Vitamin D helps calcium absorption in the gut. It is required for the regulation of calcium and phosphorus metabolism and to enable normal bone mineralisation, so is important for musculoskeletal health. Deficiency of vitamin D is associated with rickets and osteomalacia.
Vitamin D is made in the skin by the action of sunlight – the main source of vitamin D for most people. It can also be obtained from foods or dietary supplements. Vitamin D is found in relatively few foods (e.g. oily fish, red meat, liver, egg yolks fortified breakfast cereals, fortified spreads, butter and full fat milk and dairy products).
Dietary sources of vitamin D are essential when sunlight containing UVB radiation is limited (e.g. during the winter months) or exposure to it is restricted (e.g. due to lack of time spent outdoors or little skin exposure).
Vitamin D obtained from food, supplements and skin exposure to sunlight is biologically inert and must undergo two hydroxylations in the body for activation. The first occurs in the liver and converts vitamin D to 25-hydroxyvitamin D [25(OH)D], also known as calcidiol. The second occurs primarily in the kidney and forms the physiologically active 1,25-dihydroxyvitamin D [1,25(OH)2D], also known as calcitriol.
Serum concentration of 25(OH)D is the best indicator of vitamin D status, but it is not without problems as an indicator. The 25(OH)D marker reflects vitamin D produced cutaneously and that obtained from food and supplements and has a fairly long circulating half-life of 15 days.
However, while 25(OH)D functions as a biomarker of exposure, it is not clear to what extent 25(OH)D levels also serve as a biomarker of effect (i.e. relating to health status or outcomes). Serum 25(OH)D levels do not indicate the amount of vitamin D stored in body tissues.
Use of 25-hydroxyvitamin D as a biomarker is also limited by the fact that its relationship with vitamin D status may be influenced by BMI and genetics. Concentrations measured in different laboratories may also vary and there is no clear consensus on the threshold serum 25(OH)D concentration used to define vitamin D deficiency, although a level of less than 25nmol/litre is considered to be low.
Exposure of skin to UVB radiation and skin synthesis of vitamin D is influenced by many factors. These include time of day, season, latitude, altitude, cloud cover, air pollution, clothing and the use of sunscreen.
At latitudes below 37 degrees north (i.e. from southern Europe to the equator), UVB radiation is considered to be sufficient for year-round vitamin D synthesis.
At higher latitudes (e.g. the UK), vitamin D is not synthesised during the winter months. In the UK, sunlight-induced vitamin D synthesis is only effective between late March/early April and September, not from October onwards throughout the winter months.
In the SACN review of vitamin D, dietary intakes of vitamin D from all sources (including supplements) were found to be as follows:
• Breast-fed infants aged 4-18 months: 2-3mcg/day (80-120 IU/day)
• Non-breast-fed infants aged 4-11 months: 8-10mcg/day (320-400 IU/day)
• Non-breast-fed infants aged 12-18 months: 3.5mcg/day (140 IU/day)
• Children aged 18 months to adults aged 64 years: 2-4mcg/day (80-160 IU/day)
• Adults aged ≥65 years: 5mcg/day (200 IU/day)
• Institutionalised adults aged ≥65 years: 3-4mcg/day (120-160 IU/day).
Mean plasma 25(OH)D concentrations across the different age groups in the UK ranged between 40-70nmol/L. Almost a quarter of adults have a plasma 25(OH)D concentration of less than 25nmol/L.
For all age groups in the UK, mean plasma 25(OH)D concentration is lowest in winter and highest in summer. Around 30-40 per cent of the population have a plasma 25(OH)D concentration <25nmol/L in winter compared to 2-13 per cent in the summer.
Worryingly, a large proportion of some population groups do not achieve a plasma/serum 25(OH)D concentration ≥25nmol/L in summer (17 per cent of adults in Scotland; 16 per cent of adults in London; 53 per cent of women of South Asian ethnic origin in southern England; and 29 per cent of pregnant women in North West London).
