16.5: Nutrient reference values for vitamin D (18b.5)

Unlike other essential nutrients, measurements of vitamin D intake using traditional dietary assessment methods cannot be used to measure dietary exposure because some of the requirement for vitamin D can be met from skin synthesis, as noted earlier. Instead a surrogate biomarker is used to measure exposure to vitamin D; serum 25 hydroxy­vitamin D (25(OH)D) is the biomarker of choice. Serum 25(OH)D measures exposure to vitamin D from the effects of both diet and sunlight as noted earlier, and associations between concentrations of serum 25(OH)D and functional biomarkers of bone health have been considered when setting Nutrient Reference Values (NRVs) for vitamin D by several agencies. The evidence for extraskeletal outcomes has been considered inadequate, inconsistent, or insufficient to develop Nutrient Reference Values (NRVs). Nevertheless, in view of the variation in sunlight exposure and the variable response to that exposure, as well as concerns about skin cancer, several agencies, including the IOM(2011), EFSA (2016), and the UK (2016) have set their NRVs based on the assumption of minimal or no sunlight exposure. This means that in the presence of cutaneous vitamin D synthesis, the requirement for dietary vitamin D may be lower than that set, or may even be zero.

The US IOM (2011), as an example, set an Estimated Average requirement (EAR), the requirement to satisfy the need of half the population, of 10µg/d for all children and adults age 1–70y, as necessary to maintain bone health and achieve a serum 25(OH)D concen­tration of 40nmol/L (16ng/mL). The Recommended Dietary Allowance (RDA), designed to cover the requirements for 97.5% of the population, was set at 15µg/d, a level that corresponded to a serum 25(OH)D concen­tration of 50nmol/g (20ng/mL). A higher RDA level (i.e., 20µg/d) was recommended for those over 70y, due to age-related inefficiencies in vitamin D metabolism. For infants 0–12months, IOM set an Adequate Intake (AI) of 10µg/d because there was not enough information to establish an EAR for this group.

Several other countries and regions have also restricted their recommendation to a single level — the AI (or equivalent) — because of insufficient evidence, including Australia and New Zealand (NHMRC, 2006), the European Union (EFSA, 2016), and most recently, the United Kingdom (SACN, 2016), although here only for infants and children less than 4y. However, the target serum 25(OH)D concentrations on which the AIs are based vary, ranging from at least 25nmol/L (10ng/mL) in Australia and New Zealand (NHMRC, 2006), and the UK (SACN, 2016), to 50nmol/L (20ng/mL) in the EU(EFSA, 2016). As a consequence, the AI levels range from about 10µg/d to 15µg/d for most age groups, similar to those for several other European countries (Spiro and Buttriss, 2014).

The UK , like the US has also set a Recommended Nutrient Intake (equivalent to the RDA) for all life-stage groups (including pregnant women, lactating women, and the elderly), except those < 4y. However, for the UK (SACN, 2016), the RNI is 10µg/d in contrast to the 15µg/d RDA set by IOM(2011). No EAR has been set by the UK. Cashman et al. (2008), have emphasized that dietary levels of about 10–15µg/d may not be adequate to keep most of the adult population in Europe above a higher target value of 50nmol/L in winter without adequate sun exposure in the summer season.`

Several agencies have established a Tolerable Upper Intake Level (UL) for vitamin D to discourage potentially dangerous self-medication. The UL is defined as the highest average intake that is likely to pose no risk. For infants 0–6mo, the IOM set an UL of 25µg/d, rising with age through childhood to 100µg/d for persons aged 9y and older. This upper limit corresponds to an average serum total 25(OH)D levels of 125nmol/L (50ng/mL) (2011).

The UL values for the European Food Safety Authority (EFSA) start for infants aged 7–11mo with an UL of 35µg/d which rises to 100µg/d for persons aged > 11y (EFSA, 2016). Data on the long-term adverse effects of high doses of vitamin D are limited, however, and caution is necessary when setting an UL for chronic intake (Aloia, 2011).

The Endocrine Society, focusing on clinical use for patients at high risk of deficiency with rickets, osteomalacia, osteoporosis, chronic kidney disease, etc., and some special populations (e.g., pregnant women) , have set a higher adult UL of 250µg/d (Holick et al., 2011) , a recommendation that is not appropriate for healthy individuals in the population. This distinction is important, and if not recognized, could result in inappropriate dietary recommendations for healthy individuals, for whom the NRVs should be used (Aloia, 2011).