Biology of the sunshine vitamin
“Vitamin D is unique because it can be made in the skin from exposure to sunlight.[3,8–10] Vitamin D exists in two forms. Vitamin D2 is obtained from the UV irradiation of the yeast sterol ergosterol and is found naturally in sun-exposed mushrooms. UVB light from the sun strikes the skin, and humans synthesize vitamin D3, so it is the most “natural” form. Human beings do not make vitamin D2, and most oil-rich fish such as salmon, mackerel, and herring contain vitamin D3. Vitamin D (D represents D2, or D3, or both) that is ingested is incorporated into chylomicrons, which are absorbed into the lymphatic system and enter the venous blood. Vitamin D that comes from the skin or diet is biologically inert and requires its first hydroxylation in the liver by the vitamin D-25-hydroxylase (25-OHase) to 25(OH)D.[3,11] However, 25(OH)D requires a further hydroxylation in the kidneys by the 25(OH)D-1-OHase (CYP27B1) to form the biologically active form of vitamin D 1,25(OH)2D.[3,11] 1,25(OH)2D stimulates intestinal calcium absorption.[12] Without vitamin D, only 10–15% of dietary calcium and about 60% of phosphorus are absorbed. Vitamin D sufficiency enhances calcium and phosphorus absorption by 30–40% and 80%, respectively.[3,13]”
https://ods.od.nih.gov/factsheets/vitamind-healthprofessional/
This article was written by Tim Spector from King’s College London and was originally published by The Conversation.
Everyone loves D, the sunshine vitamin. Doctors, patients and the media have been enamoured with vitamin D supplements for decades. As well as their clear benefit in curing severe vitamin D deficiencies, endless headlines hail their magical ability to reduce a vast range of conditions from dementia to cancer.
Medical specialists such as myself have been promoting supplements to our patients with osteoporosis and other bone problems for decades. Many food products contain artificially added vitamin D with the aim of preventing fractures and falls and improving muscle strength although the vitamin also has been claimed to boost the immune system and reduce ageing. I used to sometimes take vitamin D myself and recommended it to my family to survive sun-starved winters.
However, a new paper on the risks that vitamin D may pose finally has convinced me that I was wrong. My view on vitamin supplements and the multi-billion dollar industry behind them altered radically after I began researching my book, The Diet Myth, in 2013. The industry and its PR is supported by celebrities who reportedly have high-dose vitamins drip fed into their veins, and around 50 percent of Americans and Britons take them regularly. But surprisingly, there is a lack of evidence to support the health benefit claims of virtually all vitamin supplements on the market.
One study based on the large SELECT trial suggested that supplements such as vitamin E and selenium actually increased prostate cancer in some men. And last year massive analyses combining 27 studies on half a million people concluded that taking vitamin and mineral supplements regularly failed to prevent cancer or heart disease. Not only are they a waste of money for the majority of us – but if taken in excessive quantities they can actually hasten an early death, increasing your risk of heart disease and cancer.
Virtually no vitamins or supplements have actually been shown to have any benefit in proper randomised trials in normal people without severe deficiencies. Rare exceptions have been lutein nutrients for macular degeneration, a common cause of blindness – and vitamin D, the golden boy of vitamins.
Since the 1980s, researchers (including myself) have written thousands of papers, associating a lack of our favourite vitamin with over 137 diseases. A 2014 BMJ report, however, found these links mainly to be spurious.
Won’t do you any harm?
Our genetic makeup influences vitamin D levels. We can use this information to tell if naturally low vitamin D levels might actually increase the risk of disease (rather than be a consequence of it). The evidence so far suggests (with the possible exception of multiple sclerosis and some cancers) that low vitamin D levels are either irrelevant or merely a marker of the disease.
Until now we haven’t worried about giving people extra vitamin D because we thought “it might help anyway and of course (as it’s a vitamin) doesn’t do you any harm”. With our increasing knowledge, we should now know better. Recent studies in the last five years have suggested that even calcium supplements as well as being ineffective in preventing fracture can increase the risk of heart disease.
While several studies in normal people failed to find any protective effects from vitamin D, others have been more worrying. One 2015 randomised study of 409 elderly people in Finland suggested that vitamin D failed to offer any benefits compared to placebo or exercise – and that fracture rates were, in fact, slightly higher.
The usual prescribed dose in most countries is 800 to 1,000 units per day (so 24,000-30,000 units per month). However, two randomised trials found that at around 40,000 to 60,000 units per month Vitamin D effectively became a dangerous substance.
One study involving over 2,000 elderly Australians, which was largely ignored at the time, and the one just published found that patients given high doses of vitamin D or those on lower doses that increased vitamin D blood levels within the optimal range (as defined by bone specialists) had a 20-30% increased rate of fractures and falls compared to those on low doses or who failed to reach “optimal blood levels”.
Explaining exactly why vitamin D supplements are often harmful is harder. Some people who don’t take supplements have naturally high blood levels which may be due to them spending large amounts of time outdoors in the sun or eating oily fish regularly – and there is no evidence that this is harmful. Higher than average levels can also be due to genes which on average influence about 50 percent of the differences between people. So our obsession with trying to bring everyone up to a standard normal target blood level is seriously flawed, in a similar way to our one-size-fits-all approach to diet.
Until now we have believed that taking vitamin supplements is “natural” and my patients would often take these while refusing conventional “non-natural” drugs. Our body may not view supplements in the same misguided way. Vitamin D mainly comes from UV sunlight converted slowly in our skin to increase blood levels or is slowly metabolised from our food.
In contrast, taking a large amount of the chemical by mouth or as an injection could cause a very different and unpredictable metabolic reaction. For example, our gut microbes are responsible for producing around a quarter of our vitamins and a third of our blood metabolites and also respond to changes in vitamin levels picked up by receptors in our gut lining. Any artificial addition of large amounts of chemicals will upset some sensitive immune processes.
The news that even my favourite vitamin can be dangerous is a wake-up call. We should be taking our worldwide abuse of these chemicals much more seriously rather than routinely adding them to foods. The billions we waste on these products, assisted by the poorly regulated but rich and powerful vitamin industry should be spent on proper healthcare – and people should be educated to go in the sunshine and eat a diverse range of real food instead. For 99 percent of people, this will provide all the healthy vitamins they will ever need.
Tim Spector is a professor of Genetic Epidemiology at King’s College London.
This article was originally published by The Conversation. Read the original article.