Amplitude of annual variation in serum 25(OH)D concentrations (nmol/L) in different supplementation groups.

1Mean amplitude of annual variation, estimated by repeated measures non-linear regression using a sine wave model, adjusted for age and gender. 2Mean difference of amplitude between 100–600 IU/day and 800 IU/day: 5.2 (95%CI-1.6 to 12.0; p = 0.13). 3Comparison of the mean amplitude between non-supplement and supplement groups.