Analysis by ethnicity shows that the annualised mean serum 25(OH)D concentration was higher in white adults aged ≥16 years (45.8nmol/L) compared to Asian (20.5nmol/L) and black (27.7nmol/L) adults. Low 25(OH)D levels are commoner in people with dark skins than lighter skins.
New research presented at this year’s European Congress on Obesity (ECO) in Portugal shows that a father’s vitamin D intake pre-conception is associated with his child’s height and weight at five years old.1
The researchers analysed data from the Lifeways Cross-Generation Cohort Study – a longitudinal database in Ireland. Information on paternal vitamin D intake and children’s height and weight measurements were available for 213 and 148 fathers and their children when the children were aged five and nine years old respectively.
Paternal vitamin D intake was positively and statistically associated with their offspring’s height and weight at five years old. Although these associations were reduced, and no longer statistically significant, they still existed when the offspring reached nine years old. These findings were independent of maternal characteristics.
The researchers think that one of the reasons this may occur is that the father’s nutrition status may somehow influence the health, quality and function of their sperm cells, which are involved in reproduction. Maternal nutrition, including vitamin D, has an important role in the musculoskeletal health of offspring – but this study shows that the father’s vitamin D intake may also affect the offspring’s growth, development and health.
1. European Association for the Study of Obesity. Fathers’ pre-conception vitamin D intake linked to child height and weight at five years
Dietary reference values (DRVs) for vitamin D were set by the Committee on Medical Aspects of Food Policy (COMA) in 1991 and were based on prevention of rickets in children and osteomalacia in adults. A reference nutrient intake (RNI) was set for children, older people and some other population groups, but not for people between the ages of four and 64 years with regular exposure to sunlight because it was thought that their skins would make enough vitamin D in the summer to cover their needs in winter.
RNIs for vitamin D (7-10mcg (280-400 IU per day) were set only for UK population groups considered to be at risk of vitamin D deficiency. These were infants (0-3 years); pregnant and breast-feeding women; adults aged 65 years and above; those with limited sunlight exposure; and women and children of Asian ethnic origin. The RNI represents the amount of a nutrient that is likely to meet the needs of 97.5 per cent of the population.
Although the DRVs were based on bone health, emerging evidence has also suggested a range of non-musculoskeletal health benefits of vitamin D, including the possibility that the vitamin could reduce the risk of bowel cancer, multiple sclerosis, types 1 and 2 diabetes, and cardiovascular disease.
Many of these trials come from in vitro, animal and epidemiological studies rather than randomised controlled trials (RCTs). Several meta-analyses have suggested that vitamin D supplementation is associated with reduced mortality. However, a clearly definitive role for vitamin D in non-musculoskeletal health outcomes is yet to be identified.
In 2010, SACN agreed to review the DRVs for vitamin D because a substantial amount of published data had accumulated since its previous considerations, but it has remained of the opinion that the attributes of vitamin D focus mainly on musculoskeletal health.
Vitamin D is essential for musculoskeletal health at all stages of life. In its review of the most recent evidence (2016), SACN found a positive link between 25(OH)D levels in pregnancy and indices of bone health in an infant. In older people, however, it concluded that vitamin D supplementation does not reduce fracture risk.
SACN also concluded that vitamin D supplementation improves muscle strength and function and reduces the risk of falls in adults over the age of 50 years living in the community with mean baseline serum 25(OH)D concentrations across a range of values. However, doses of vitamin D associated with reduced risk of falls are often very high, which may produce different effects from daily supplementation at lower doses.
Because of the role of vitamin D in improving the absorption of calcium, it is difficult to distinguish between the effects of the two nutrients. Among postmenopausal women and older men, supplements of both vitamin D and calcium result in small increases in bone mineral density throughout the skeleton.
Calcium and vitamin D in combination also help to reduce fractures in institutionalised older populations, although the benefit is inconsistent in community-dwelling individuals.
