Vitamin A

Colostrum is particularly rich in vitamin A. The content of vitamin A in human milk depends on maternal stores and is transported in the lipid fraction of human milk primarily as retinyl ester. It has been estimated that over the first 6 months of lactation, infants receive 60 times the amount of vitamin A that they received during the 9 months of pregnancy. However, women lacking food sources of vitamin A demonstrate low concentrations in both plasma and milk. Milk concentrations correlate with plasma concentrations and can be useful as a biomarker of maternal vitamin A status. In populations with low levels of vitamin A, breastfeeding is associated with significant protection of infants against xerophthalmia. Postnatal supplementation of mothers may improve maternal nutritional status and infant stores of vitamin A.

The B vitamins except for folate

Vitamins B1, B2, B3, B6, and B12 are not stored and are therefore necessary in the diet. These vitamins are readily transported across the mammary gland, but grain diets that are not fortified with B vitamins, diets low in animal products and thus in B12 intake, or disease conditions that influence a mother’s B12 status may result in low concentrations of these B vitamins in human milk. Likewise, the use of oral contraceptives before pregnancy and lactation can adversely affect vitamin B6 concentrations in mother’s milk. Women living in areas of the world that lack food fortification strategies are particularly vulnerable. In developed nations, mothers consuming specialty diets or fad diets, mothers who have had an intestinal injury or gastric bypass, or mothers who are consuming gluten-free diets that are not replacing the vitamin B usually obtained from wheat products could also be at risk.

Vitamin C

Maternal dietary vitamin C has been correlated with maternal milk concentrations in some studies. In a study of 200 women a significant seasonal variability in concentrations of vitamin C was noted, with the higher concentration evident in summer months. In supplementation studies, however, the supplement in the maternal diet did not correspond to maternal milk concentrations, implying a regulatory mechanism for vitamin C in milk. Nevertheless, mothers should be encouraged to consume a healthy diet that is replete with vitamin C.

Vitamin D

This vitamin is a precursor hormone, and unlike other nutrients that can only be obtained from diet, the vast majority of vitamin D is derived from its synthesis within the skin following ultraviolet B exposure. Only a small portion of the daily requirement comes from the diet, mainly from fatty fish, organ meats such as liver, eggs (in the form of vitamin D3 or cholecalciferol), and mushrooms (in the form of vitamin D2 or ergocalciferol). In a recent vitamin D supplementation trial conducted during pregnancy (sponsored by the National Institute of Child Health and Human Development [NICHD]), the average daily intake of women was approximately 200 IU/d compared with the 10,000 to 20,000 IU that are generated within 24 hours of whole-body sunlight exposure (without sunscreen). It was thought for decades that the sole purpose of vitamin D was for calcium homeostasis and to prevent rickets in children. However, advances made in the past decade using molecular techniques demonstrate the significant role that vitamin D plays in immune function, both innate and adaptive. Given this recently expanded view of vitamin D, it is clear that its role in immune modulation during lactation and in the breastfeeding infant is just beginning to be understood.

The content of vitamin D in human milk itself has also been a source of controversy. Studies of the vitamin D or antirachitic content of human milk showed that on average there was approximately 70 IU/L and that this amount was barely able to provide for the breastfeeding infant who had no sunlight exposure. The AAP revised their recommendations in 2008 to include vitamin D supplementation of 400 IU/d within the first few days after delivery in all breastfeeding infants and in any infant who consumes less than 1 L of formula per day (which contains ∼400 IU/L). Such a recommendation provides adequate vitamin D to the breastfeeding infant but does not address the needs of the mother, nor does it address the issue of why breast milk has marginal sufficiency of vitamin D.

Recent studies have demonstrated that vitamin D supplementation in the mother improves her vitamin D status, thereby improving her milk antirachitic activity and, thus, the transfer of vitamin D to her infant. In an initial pilot study, vitamin D2, 2000 IU/d versus 4000 IU/d, was given to mothers and the transfer to their fully breastfeeding infants was tracked. There was an increase in maternal total circulating 25(OH)D, a concomitant increase in milk antirachitic activity, and improved infant vitamin D status that significantly correlated with maternal vitamin D status. A second pilot study performed by this group compared maternal supplementation vitamin of 400 IU/d (the amount found in most prenatal vitamins) with 6400 IU/d. The infants whose mothers were randomized to 400 IU/d also received 400 IU/d, and those infants whose mothers were randomized to 6400 IU/d received placebo (0 IU/d). Mothers who received the 6400 IU/d dose had improved vitamin D status that resulted in 25(OH)D levels in their infants comparable with levels of those infants who were receiving 400 IU/d. There was no toxicity associated with the higher dosing regimen; however, the sample size for this pilot study was small. A larger, 2-site NICHD trial initiated in 2006 was recently completed. Although the results of this larger study are not yet fully available, there was no reported toxicity in either the mother or the infant, and the preliminary analyses are consistent with the findings of the earlier pilot study. These studies clearly indicate that breast milk is only deficient or “minimally sufficient” in vitamin D if the mother is deficient or “minimally sufficient” herself. There is a strong correlation between maternal vitamin D status, milk antirachitic activity, and infant vitamin D status.

