Vitamin D3 is one of the most deficient nutrients in the Spanish population: more than 40% of adults have insufficient levels, even in a sunny country like Spain. This fat-soluble vitamin is essential for calcium absorption and bone health, the proper functioning of the immune system, mood, and muscle function. Unlike vitamin D2, the D3 form (cholecalciferol) is what the body naturally synthesizes with sunlight and is absorbed more efficiently. Combine it with vitamin K2 to optimize its utilization. Find the best vitamin D3 supplements from brands like Solgar, Solaray, and Physalis here.