Evidence suggests that the risk of poor musculoskeletal health is increased at serum 25(OH)D concentrations below 25nmol/L. This concentration therefore represents a ‘population protective level’ (i.e. the concentration that individuals in the UK should be above, throughout the year, in terms of protecting musculoskeletal health).
It was not possible to quantify how much sunlight exposure would be required in the summer to achieve a winter serum 25(OH)D concentration ≥25nmol/L because of the number of factors that affect cutaneous vitamin D synthesis. SACN therefore derived a RNI for vitamin D by estimating the average vitamin D intake that would be required for individuals in the UK to achieve a serum 25(OH)D concentration ≥25nmol/L (i.e. an average intake over the long-term that takes account of day-to-day variations in vitamin D intake).
The RNI was estimated by modelling data from individual RCTs conducted in winter (so that cutaneous vitamin D synthesis arising from current UVB exposure was minimal) with adults (20-40 years and ≥64 years) and adolescent girls (11 years).
It was estimated that the average daily vitamin D intake required to maintain a serum 25(OH)D concentration ≥25nmol/L in winter by the majority (97.5 per cent) of the population was around 10mcg (400 IU). Data from these RCTs were extrapolated to younger age groups.
Data were not available to relate serum 25(OH)D concentration in infants clearly to current or long-term health. ‘Safe intakes’ rather than RNIs were therefore recommended for infants and children aged under four years in the range of 8.5-10mcg/day (340-400 IU/d).
In its review, SACN came to the following conclusions:
• Serum 25(OH)D concentration is an indicator of exposure to vitamin D (from skin synthesis and dietary intake)
• In order to protect musculoskeletal health, serum 25(OH)D concentration of all individuals in the UK should not fall below 25nmol/L at any time of the year
• A reference nutrient intake (RNI) of 10mcg of vitamin D per day should be taken throughout the year for everyone in the general population aged four years and older
• An RNI of 10mcg of vitamin D per day should be taken by pregnant and lactating women and population groups at increased risk of vitamin D deficiency
• A ‘safe intake’ of 8.5-10mcg per day should be taken by all infants from birth to one year of age
• A ‘safe intake’ of 10mcg per day should be taken for children aged one to 4 years
• In the UK, individuals in population groups at increased risk of having a serum 25(OH)D concentration <25nmol/L are those with minimal sunshine exposure as a result of not spending time outdoors (e.g. frail and institutionalised people) or habitually wearing clothing that covers most of the skin while outdoors,
and those from minority ethnic groups with dark skin
As a result of these conclusions, PHE advises that:
• In spring and summer, the majority of the population get enough vitamin D through sunlight on the skin and a healthy, balanced diet. (However, it should be noted that data from the National Diet and Nutrition Survey shows that the population is insufficient for nine months of the year and is barely above adequate even in summer. As already mentioned above, SACN’s recommendation is for all-year-round supplementation.)
• During autumn and winter, everyone will need to rely on dietary sources of vitamin D
• Since it is difficult for people to meet the 10mcg recommendation from consuming foods naturally containing or fortified with vitamin D, people should consider taking a daily supplement containing 10mcg of vitamin D in autumn and winter
• People whose skin has little or no exposure to the sun, like those in institutions such as care homes, or who always cover their skin when outside, risk vitamin D deficiency and need to take a supplement throughout the year
• Ethnic minority groups with dark skin, from African, Afro-Caribbean and South Asian backgrounds, may not get enough vitamin D from sunlight in the summer and therefore should consider taking a supplement all-year-round
• Children aged one to four years should have a daily 10mcg vitamin D supplement
• All babies under one year should have a daily 8.5-10mcg vitamin D supplement to ensure they get enough of the vitamin
• Children on more than 500ml of infant formula a day do not need any additional vitamin D, as the formula is already fortified.
Learning module: Recommending vitamin D to your customers. Click here.
Adults and children over one year should take a daily vitamin D supplement