The current recommendation of the AAP, reiterated by the Institute of Medicine (IOM), is to supplement the infant with 400 IU/d to achieve vitamin D sufficiency in the breastfeeding infant. It is suggested that the mother continues to take her prenatal vitamin containing between 400 and 600 IU. Data from recent vitamin D supplementation trials that have the potential to shift the paradigm of care are awaited. In the meantime, it is essential to educate women who choose to breastfeed about the various options to improve their vitamin D status and that of their breastfeeding infant.

Fatty acids are made by the mammary gland but the content of fatty acids in human milk largely depends on maternal diet and body stores. The essential fatty acids, including the ω-6 linoleic acid series and ω-3 linolenic acid series, are required in the diet to avoid essential fatty-acid deficiency in both mother and infant. Over the last century dietary habits have changed in North America and elsewhere, increasing the ratio of ω-6 to ω-3 long-chain polyunsaturated fatty acids (LCPUFAs). These LCPUFAs exert unique biological effects, and thus should not be grouped together to examine their effects. Immune homeostasis is affected in the mother and infant as the different fatty acids modulate a very different prostaglandin and cytokine expression. Perhaps the most potent of the ω-3 fatty acids is docosahexaenoic acid (DHA), which has beneficial anti-inflammatory activity. Another important biological effect of DHA pertains to brain development and cognition. A provocative study by Jensen and colleagues examined supplementation with a modest 200 mg/d of DHA in the nursing mother’s diet for the first 4 months postpartum, and demonstrated that sustained attention scores at 5 years of age improved in the intervention offspring compared with controls.

Careful attention to reading the label on dietary supplements is necessary, as it has been observed that DHA supplements can consist of a mere 23 mg/d in some fish-oil products. In the Midwest, motivated nursing women taking a varied diet and their prenatal vitamins have a low concentration of DHA in their milk at 0.1%, similar to women in developing countries on meager food sources. Supplementing these breastfeeding mothers with 1 g DHA in comparison with placebo increased milk concentrations significantly for the nursing infant. Most expert panels suggest that adult women ingest 3 ω-3–rich fish sources per week or a minimum of 300 to 1000 mg of DHA per day while pregnant or breastfeeding.

Finally, the forms of fats consumed by the mother are also consumed by the breastfed infant. That is, as for unsaturated fats, if saturated or trans fats predominate in the mother’s diet, these fats are also transferred to the infant through breast milk. The murine model of obesity demonstrates a direct relationship between saturated fats in the mouse pup and fat deposition, thus prompting careful examination of the mother’s diet and recommendations for a healthy balance of dietary sources of ω-6 and ω-3 fatty acids.

Iodine

Iodine concentration in breast milk is strongly influenced by the mother’s iodine status. Iodine is essential for thyroid and developmental function. Fortunately, the mammary gland is able to concentrate iodine and often provides adequate iodine to the infant even in mothers of insufficient status. Iodine status is measured by median urinary iodine concentration in response to salt iodization. The current World Health Organization (WHO) recommendation is a daily maternal intake of 250 μg/d during lactation to ensure that deficiency does not occur. Women who live where iodized salt programs exist appear to have reasonable iodine status.

Fig. 1 describes the targets for both maternal and infant diets during lactation.

The targeted approach to nutrients for the nursing infant.

Energy and protein

These components are provided to the breastfed infant through human milk almost regardless of the mother’s diet. Energy restriction or exercise during lactation that is modest (deficits of 500 kcal/d or exercise 45 minutes 4–5 times per week) does not affect milk volume, lactose, or protein concentrations. Rather, if the energy costs of milk production are not met by maternal diet, maternal stores will be depleted to ensure adequate milk production. It appears that maternal plasma prolactin increases with negative energy balance and may protect lactogenesis.

Calcium

Calcium homeostasis in the mother is another good example of maternal use of stores to protect lactation. The calcium found in milk is regulated in the mammary gland by citrate and casein unrelated to maternal dietary intake of calcium or vitamin D. Based on stable isotope studies, Mohammad and colleagues demonstrated that calcium stores from bone are used during lactation to assure availability to milk production. Overall, the bone loss during lactation is transient, and does not increase the risk of osteoporosis or bone fractures in later life. However, women may experience bone loss if their calcium intake is less than 500 mg/d and should therefore include good sources of calcium in their diet.

Vitamin E

This vitamin is present in 3 forms: α-tocopherol, β-tocopherol, and γ-tocopherol, with α-tocopherol the most active form. The level of α-tocopherol is significantly higher in early milk and correlates with lipid concentrations, but does not appear to be affected by maternal diet or smoking.

Folate and the minerals zinc, copper, iron, and zinc

These nutrients are similarly regulated, so that their concentrations in milk do not depend on maternal diet. Thus, for these nutrients the quality the mother’s diet does not affect the quality of her milk but is important for maintaining her own health.