Pregnant and lactating women and breastfed infants are at risk of vitamin D deficiency. The supplemental vitamin D dose that optimises maternal vitamin D status and breast milk antirachitic activity (ARA) is unclear. Healthy pregnant women were randomised to 10 (n 10), 35 (n 11), 60 (n 11) and 85 (n 11) µg vitamin D3/d from 20 gestational weeks (GW) to 4 weeks postpartum (PP). The participants also received increasing dosages of fish oil supplements and a multivitamin. Treatment allocation was not blinded. Parent vitamin D and 25-hydroxyvitamin D (25(OH)D) were measured in maternal plasma at 20 GW, 36 GW and 4 weeks PP, and in milk at 4 weeks PP. Median 25(OH)D and parent vitamin D at 20 GW were 85 (range 25–131) nmol/l and ‘not detectable (nd)’ (range nd–40) nmol/l. Both increased, seemingly dose dependent, from 20 to 36 GW and decreased from 36 GW to 4 weeks PP. In all, 35 µg vitamin D/d was needed to increase 25(OH)D to adequacy (80–249 nmol/l) in >97·5 % of participants at 36 GW, while >85 µg/d was needed to reach this criterion at 4 weeks PP. The 25(OH)D increments from 20 to 36 GW and from 20 GW to 4 weeks PP diminished with supplemental dose and related inversely to 25(OH)D at 20 GW. Milk ARA related to vitamin D3 dose, but the infant adequate intake of 513 IU/l was not reached. Vitamin D3 dosages of 35 and >85 µg/d were needed to reach adequate maternal vitamin D status at 36 GW and 4 weeks PP, respectively.

Breast-fed infants are susceptible to vitamin D deficiency rickets. The current vitamin D ‘adequate intake’ (AI) for 0–6-month-old infants is 10 µg/d, corresponding with a human milk antirachitic activity (ARA) of 513 IU/l. We were particularly interested to see whether milk ARA of mothers with lifetime abundant sunlight exposure reaches the AI. We measured milk ARA of lactating mothers with different cultural backgrounds, living at different latitudes. Mature milk was derived from 181 lactating women in the Netherlands, Curaçao, Vietnam, Malaysia and Tanzania. Milk ARA and plasma 25-hydroxyvitamin D (25(OH)D) were analysed by liquid-chromatography-MS/MS; milk fatty acids were analysed by GC-flame ionisation detector (FID). None of the mothers reached the milk vitamin D AI. Milk ARA (n; median; range) were as follows: Netherlands (n 9; 46 IU/l; 3–51), Curaçao (n 10; 31 IU/l; 5–113), Vietnam: Halong Bay (n 20; 58 IU/l; 23–110), Phu Tho (n 22; 28 IU/l; 1–62), Tien Giang (n 20; 63 IU/l; 26–247), Ho-Chi-Minh-City (n 18; 49 IU/l; 24–116), Hanoi (n 21; 37 IU/l; 11–118), Malaysia–Kuala Lumpur (n 20; 14 IU/l; 1–46) and Tanzania-Ukerewe (n 21; 77 IU/l; 12–232) and Maasai (n 20; 88 IU/l; 43–189). We collected blood samples of these lactating women in Curaçao, Vietnam and from Tanzania–Ukerewe, and found that 33·3 % had plasma 25(OH)D levels between 80 and 249·9 nmol/l, 47·3 % between 50 and 79·9 nmol/l and 19·4 % between 25 and 49·9 nmol/l. Milk ARA correlated positively with maternal plasma 25(OH)D (range 27–132 nmol/l, r 0·40) and milk EPA+DHA (0·1–3·1 g%, r 0·20), and negatively with latitude (2°S-53°N, r −0·21). Milk ARA of mothers with lifetime abundant sunlight exposure is not even close to the vitamin D AI for 0–6-month-old infants. Our data may point at the importance of adequate fetal vitamin D stores.