Dr. Bikle, Professor in Residence, Medicine, at the University of California, San Francisco, has a wealth of experience in the basic and clinical aspects of vitamin D metabolism and function. His session was titled, "Screening for & Treating Vitamin D Deficiency." In 50 minutes, his talk could not be a comprehensive analysis of vitamin D insufficiency and deficiency, so he focused his comments on audience questions. Here are some highlights:
There is no single target level of serum 25-hydroxyvitamin D [25(OH)D] acceptable to all physicians, but Dr. Bikle defends the target of 80 nmol/L or the equivalent of 32 ng/mL. "I believe about 30 ng/mL is a decent number, and-this is controversial and I'll have vitamin D zealots on my head for what I'm about to say-probably the juice is not worth the squeeze to get much above!" He noted that in Scandinavia and the Netherlands, a target of 20 ng/mL might be regarded as acceptable, given that the typical diet is calcium rich with cheese and bony fish, but a higher target is appropriate in the United States.
It is helpful to measure both 25(OH)D2 and 25(OH)D3 to monitor the full effect of vitamin D2 supplementation on total vitamin D levels. One widely used assay is the DiaSorin radioimmunoassay, but lab results have tended to vary greatly. However, labs are standardizing more now, Dr. Bikle said. An excellent test used in the past was the quantitative high performance liquid chromatography (HPLC) method, but it is too labor intensive to be used much today. New tests in development are based on mass spectrometry.
Vitamin D3 comes from diet or is formed in the skin after ultraviolet irradiation. It is further hydroxylated in the liver to 25(OH)D3 as the first step of its conversion. Dr. Bikle noted that as a person ages, the skin becomes less effective in this function, so an older person needs more sunlight exposure. There are no data to suggest that 25(OH)D absorption or conversion in the liver declines with age. However, the kidney is less effective at making dihydroxyvitamin D [1,25(OH)2D]) for any given parathyroid hormone (PTH) level, so PTH production rises to compensate. Additionally, the PTH level negatively correlates with 25(OH)D levels at least until 25(OH)D rises above 30 ng/mL, suggesting that 25(OH)D itself or its conversion to 1,25(OH)2D in the parathyroid gland might be a controlling factor. In an ideal world, all patients of any age would be tested for vitamin D during routine medical visits. But physicians typically measure those who are most at risk, including:
The main treatment is vitamin D, provided the patient does not have hypercalciuria that would be revealed in urine calcium testing. To urgently raise a patient's 25(OH)D level from near 15 ng/mL to 30 ng/mL, Dr. Bikle said he uses 50,000 IU of D2 weekly and continues for a month before checking the urine calcium to determine the next step. For someone deficient-at 10 ng/mL or less-he would continue that dose for at least 6 to 8 weeks. Although not readily available in high doses, D3 should be given in preference to D2 because it is more biologically active, in part because it is less rapidly cleared. But either form of vitamin D will work. When asked about the benefit of giving mega doses every 3 months, he recommended against it, although the data are scant for this means of administration. Because of its quicker clearance, if D2 is given only every 3 months, during much of that time the vitamin D levels may not be in the desired range. He added that taking a dose of 800 IU daily is not toxic, given that it translates into 25(OH)D increments of only 8 ng/mL, and substantially higher doses (e.g., 2,000 IU daily) are probably safe for most individuals.
During wintertime, with less sunlight, routine vitamin D supplementation should be greater than in summer. "Unless you are very fair and very sensitive to sunlight, a moderate amount of exposure to sun is the best way to get your vitamin D during the time you can make it, but you can't make it all year long," he said. "Skin is the best source of vitamin D in my humble opinion," he elaborated. "I can't prove to you that the vitamin D made in the skin is any better than the vitamin D you can buy at the drugstore?hopefully, one day I will."