Essential for calcium absorption and thus for strong bones and teeth. Also plays a role in immune system function.
Fatty fish (salmon, mackerel, herring), egg yolk, and fortified products. The body produces vitamin D itself under the influence of sunlight on the skin — in countries with little sun (such as Belgium and Netherlands, especially in winter), a supplement is often recommended.
In children: Rickets (soft/deformed bones). In adults: bone demineralization and muscle weakness.