health organisations such as the NHS in the UK have no such recommendation. Breast milk contains less iron than formula, because it is more bioavailable as lactoferrin, which carries more safety for mothers and children than ferrous sulphate. Both the AAP and the NHS recommend vitamin D supplementation for breastfed infants. Vitamin D can be synthesised by the infant via exposure to sunlight, however, many infants are deficient due to being kept indoors or living in areas with insufficient sunlight. Formula is supplemented with vitamin D for this reason. Production Under the influence of the hormones prolactin and oxytocin, women produce milk after childbirth to feed the baby. The initial milk produced is referred to as colostrum, which is high in the immunoglobulin IgA, which coats the gastrointestinal tract. This helps to protect the newborn until its own immune system is functioning properly. It also creates a mild laxative effect, expelling meconium and helping to prevent the build-up of bilirubin (a contributory factor in jaundice). Male breastfeeding can occur and the production of the hormone prolactin is necessary to induce breastfeeding (see male lactation). Actual inability to produce enough milk is rare, with studies showing that mothers from malnourished regions still produce amounts of milk of similar quality to that of mothers in developed countries. There are many reasons a mother may not produce enough breast milk. Some of the most common reasons are an improper latch (i.e., the baby does not connect efficiently with the nipple), not nursing or pumping enough to meet supply, certain medications (including estrogen-containing hormonal contraceptives), illness, and dehydration. A rarer reason is Sheehan's syndrome, also known as postpartum hypopituitarism, which is associated with prolactin deficiency and may require hormone replacement. The amount of milk produced depends on how often the mother is nursing and/or pumping: the more the mother nurses her baby or pumps, the more milk is produced. It is beneficial to nurse when the baby wants to nurse rather than on a schedule. A Cochrane review came to the conclusion that a greater volume of milk is expressed whilst listening to relaxing audio during breastfeeding, along with warming and massaging of the breast prior to and during feeding. A greater volume of milk expressed can also be attributed to instances where the mother starts pumping milk sooner, even if the infant is unable to breastfeed. Sodium concentration is higher in hand-expressed milk,