To maintain those levels, the current RDA for vitamin D in adults is 600 International Units or 15 micrograms (vitamin D can be measured either way; 1 microgram equals 40 International Units). But many experts believe that blood levels should be much higher and that levels above 30 ng/ml are preferable for optimal health.37 Until the issue is resolved, taking a supplement providing between 600 and 1,000 International Units of vitamin D is a reasonable choice. It’s always a good idea to include an assessment of vitamin D levels when your doctor orders routine blood tests.
