Vitamin D: A Rapid Review

Mark A. Moyad, MD, MPH

Disclosures

Dermatology Nursing. 2009;21(1) 

In This Article

The "Ideal" Vitamin D Blood Level

Over the past few decades, the "normal" blood level of vitamin D (25-OH vitamin D) was based on the amount needed to keep PTH from becoming abnormally high. Again, PTH at high levels can cause calcium loss from the bone, so this would make sense that vitamin D could maintain or improve bone health at these levels. However, PTH can change due to renal function, exercise level, the time of day, or even diet. There has been no consensus on the optimal level of vitamin D intake to reduce PTH, and this is why many laboratories report the normal range of vitamin D to be so wide (20 to 40 ng/ml, or in some cases, 50 to 100 nmol). However, this is tantamount to saying a normal total cholesterol level is between 100 to 500.

What is the best blood level of vitamin D? Several prominent experts reviewed a large number of past studies to arrive at an answer to this question (Bischoff-Ferrari, Giovannucci, Willett, Dietrich, & Dawson-Hughes, 2006). Their findings were satisfactory in this author’s opinion. A variety of health changes not specific to bone health were evaluated, and the researchers sought to determine what level of vitamin D could maintain muscle strength, prevent falls, improve dental health, and prevent cancer (especially colorectal cancer). Weaker evidence for vitamin D includes preventing multiple sclerosis, other cancers, arthritis, hypertension, and tuberculosis, as well as solving insulin problems (diabetes mellitus). These researchers also reviewed a variety of other areas apart from keeping PTH normal and looked at studies that included a variety of ethnic groups. These experts found a consistent answer, which is that most clinical studies in a variety of health areas point toward a blood level of vitamin D that is between 90 to 100 nmol/L, or 35 to 40 ng/ml, for preventive health.

Why not surpass the number of 35 to 40 ng/ml as some experts have suggested? Unfortunately, higher does not mean better. Medical research is replete with examples of where a little higher helped, but more was not necessarily better. Supraphysiologic levels beyond what is now recommended in this manuscript is not yet supported in medical literature. It is interesting that some studies (for example, in the area of prostate cancer) have not yet found considerable benefits to achieving such higher vitamin D levels (Mucci & Spiegelman, 2008). In fact, it has been suggested that long-term significant increases in vitamin D could be detrimental. Thus, some experts suggest that there is no harm of carrying high vitamin D levels (70 ng/ml or more for example), but this recommendation is based on acute and not chronic observations. Not long ago, this same philosophy was applied to selenium or vitamin E, and ample evidence now exists to suggest that toxicity can occur when these nutrients are given chronically in mega-doses to achieve higher-thannormal blood levels of these nutrients.

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