Over 40% of US adults are vitamin D deficient — the most common nutritional deficiency in America. This 2026 complete guide covers all symptoms (bone pain, weakness, fatigue, low mood), the D3 vs D2 debate resolved, the 50,000 IU weekly loading protocol, a dosage table by risk group, 8-12 week recheck rule, toxicity warning signs, and exactly why sunscreen and glass block all vitamin D production.