The benefits of vitamin D were once thought to relate only to calcium absorption and healthy bones and teeth. Now researchers recognise that vitamin D is also important for immunity, muscle strength, a healthy circulation and for regulating mood.
Vitamin D3 benefits
Vitamin D, or calciferol, is the collective term for five different, fat-soluble vitamins. The most important for human health is vitamin D3 (cholecalciferol) which is obtained from animal sources.
Vitamin D2 (ergocalciferol) is derived from plant foods such as algae and mushrooms that have been exposed to ultraviolet light.
These two forms differ in their side chain structure which makes vitamin D2 less beneficial for health than vitamin D3.
Before it can provide health benefits vitamin D must first be converted into the active form, calcitriol, in the body.Active vitamin D regulates calcium and phosphate metabolism by:
- increasing calcium and phosphorus absorption in the small intestines
- regulating calcium reabsorption in the kidneys
- regulating secretion of parathyroid hormone which releases calcium from bones.
Vitamin D is now known to have more widespread effects within the body. Vitamin D stimulates the production of cartilage building blocks (proteoglycans) within joints, plays a role in learning, memory and mood, as well as reducing calcium deposition in arteries to reduce hardening of the arteries (arteriosclerosis).
Vitamin D also has a protective effect against infections by increasing the antimicrobial action of white blood cells that fight infection. This partly explains why sun exposure was a popular sanatorium treatment for TB before antibiotics became available.
Within the EU, the European Food Safety Authority has authorised health claims that vitamin D contributes to:
- Normal absorption/utilisation of calcium and phosphorus
- Normal blood calcium levels
- The maintenance of normal bones
- The maintenance of normal muscle function
- The maintenance of normal teeth
- The normal function of the immune system
- The process of cell division
- The normal growth and development of bone in children
- Reduce the risk of falling associated with postural instability and muscle weakness (a risk factor for bone fractures among men and women aged 60 and older)
And, together with calcium, helps to reduce the loss of bone mineral in post-menopausal women (a risk factor for osteoporotic bone fractures).
Vitamin D production
While you can make some vitamin D in your skin, this is usually insufficient to meet your needs – otherwise it would not be classed as a vitamin. Vitamin D is made in your skin by a photochemical reaction between a cholesterol-like molecule (7-dehydrocholesterol) and UVB ultraviolet sunlight (wavelengths between 290-315 nm). This creates the previtamin form of D3 (cholecalciferol) which is inactive and must undergo two further conversions before it is activated.
Once you’ve made cholecalciferol in your skin, it is absorbed into the circulation and travels to the liver, where it is converted to 25-hydroxyvitamin D (calcidiol). This form of vitamin D is also inactive and is sent back out into the circulation to reach the kidneys. This form of vitamin D, calcidiol, is the form measured in vitamin D blood tests as it is the most stable, and is the best measure of your vitamin D status.
Calcidiol is converted into the biologically active hormone, calcitriol (1,25-dihydroxyvitamin D) within the kidneys. This conversion only occurs when active vitamin D3 is needed, and is tightly regulated depending on your levels of parathyroid hormone, phosphate, and a growth factor called FGF23.
Because of the complexity of this series of reactions, it’s not surprising that many people have low vitamin D status, as there are so many stages where things could go wrong.
Vitamin D and UVB levels
The main challenge in maintaining good levels of vitamin D is in making sufficient previtamin D3 (cholecalciferol) in your skin. This reaction can only occur when the UV index is greater than 3 and there is a wide seasonal and regional variation in UV levels in different countries.
People living at a latitude of 52 degrees N (which passes through the centre of the UK and Canada) are not exposed to enough UVB radiation to make previtamin D3 between the months of October to April.
Those living at a latitude 42 degrees N (which forms the northern limit of Spain and part of the border between Canada and North America) are unable to synthesise previtamin D3 between November to February.
Low vitamin D status is widespread at northern latitudes and, with the exception of Norway (where intakes of vitamin-D rich fish are high) most Europeans have low vitamin D levels during winter. In the UK, for example, the UV index only reaches 3 and above during spring and summer – but not on cloudy, rainy days.
