Page 34 - Summer 2017 Journal

that the average older man and woman will need intakes of at least 800 to 1,000 IU per day of vitamin D3 to reach a serum 25(OH)D level of 30 ng/mL.21 In addition, vitamin D supplementation at 600-800 IU per day has been shown in studies to reduce fracture risk when calcium is also supplemented.22 Caution with higher levels accompanied by calcium supplementation is advised: according to the Merck Manual, “target 25(OH)D levels are > 20 to 24 ng/mL for maximal bone health; whether higher levels have other benefits remains uncertain, and higher absorp- tion of calcium may increase risk of coronary artery disease.”23
The efficiency of calcium absorption in the small intestine increases with higher levels of serum 25(OH)D, reaching a plateau at 32 ng/mL.24 Some studies suggest that the lower end of the IOM reference range for 25(OH)D, 20 ng/mL, may not optimize calcium absorption, and that levels of at least 32 ng/mL may be required.25
Bone mass density (BMD) has been positively associated with 25(OH)D levels, as one might expect. In the Multi-Ethnic Study of Atherosclerosis involving 1,773 adult participants, this association was significant for Caucasian and Asian participants alone; in Hispanics there was a non-statistically significant association. An inverse association was actually found in Black participants: those having 25(OH)D levels less than 20 ng/mL had higher BMD than those with levels greater than 30 ng/mL. Even more surprising may be the finding that 25(OH)D levels were highest among Whites and lowest among Blacks, but BMD was highest among Blacks. The IOM states that “African-Americans pres- ent a conundrum, because, although their serum values are lower than those of white counterparts, their rates of osteoporosis and fractures are lower than Caucasians.”22 Thus it appears that when interpreting blood test results, racial differences should be considered as they may influence recommendations for vitamin D supplementation.26
The level of 25(OH)D required for maximum suppression of para- thyroid levels is also an indicator of vitamin D requirement. Chronic elevations of serum PTH increase osteoclast (bone cell breakdown) activity, negatively affecting bone density. Besides low vitamin D lev- els, low dietary calcium, skeletal muscle wasting, primary or secondary hyperthyroidism, and chronic kidney disease can all be independent con- tributors to high serum PTH concentrations.27 The level of 25(OH)D that maximally suppresses parathyroid hormone blood levels is somewhere around 32 ng/mL for adults.28,29 A meta-analysis of available clinical trials indicates that vitamin D supplementation of 1000 IU per day can best suppress serum PTH levels.27
While many studies suggest extraskeletal benefits, overall the evidence does not strongly support the use of supplementation with vitamin D to address the myriad of diseases that have been linked to low serum 25(OH)D levels.30 At the same time, a systematic review of studies showed that supplementation using vitamin D3 alone (but not D2) reduced all-cause mortality by 11 percent.31 Being deficient in vitamin D is not good for one’s health, but in the medical community the jury is out on whether levels above those needed for bone health offer clear benefits.
MISSING PIECES OF THE PUZZLE
Any student who has taken an introductory nutrition course learned that the fat-soluble vi- tamins, and especially vitamins A and D, can produce toxicity at high levels of intakes. The IOM has set the Tolerable Upper Intake Level for vitamin D at 4,000 IU per day for adults,5 identical to the amount the Geriatric Society generally recommends to support seniors’ bone and muscle strength.8 The Vitamin D Council states that toxicity is highly unlikely until in- takes are above 10,000 IU per day for several months.32 Why do the expert opinions vary, and what risks might there be for higher-dose
vitamin D supplementation?
Becoming acutely toxic from vitamin D is
thought to be extremely difficult, identified by a marked elevation of 25(OH)D levels (> 150 ng/ mL) in conjunction with elevated blood calcium and high normal or elevated serum phospho- rus, and clinically accompanied by symptoms such as constipation, confusion, fatigue, and increased thirst and urination.33 When levels exceed 600 ng/mL symptoms progress to pain, anorexia, fever, chills, vomiting and weight loss, requiring complete avoidance of sun exposure
SERUM (BLOOD) 25(OH)D TEST RESULTS
Organization & Recommendation
< 20 ng/mL
≥ 20 ng.mL < 30 ng/mL
≥ 30 ng/mL < 40 ng/mL
≥ 40 ng/mL < 50 ng/mL
> 50 ng/mL
Considered Ideal
Institute of Medicine
Deficient
SUFFICIENT
Toxicity Possible
Endocrine Society
Deficient
Insufficient
SUFFICIENT
40-60
Vitamin D Council
Deficient
Deficient
Deficient
Insufficient
SUFFICIENT
50-80
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Wise Traditions SUMMER 2017