It is involved in the photochemical reactions of the retina, it is an antioxidant, and has antimicrobial properties. The vitamin A content in milk decreases as lactation progresses. The intake obtained with a balanced diet is adequate and supplementation is not necessary. However, in developing countries it is recommended that all mothers take a single supplementary dose of 200,000IU of vitamin A as soon as possible after delivery.

Vitamin D deficiency is fairly frequent in pregnant and lactating women. Mothers that have restricted diets, such as strict vegetarians, and those with limited exposure to UV radiation (mothers with limited exposure to sunlight, with dark skin, or that wear a veil) may have very low plasma levels. The transfer of maternal vitamin D to milk is poor, so it is recommended that all breastfed infants younger than 1 year receive supplementation with 400IU/day of vitamin D, starting its administration within a few days from birth. Supplementation will continue until the child starts consuming one litre of vitamin D-enriched formula a day.24–28

The concentration of vitamin E in breast milk is sensitive to maternal intake, so the maternal diet must be assessed and supplemented if intake is inadequate.3,28

Vitamin K is also synthesised by bacteria lining the gastrointestinal tract. If the diet is adequate, the lactating mother does not require vitamin K supplementation. Newborns usually have low levels of vitamin K, as this vitamin is not easily mobilised through the placenta and the bacterial flora of the newborn is inadequate for its synthesis in the first days of life. Following birth, intramuscular administration of 1mg of vitamin K is recommended for the prevention of haemorrhagic disease of the newborn. Parents that refuse intramuscular administration of vitamin K are offered the oral route as an alternative (2mg of oral vitamin K at birth, followed by 1mg weekly until week 12 in infants that are partially or exclusively breastfed).

The concentration of water-soluble vitamins in milk depends to a great extent on maternal levels, so that deficiencies in the mother can lead to deficiencies in the infant.3

In the early weeks of life, the vitamin B6 stores accumulated during pregnancy are crucial in maintaining adequate levels in breastfed children. The manifestations of vitamin B6 deficiency in infants also depend on its severity, although in general it presents with neurologic symptoms and different forms of dermatitis.29

Vitamin B12 concentrations in the milk of well-nourished mothers are adequate. However, its levels are low in mothers that are strictly vegetarian (vegan), are malnourished or have pernicious anaemia, even if they do not show signs of deficiency. In these cases, it is important that mothers receive a vitamin B12 supplement the entire time they are breastfeeding, as vitamin B12 deficiency in the infant can have short- and long-term neurological effects.30

The plasma and tissue concentrations of vitamin C in smokers are lower than in nonsmokers, so an increase in vitamin C intake is recommended in mothers that smoke.

The recommended concentration of folic acid in breast milk can be easily achieved through dietary intake or supplementation, if needed.31

Iron supplementation is usually recommended to make up for losses sustained during childbirth, although it must be noted that women that practise exclusive breastfeeding usually experience amenorrhoea for a minimum of six months and thus do not lose iron through menstruation during that time. Therefore, it could be said that breastfeeding exerts a protective effect against maternal iron deficiency.9,32

Calcium is essential during lactation, during which it is subject to special regulatory mechanisms that lead to increased absorption, decreased renal excretion and greater mobilisation of bone calcium. To meet maternal calcium requirements, the American Academy of Paediatrics recommends lactating mothers to consume five servings a day of calcium-rich foods of any kind, such as low-fat yoghourt or cheese, and other nondairy foods that contain calcium, such as fish consumed with its bones (for example, canned sardines), salmon, broccoli, sesame seeds or cabbages, which may provide the 1000–1500mg daily recommended allowance for lactating women.33,34

Zinc is essential to growth, cell immunity and enzyme synthesis. While zinc concentrations in human milk are not high, they suffice to satisfy the needs of the child due to its high bioavailability. We recommend increasing zinc intake by 50% during lactation.

Selenium is a mineral involved in the immune system, cholesterol metabolism and thyroid function. The concentration of selenium in breast milk is three times that in artificial formulae.

The iodine requirements of lactating women nearly double those of healthy adult women, as in addition to meeting maternal requirements, iodine levels must guarantee that the baby receives sufficient iodine from the milk to synthesise thyroid hormones.35–37 The iodine content of human milk is variable and depends on maternal intake. In iodine-sufficient regions, the iodine content can reach 200μg/L in colostrum and 100–150μg/L in mature milk.

Since the requirements of the infant are approximately 90μg/day, and the volume of milk ingested ranges between 600 and 1000mL/day, the minimum iodine concentration in breast milk should be 100μg/L. To achieve this, the mother must consume at least 250μg of iodine a day.

Iodised table salt (the source of iodine recommended for all individuals) only contributes half of the required intake during lactation, while salt consumption may decrease during this period. Consequently, and in adherence with the recommendations of the American Thyroid Association, we recommend that breastfeeding women take 200μg a day of supplementary iodine in the form of potassium iodide. When the iodisation of the water supply becomes nationwide and has been established for at least two years, and once its consumption has become widespread in the population, iodine supplementation will no longer be necessary in pregnant and lactating women.