Vitamin D status is also low in those who habitually wear clothes that cover most of their skin, or who stay indoors most of the time.
Blood levels for vitamin D
Vitamin D (25-hydroxyvitamin D) blood level ranges vary slightly from lab to lab, but the following provides a useful general guide:
- Deficiency: below 30 nmol/L (12 ng/ml)
- Insufficiency: 30 – 50 nmol/L (12 – 20 ng/ml)
- Adequate: 50 – 80 nmol/L (20 – 32 ng/ml)
- Optimum: > 80 nmol/L (32 ng/ml)
- Risk of toxicity: > 125 nmol/L (50 ng/ml)
- Toxicity: > 220nmol/L (88 ng/ml)
NB Low 25-hydroxyvitamin D plus a raised calcium suggests a parathyroid tumour, and vitamin D supplements should not be taken until the tumour is surgically removed and levels rechecked.
Home Vitamin D Test Kits
Your doctor can test your vitamin D status if you have symptoms such as muscle aches and bone pain. Otherwise, home test kits are available which involve sending a fingerprick sample of blood to an accredited laboratory. The results will let you know if you need a vitamin D3 supplement, and usually suggest the best dose based on your readings.
Vitamin D and sunscreen
Used properly, a sunscreen with a sun protection factor of SPF8 reduces previtamin D3 production in the skin by 95%, while SPF15 reduces previtamin D3 production by 99%.
To balance adequate production of previtamin D3 against skin cancer risk, usual advice is to obtain 10 to 15 minutes sun exposure to face, arms, hands and/or back, without sunscreen, two or three times a week.
Most people do not apply enough sunscreen, however, and the development of a tan suggests that enough UVB radiation strikes the skin to stimulate production of both melanin (a natural sunscreen produced in response to UV damage) and some previtamin D3 regardless of sunscreen use.
Even when the sun is shining, not everyone produces previtamin D3 efficiently. Among 93 fit, healthy surfers in Hawaii, for example, more than half had low vitamin D status despite achieving 29 hours of sunshine exposure per week. This may partly be due to genetic inefficiencies of vitamin D3 metabolism, and partly because longer sun exposures cause the rapid breakdown of previtamin D3 in the skin.
Vitamin D and age
Over the age of 50, the ability to synthesise previtamin D3 reduces so that skin concentration fall by at least half between the ages of 20 to 80 years – often more. In one study, researchers found that people aged 62 to 80 years synthesised four times less natural vitamin D than those aged 20 to 30 years. This makes dietary sources particularly important for older people.
Food Sources of vitamin D
Vitamin D3 (cholecalciferol) is obtained from eating:
- oily fish (sardine, herring, mackerel, salmon, tuna)
- fish liver oils
- fortified milk
- fortified margarine
Vitamin D2 (ergocalciferol) is mainly found in mushrooms that have been exposed to ultraviolet light (eg maitake, chanterelle, morel, oyster, shiitake, enoki, portobello and chestnut mushrooms) and in some fortified foods such as soy milk.
Vitamin D deficiency
Lack of vitamin D is associated with reduced bone mineralisation, which can lead to rickets in children and osteomalacia in adults.
Bone thinning (osteoporosis) in menopausal women is also partly regulated by vitamin D status as, when vitamin D is in short supply and calcium absorption from the diet is reduced, blood levels are maintained by leaching calcium from bones. As a result, four out of five people with hip fracture have evidence of vitamin D deficiency.
Symptoms that may be due to vitamin D deficiency include:
- muscle weakness
- increased susceptibility to infection.
Symptoms that may be due to major vitamin D deficiency are:
- poor growth
- bone pain
- bone deformities (in rickets)
- deafness (in osteomalacia).
Who is most at risk of vitamin D deficiency?
People most at risk of vitamin D deficiency include:
- Pregnant and breast-feeding women
- Breastfed infants (if their mothers have a low vitamin D status)
- Children under the age of 5 years
- Older adults – skin synthesis of vitamin D reduces with age so you produce at least 4 times less in your 60s compared to your 20s.
