Vitamin D: Sunshine vs. diet vs. pills

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Abstract

One in five people in the UK is known to have a low serum vitamin D level (25-hydroxy vitamin D below 25 nmol/l) according to the National Diet and Nutrition Survey. The Summer of 2015 saw publication of a draft report from the government's Scientific Advisory Committee on Nutrition (SACN), which proposes introduction of dietary reference values (DRVs) for all age groups (not just those considered as vulnerable).

The health outcome identified as the basis for setting DRVs for vitamin D was musculoskeletal health (based on rickets, osteomalacia, falls, risk of falling and muscle strength). The data were not sufficient to establish a distribution of serum 25(OH)D concentrations or a clear threshold serum 25(OH)D concentration to support musculoskeletal health outcomes, but the evidence overall suggests that the risk of poor musculoskeletal health is increased at serum 25(OH)D concentrations below 25 nmol/l. Therefore, SACN selected a serum 25(OH)D concentration of 25 nmol/l, on a precautionary basis, as the target concentration to protect all individuals from poor musculoskeletal health. This concentration was considered to be a ‘population protective level’ (i.e. the concentration that 97.5% of individuals in the UK should be above, throughout the year, in order to protect musculoskeletal health).

After establishing the health outcomes linked with low vitamin D status, the next step in estimating DRVs for vitamin D was translation of the serum 25(OH)D concentration of 25 nmol/l into a dietary intake value that represents the reference nutrient intake (RNI) for vitamin D [i.e. the average daily vitamin D intake that would be sufficient to maintain a serum 25(OH)D concentration of at least 25 nmol/l in 97.5% of individuals in the UK]. The average vitamin D intake refers to the mean or average intake over the long-term and takes account of day-to-day variations in vitamin D intake. It was not possible to quantify the sunlight exposure required in the summer months to maintain a winter serum 25(OH)D concentration of at least 25 nmol/l because of the number of factors that affect endogenous vitamin D synthesis, storage and utilisation. Instead, use was made of a series of three randomised controlled trials, conducted in the winter months, to estimate directly the amount of vitamin D required daily to achieve a serum threshold of 25 nmol/l throughout the year. The RNI proposed by SACN for all people aged 4 and above is 10 μg/day. For younger children, a Safe Intake of 8.5–10 μg/day (depending on age) is proposed.

These recommendations bring alignment with many other countries of the world. As dietary intakes from food are typically well below the 10 μg/day proposed by SACN for most age groups, media reports speculated on how this advice might be achieved in practice.

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