In recent years, some research has suggested a link between vitamin D deficiency and heart disease.
So some doctors have begun checking their heart patients for vitamin D deficiency. If they find low levels, supplements of "the sunshine vitamin" are prescribed.
But the body produces it when the skin is exposed to sunlight, so should anyone living here need them? About 10 minutes a day in moderate sun here should give you enough vitamin D for the day.
This vitamin is not found widely in foods. It is mainly found in costly salmon, tuna and mackerel and also in beef liver, egg yolk and some mushrooms. Still, there are brands of milk, orange juice and breakfast cereals that are fortified with vitamin D.
People do spend more time indoors nowadays, while also using more sunscreen for fear of skin cancer.
And people are getting fatter: vitamin D dissolves in fat, so it is stored in fat cells and prevented from circulating in the blood. Both gut absorption and skin synthesis of vitamin D drop with age.
In the body, vitamin D promotes calcium absorption in the small intestine.
It may also matter in the regulation of blood pressure via the kidney and of blood sugar levels via the pancreas.
But patients are right to worry about supplementation. Being fat-soluble, vitamin D can accumulate in fat cells to cause toxicity.
Excessive vitamin D causes excess calcium to be absorbed in the small intestines. This leads to excessive blood calcium levels, which may cause the mineral to be deposited in soft tissues - a painful condition - or form kidney stones and cause kidney damage.
Right now, the long-term effects of high daily doses of vitamin D are not known for certain.
In the body, vitamin D is converted into an active form known as calcitriol, which acts as a hormone that may impact hundreds of genes.
But how many gene variants control the way vitamin D and calcium are metabolised in the body is not known for sure.
For example, some gene variants that make vitamin D receptors are associated with calcification in some heart valves (called aortic valve stenosis). What if you have those gene variants that cause your aortic valves to calcify if you take too much vitamin D?
So whether to supplement right now is a gamble as biological systems are extremely complex. There is always another level of complexity or another set of interactions that scientists did not know about previously.
Only rigorous clinical trials can help determine if vitamin D supplementation is worth the while. Just because its low levels are associated with some conditions may not be reason enough. What it actually does to people matters.
There are indeed large ongoing trials with results expected in five to eight years. Meanwhile, fans argue that toxic levels of the vitamin are far above those achieved with recommended doses.
Fans also point to an old practice: Many children used to be given Scott's Emulsion to prevent rickets, a weakening of the bones caused by vitamin D deficiency. That nasty tasting concoction was cod liver oil rich in vitamin D, so they argue that this old practice proves that vitamin D pills should be safe.
In Britain, the government advises supplementation for kids under five, adults over 65, pregnant and breast-feeding women and if you are dark-complexioned. But the background is that parts of Britain get little sun in winter.
It seems unwise to pop a pill just because it might do some good when the evidence that it really does so is not undisputed.
With supplements being a big business, a pill that gains a foothold may be impossible to dislodge even if evidence it may do some harm should appear later.
For example, a huge market for anti-oxidants grew just because manufacturers conveniently extrapolated from the basic science evidence that anti-oxidants reduce oxidative stress in the lab.
When the claimed health benefits did not show up in good clinical studies, manufacturers said the doses weren't sufficient. Later, studies showed high doses of anti-oxidants being linked to heightened heart risks and worse prognoses in breast and lung cancer.
Biological systems are very complex: Oxygen free radicals portrayed as cellular stresses in the lab also function as chemical signals that initiate important cell processes in life.
So anti-oxidant supplementation could suppress such life processes with adverse consequences. Yet many popping these pills may not be aware and simply continue doing so. What if the same holds for vitamin D?
Still, fans point to the Vitamin D Consensus Letter signed by 12 top researchers in 2008 that recommended testing for vitamin D deficiency and taking supplements.
The 12 helm GrassrootsHealth that will test your vitamin D level once for US$70 (S$87) or twice a year for five years at US$65 a test. These tests are for "the world's largest vitamin D project to solve the deficiency, called D*action", that it runs.
But their consensus statement, a press release, is mere opinion. It is not evidence-based: No references are cited for its claims.
By comparison, the 2010 report from the Institute of Medicine (IOM) is a book-length one, replete with citations of over 1,000 studies which its independent experts actually sat down to read and analyse, besides asking for views from all stakeholders.
Part of the United States National Academies, the IOM - a non-profit that provides independent advice on health issues - was commissioned by the US and Canadian governments to assess the health roles of vitamin D and calcium supplements, if any.
The IOM took pains to stress that there was no reliable evidence for the cut-off points that labs were using to determine vitamin D deficiency.
Blood samples from one patient may give widely different results if sent to different labs, adding: "The number of people with vitamin D deficiency... may be overestimated."
In fact, what levels of vitamin D actually mark deficiency are not settled, it noted. (Its deficiency is defined using a randomly chosen statistical parameter such as the level leading to osteoporosis or bone-thinning through calcium loss.)
The IOM found that vitamin D, if taken together with calcium pills, did have a role in bone health but not other conditions. Otherwise, there is not enough evidence that most people need vitamin D supplements, it concluded.
Clearly, those suspected of having osteoporosis should be screened and treated (with both calcium and vitamin D pills).
Perhaps not all heart patients, but only those whose hearts are actually failing may need their vitamin D levels monitored and supplements considered.
Otherwise, most shouldn't be taking vitamin D pills just yet.
This article was published on April 20 in The Straits Times.Get a copy of The Straits Times or go to straitstimes.com for more stories.