- People who are frail and may have reduced appetite or poor diet.
- People with limited sun exposure (housebound or those wearing long robes and head covers or using copious amounts of sunblock)
- People with darker skin – the pigment, melanin, reduces the skin’s ability to synthesis vitamin D by neutralising some of the effects of UV light.
- People with inflammatory bowel disease or other bowel conditions associated with malabsorption of fat (vitamin D is a fat-soluble vitamin).
- Obesity – as vitamin D is fat-soluble, a larger intake of vitamin D is needed to maintain adequate blood and tissue levels
- People taking a statin drug as this lowers levels of cholesterol from which vitamin D3 is made.
Vitamin D and arthritis
People with low vitamin D intakes are three to four-fold more likely to experience progressive osteoarthritis than those with high intakes, and as many as 85% of people requiring total hip or knee replacement are deficient in vitamin D. This compares with a background level of around 15% of men and women of similar age having a low vitamin D intake.
Vitamin D and osteoporosis
The results from 29 clinical trials, involving almost 64,000 people aged 50, and over suggests that calcium supplements can reduce the risk of bone fracture by 12% to 24% with best results obtained when also taking vitamin D at doses of at least 20mcg (800 IU).
Another 12 studies, involving over 42,000 adults, showed that vitamin D supplements could reduce the risk of hip fractures by 9% and other non-vertebral fractures by 14%. Higher doses of over 10 mcg (400 IU) vitamin D per day reduced the risk of non-vertebral bone fractures by at least 20%.
Older people are also more likely to experience a fall if they have low vitamin D levels, due to effects on muscle fibres, which will increase the risk of an osteoporotic fracture. Ten studies involving older adults also show that vitamin D supplements of 5mcg to 25mcg daily can reduce falls by 14% compared with placebo.
Vitamin D and cardiovascular disease
Vitamin D is involved in blood pressure regulation and calcium metabolism. Having an optimum vitamin D level may protect against arterial calcification to reduce the risk of a heart attack. Vitamin D also protects against stroke.
Research from 19 studies, involving almost 66,000 people found that those with the lowest vitamin D levels were 52% more likely to have cardiovascular disease than those with the highest blood levels, and more likely to have a fatal heart attack.
Another analysis of 10 studies, involving over 58,000 people confirmed a stepwise increase in the risk of stroke with a stepwise decrease in vitamin D blood levels. Those with the highest levels were up to 82% less likely to have a haemorrhagic stroke than those with low levels.
Having a low level of vitamin D appears to increase the risk of ischemic stroke due to poor blood flow to parts of the brain. Vitamin D also has a strengthening effect on muscle fibres, and taking supplements (4000 IU per day) for six months can improve heart pump efficiency in people with congestive heart failure.
Vitamin D and diabetes
Data from 21 studies, involving over 76,000 people, showed that those with the highest vitamin D levels were 62% less likely to develop type 2 diabetes than those with the lowest levels. Each 10 nmol/L increase in vitamin D (calcidiol) levels was associated with a 4% lower risk of type 2 diabetes – possibly because vitamin D3 improves insulin sensitivity to achieve better glucose control.
Vitamin D and infections
Low vitamin D levels can suppress immunity and increase the risk of bacterial and viral infections.
Research involving over 19,000 adults found those with the lowest vitamin D levels were 36% more likely develop a common cold than those with high levels.
Lack of vitamin D is also associated with an increased risk of bacterial vaginosis in pregnancy. As BV is associated with an increased risk of miscarriage and premature labour, this is one reason why pregnant women are now advised to routinely take vitamin D supplements in addition to folic acid supplements.
Vitamin D and cancer
Data from 14 studies, involving over 25,000 women, showed that women with the highest vitamin D levels were less likely to develop breast cancer than those with low levels. Every 10 ng/mL increment in vitamin D (calcidiol) concentration was associated with a 3.2% reduction in breast cancer risk.
