From the September 2005 edition of Dr. Fuhrman's Healthy Times:
Assessment of vitamin D status is usually made by measuring 25-hydroxy-vitamin D; however, the optimal serum concentration is somewhat controversial. The data sheets from most blood laboratories list 20 ng/ml as the cutoff for vitamin D deficiency. However, recent studies have demonstrated that parathyroid hormone (PTH) levels begin to rise as 25-hydroxy-vitamin D falls below 30, and recently there has been a growing consensus that 30 ng/ml should be used as a cutoff for the diagnosis of vitamin D inadequacy. As pointed out by T. Colin Campbell, Ph.D., in The China Study, it may be true that vegetarians and those eating little animal protein have a more efficient conversion of the 25-hydroxy-vitamin D into its biologically active form, 1,25-dihydroxy, and perhaps a blood level below 30 is not so bad in a vegan or vegetarian. I do not think there is enough evidence to take a chance with running a D level below 20, unless your level of 1,25-dihydroxyvitamin D comes out on the blood test above 40. Otherwise, I think a blood level of at least 25 ng/ml is still advisable in those who consume little or no animal products.
Recently, a large study assessed the vitamin D status of postmenopausal women receiving therapy to treat or prevent osteoporosis. Amazingly, they found that 52 percent of the 1,536 women had inadequate vitamin D levels—and these were women being treated with drugs for osteoporosis. We know that vitamin D levels are inadequate in the vast majority of American women, and that this deficiency is a major cause of disability and death.
For more on vitamin D, check out these previous posts: