Vitamin D is one of the most commonly deficient nutrients. About 40% of Americans have inadequate levels. The official RDA (600-800 IU) was set to prevent bone disease, not optimize health. Most researchers think it's too low.
General Dosing Guidelines
Maintenance dose (if not deficient): 1000-2000 IU daily
Common dose for most adults: 2000-5000 IU daily
If deficient (under 30 ng/mL): 5000 IU daily until levels normalize, then drop to maintenance
If severely deficient (under 20 ng/mL): Some doctors prescribe 10,000 IU daily or weekly high-dose for a period
Important: Vitamin D is fat-soluble. Take it with a meal containing fat for better absorption.
Factors That Increase Your Need
Darker skin - More melanin means less vitamin D production from sun
Northern latitude - Above 37°N (roughly San Francisco, Philadelphia), sun angle limits D production October-March
Obesity - Vitamin D gets sequestered in fat tissue, requiring higher doses
Age over 60 - Skin produces less vitamin D
Limited sun exposure - Indoor work, sunscreen use, covered clothing
Certain conditions - Malabsorption, kidney disease, some medications
Testing and Target Levels
Get a 25-hydroxy vitamin D blood test. It's cheap and tells you exactly where you stand.
Target range: Most experts suggest 40-60 ng/mL (100-150 nmol/L) for optimal health. The "normal" lab range (30-100) is very wide.
Under 20 ng/mL: Deficient 20-30 ng/mL: Insufficient 30-50 ng/mL: Adequate 40-60 ng/mL: Optimal (per many researchers) Over 100 ng/mL: Potentially too high
Can You Take Too Much?
Yes, but it's hard to do accidentally. Toxicity typically requires chronic intake above 10,000 IU daily for months. Symptoms include nausea, weakness, kidney problems from excess calcium.
D3 vs D2: D3 (cholecalciferol) is more effective at raising blood levels than D2. Choose D3.
Consider K2 with D. Vitamin K2 helps direct calcium into bones rather than arteries. Many D supplements now include it.