Vitamin D plays an important role in regular bone growth and in adequate function of the innate immune system, including barrier functions of mucous membranes. A sufficient supply during pregnancy and lactation protects the child from infectious diseases.
Clinical symptoms of severe vitamin D deficiency (rickets) are well known and can be easily detected.

Signs and symptoms beyond deficiency, however, remain to be elucidated. Based on clinical and observational data, the plasma level of 25(OH)D may serve as a 'marker' to detect or define a subclinical deficiency. Levels below 50 nmol/l might be insufficient to maintain the non-bone-related activities of vitamin D.

Finally, it has to be considered that all of the non-bone activities of vitamin D are in concert with vitamin A (9-cis retinoic acid). Studies combining both vitamins in sufficient amounts (cod liver oil) demonstrated a beneficial effect on the prevention of respiratory tract infections. Consequently, it should be strongly recommended to increase the intake of vitamin D and to ensure a daily intake of vitamin A as counseled.