It’s more than 10 years since I last wrote about vitamin D for the Kangaroo Valley Voice and lots of research has been published since then.
Vitamin D is better described as a steroid hormone than a vitamin. It occurs in several forms and has a wide variety of roles in the body. Its best-known function is in the absorption and control of calcium and phosphate in building and maintaining bones but various organs also depend on adequate levels of vitamin D.
Checking the evidence
Vitamin D’s multiple roles have played a big part in the idea that it may be a panacea for many health problems.
It is true that some people with higher blood levels of vitamin D have lower levels of some health problems. However, before concluding that vitamin D is the cause of these correlations, we also need to check factors such as whether those with higher levels of vitamin D spend more time exercising outdoors, or perhaps have healthier diets. Many studies quoted by supplement sellers did not control for these and other factors.
The results for many newer well-conducted randomised controlled trials have now been published and show that vitamin D is not quite the miracle some sellers of supplements have led us to believe.
These studies have now reported a lack of evidence that extra vitamin D helps prevent or cure heart diseases, diabetes, cancers, depression, cognitive decline – and even bone fractures. Recent trials of vitamin D for preventing or treating COVID have also generally not shown benefit.
This does not mean that vitamin D is not important for health. Vitamin D is indeed essential, but the body regulates its levels carefully so that even small amounts meet the requirements for healthier outcomes in these conditions.
The VITAL study, possibly the largest randomised controlled clinical trial ever done, gave almost 26,000 men and women large doses of vitamin D and omega 3 fatty acids or a placebo for more than five years and monitored them for cancer and cardiovascular diseases.
Those given vitamin D had no reduction in development of cancers of the breast, bowel or prostate, nor in heart attacks, stroke or deaths from heart from cardiovascular disease. The good news from this study is that the high dose of vitamin D given (more than 10 times the level considered adequate for Australians under 50 and three times the highest level considered adequate for Australian adults over 70 years of age) did not cause side effects such as high levels of calcium in the blood, kidney stones or gastrointestinal symptoms.
How much vitamin D?
Australia has not set a recommended dietary intake (RDI) for vitamin D. This is partly because most of our vitamin D comes from incidental exposure of skin to the sun rather than coming from food.
When skin is exposed to ultraviolet rays from sunlight, a substance called 7-dehydrocholesterol absorbs the UVB radiation. This then undergoes two major changes. The first occurs in the liver where the vitamin is converted to a form known as calcidiol (chemically 25-hydroxyvitamin D). The second change occurs in the kidneys to form calcitriol (1,25-dihyroxyvitamin D), the physiologically active form of the vitamin that is vital for bone growth and the maintenance of healthy bones. Many cells and organs in the body have a vitamin D receptor that can also produce this form of vitamin D.
Australia has set the levels for adequate intake (AI) of vitamin D from the diet at 5 micrograms (mcg) from birth to age 50, 10 mcg for 51-70 year-olds and 15 mcg for those older than 70.
Some countries use International Units as their measurement: 1 mcg=40 IU and in areas where the weather is rarely warm enough to expose skin to sun exposure higher levels of dietary intake may be recommended.
Vitamin D is ‘fat-soluble’, which means the body can store it, as occurs with fat-soluble vitamins A, E and K. By contrast, vitamins C and the eight forms of vitamins in the B group are water soluble and are not well-stored in the body.
Vitamin D in foods
Vitamin D occurs in few foods and is not well-supplied by the usual diet. The main forms found in foods are ergocalciferol (D2) and cholecalciferol (D3). Both these forms are absorbed from the small intestine, assisted by foods that contain fat, although some vitamin D is absorbed even without any dietary fat. Contrary to popular belief, neither ageing nor obesity alter the absorption of vitamin D from the intestine.
Food sources include fish liver oils (such as cod liver oil), liver from calves, sheep or chickens, salmon, herrings and sardines. Small amounts are found in egg yolk, cheese and butter. Table margarines sold in Australia must have vitamin D added to bring their level to that found naturally in butter. Some milk producers also add vitamin D to their products.
Mushrooms are usually grown in darkness but, if they are produced with deliberate exposure to ultra violet light, they become a good source of vitamin D2, making these special mushrooms the only unfortified plant food to contain the vitamin. The levels depend on how long they have been exposed to UV-radiation, but last well for about a week in the refrigerator. Cooking losses are moderate.
Cancer Councils throughout the world advise avoiding too much sunlight on skin because of the well-known increase in skin cancers, including melanoma. With Australia being the skin-cancer capital of the world, we need to take care with all skin exposure.
Fortunately, the body’s vitamin D needs are satisfied with short periods of skin exposure (5-9 minutes) for arms and hands in summer, preferably before 10am or after 3pm and only when the UV levels are under 3. In winter, Tasmanians or people in other cold areas may need 2-3 times as long. More is not better and sunbaking is never recommended. Indeed, if you are tanned, your ability to make vitamin D decreases.
As I explained in my previous article, tests for vitamin D levels are not always accurate or needed. For those with no skin exposure to sunlight – as occurs in many aged care facilities – the ideal is to organise appropriate exposure, or if that’s not possible, an appropriate supplement can be used.
In Australia, as vitamin D tests increased dramatically (from 0.4 to 36.5 tests/1000 people, Medicare claims for testing vitamin D levels now require the patient to have signs or symptoms of recognised health problems caused by vitamin D deficiency. That’s fair enough and we should also recognise that ‘ideal’ blood levels of vitamin D are also often exaggerated by those with a conflict of interest in selling supplements.
Dr Rosemary Stanton OAM