Vitamin D, D3 or 'cholecalciferol' for scientists, is a fat-soluble vitamin with numerous functions. This vitamin, already well known to the general public, is also important for the regulation of immune function2, especially the differentiation and proliferation of hematopoietic cells2. In addition to a healthy and balanced diet, an extra intake of vitamin D can be useful in case of a deficiency, especially when there is little sunshine, such as in autumn and winter.