In one study, women taking 25mcg (1000 IU) vitamin D daily had a lower incidence of cancer by as much as 60% over a 4 year follow-up period. As vitamin D levels are low in between 20% and 60% of cancer patients at diagnosis, vitamin D supplements may be advisable to support cancer treatment.
Vitamin D and brain health
It now seems that vitamin D is important for optimal brain function and mood. Data from 14 studies, involving around 31,500 people, shows that low vitamin D levels can double the risk of developing depression compared with high levels.
Eleven studies, involving almost 4,000 people, also suggest that vitamin D levels (calcidiol) are a third lower in people who have Alzheimer’s or Parkinson’s disease when compared with healthy controls. This may reflect a protective effect of vitamin D, although differences in diet and sun exposure also play a role.
Can vitamin D help you live longer?
Surprisingly, the answer could be yes. International studies involving over 26,000 adults aged 50 or more, found that those with the highest vitamin D levels were 57% less likely to die from any medical cause during the study follow-up periods than those with the lowest levels.
Even though vitamin D3 levels varied according to country latitude, gender and season, the difference in longevity between those with the lowest and highest vitamin D levels within each country remained remarkably consistent.
Vitamin D3 dose
Vitamin D is often expressed in International Units (IU) rather than micrograms.
- 1mcg vitamin D = 40 IU.
The EU RDA for vitamin D is just 5 mcg (200 IU) while the US DV is 10 mcg (400 IU).
Requirements increase during pregnancy and lactation.
These recommended intakes are based on the old understanding that vitamin D3 was just about calcium absorption to maintain healthy bones.
In the UK, Public Health England now advises that everyone (from the age of 1 year) should take a supplement supplying 10mcg vitamin D during autumn and winter as we do not receive enough sunshine to synthesise adequate amounts during the colder months of the year.
This is very much a minimum. Many experts now suggest that, in the absence of exposure to sunlight, an intake of 25mcg (1000 IU) is more appropriate for optimum heart, brain and immune health – at least in those under the age of 50.
A higher intake of at least 50mcg vitamin D3 may be more appropriate for older people – both to offset lower production in the skin and because of reduced dietary absorption.
In the United States, for examples, studies show that an intake of 100 mcg (4000 IU) is needed to maintain vitamin D levels in all older women.
The average Western diet supplies just 3mcg vitamin D per day.
Vitamin D supplements
Supplements supplying vitamin D3 (cholecalciferol) are at least twice as effective in maintaining vitamin D status as sources providing vitamin D2 (ergocalciferol).
A daily dose of 250 mcg vitamin D2 is needed to achieve the same blood serum calcidiol levels as a daily dose of 100 mcg vitamin D3 in postmenopausal women with osteoporosis. This suggests that vitamin D2 has a 2.5 fold lower bioavailability and vitamin D3.
I usually advise against using a liquid form of vitamin D as it is easy to over-dose on these. One UK laboratory found that 3.1% of the samples they tested were within the toxic range and, in most cases, this was due to taking vitamin D supplements at doses above 100mcg (4,000 IU) per day. These highest risk of toxicity was in people who had taken crazily high doses, in liquid supplement form, without medical supervision.
Vitamin D safety
Excess vitamin D can cause disturbances in calcium metabolism with headache, loss of appetite, nausea, vomiting, diarrhoea or constipation, palpitations and fatigue.
The European Food Safety Authority has suggested a tolerable upper safe level for long-term use from supplements of 100mcg.
Although higher intakes of vitamin D3 are recommended by some experts, these are best taken under medical supervision.
The best vitamin D supplements
When taking a vitamin D3 supplement, it’s important to choose one made to a pharmaceutical standard known as GMP (Good Manufacturing Practice). This ensures it’s tested at all stages of production for purity and consistency of dose.
Most evidence suggests that the vitamin D3 (cholecalciferol) is more effective for maintaining vitamin D status than the vitamin D2 (ergocalciferol form).
Vitamin D is also available as an oral spray and as a cream to rub into the skin.
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