Background: The rate of non-compliance with vitamin D supplementation is as high as 45%. This is why randomised controlled trials are needed to analyse the response to low doses of vitamin D3.

Objective: (1) To compare supplementation with 250 versus 500 units of vitamin D3 and (2) to analyse sun exposure time/ultraviolet B (UVB) exposure during the first 6 weeks of life.

Design: 40 breastfed infants (skin photo-types I, II) were recruited in Berlin, Germany (52.5°N), during summer (n=20) and winter (n=20) and randomised into equal groups on either 250 or 500 units of vitamin D3 per day. Outcome measures were: parameters of vitamin D and bone metabolism at delivery and 6 weeks later, sun exposure time, UVB dosimetry and surrounding factors including maternal diet.

Results: At delivery 25-hydroxy vitamin D levels were insufficient: 68 (53–83) nmol/l in each group. 6 weeks later levels were sufficient: 139 (114–164) nmol/l on 250 units of vitamin D3 per day and 151 (126–176) nmol/l on 500 units/day. There was no seasonal variation. Daily sun exposure time was 0.4–3.5 h and higher in summer. UVB exposure was 0.01–0.08 minimal erythema dose/day. Calcium levels were within normal.

Conclusions: In Berlin, Germany, supplementation with 250 units of vitamin D3 is sufficient for breastfed infants during their first 6 weeks of life in summer and winter. UVB exposure is very low throughout the year.