 1 year of age (8,9). This is particularly important to consider due to the ongoing debate about what levels of serum 25(OH) vitamin D (20–30 ng/mL) should be used to define optimal levels to promote overall health. Current findings in this study suggest that levels higher than 30 ng/mL may be needed to prevent increased risk for comorbid conditions, but due to the aforementioned limitations within the current study design should be interpreted with caution.

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