The fastest safe way to improve vitamin D depends on your baseline 25-hydroxyvitamin D (25(OH)D). For most people, ≥20 ng/mL (≥50 nmol/L) is generally considered adequate, while <12 ng/mL (<30 nmol/L) increases deficiency risk. Sun exposure can stimulate skin synthesis, but season, latitude, skin pigmentation, age, clothing, sunscreen, and lifestyle strongly affect results. Foods (fatty fish, fortified dairy/plant milks, egg yolk) help, yet diet alone may not be enough. If supplementation is needed, consistency matters more than “mega-dosing.” Response varies by starting level and dose, and very high 25(OH)D (e.g., >50 ng/mL, especially >60 ng/mL) is linked to higher risk of adverse effects—so more is not always better. A practical approach is: test → personalize dose → recheck in ~8–12 weeks → maintain with a sustainable plan.