Page 36 - Summer 2017 Journal

ARE VITAMIN D TESTS RELIABLE?
Can we be confident that our 25(OH)D test results represent what is going on in the body?
In this article we have made a broad assumption, namely that we can and should rely on the 25(OH)D blood test to accurately reflect the body’s level of circulating vitamin D, then use this to make a personal or clinical decision on how much vitamin D to supplement, or whether to increase the consumption of vitamin-D rich foods or time spent in the sun. In many cases, this assumption is likely a good one. However, exceptions have been found in individuals who have obesity, certain genetic variations, and/or acute or chronic inflammatory disease, raising questions on the broad applicability of the 25(OH)D test. In addition, some of the laboratory protocols used to test vitamin D have inherent limitations.
The blood test for 25(OH)D measures only the hydroxylated forms of vitamin D3 and D2 that circulate in the bloodstream. While considered the best test for vitamin D, is it a reliable indicator of whole body vitamin D status? Unfortunately, not always. It does not measure the amount of vitamin D itself (in the non-hydroxylated form) that is stored in the body fat, which for some people may be considerable. Obese persons have lower blood levels of vitamin D, yet at the same time can have considerable quantities stored in their body fat. For example, one study found after taking a supplement of 20,000 IU of vitamin D3 once per week for 3-5 years, serum 25(OH)D was 39.6 ng/mL versus 24.8 ng/mL in the placebo group, but abdominal fat contained 209 ng/g of vitamin D3 versus 32 ng/g in placebo group. Assuming that the 209 ng/g concentration was the same in all body fat, the supplemented group had an average of 264,000 IU stored vitamin D in their bodies.44 Whether this presents a problem is unknown, but there is the potential for stored vitamin D to be released during even modest weight loss.22
GENETIC VARIATION
Serum 25(OH)D levels are influenced by a number of genes that govern the vitamin D binding and receptor pro-
teins and the enzymes that hydroxylate vitamin D. Increases in serum 25(OH)D with vitamin D supplementation vary according to common differences (polymorphisms) in these genes, with potential implications for vitamin D-related health outcomes.45 The differences in 25(OH)D levels attributed to polymorphisms has been estimated to range from 23 to 77 percent, rivaling differences due to sun avoidance, and may be related to the season of the year.46 This is an emerging area of research, but it appears that in those affected, serum 25(OH)D may actually “overestimate” actual vitamin D intakes from food or sunlight.10
INFLAMMATION AND INFECTION
Multiple observational studies have reported a correlation of vitamin D deficiency with inflammation and in-
flammatory diseases, however cause and effect has yet to be conclusively demonstrated for the majority of these relationships. For example, a review of several clinical trials of vitamin D supplementation with overweight or obese individuals found no overall improvement in inflammatory markers.47 Evidence has emerged to suggest another hy- pothesis—that low 25(OH)D is a result of the chronic disease process, provoked by a chronic bacterial infection.48
LABORATORY TEST VARIABILITY
Between different laboratories, substantial variations in results for serum 25(OH)D have been reported.49 The
Office of Dietary Supplements of the National Institutes of Health has instituted the Vitamin D Standardization Pro- gram (VDSP), a collaborative venture of several governmental and non-governmental organizations, to address this problem.50 It has been suggested that the preferred assay is mass spectrometry for a number of reasons, but costs and high levels of expertise required are barriers to its availability. In the future, the CDC plans to post on their website a list of laboratories that meet the VDSP certification standards.
INTERFERING COMPOUNDS
Many of the commonly available 25(OH)D laboratory tests are unable to distinguish true 25(OH)D from related
forms called epimers and isobars. In some individuals, especially those with autoimmune diseases, there can be relatively high amounts of these forms leading to a high degree of uncertainty on what is actually being measured by the blood test.51
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Wise Traditions SUMMER 2017