We are always warned that a statistical correlation should not be used to infer causation because either the causal relationship can go either way or both effects may be due to a third cause. The only way to truly determine the direction of causality is to design experiments that are specifically meant to tease it out. But sometimes we can be fooled by multiple, seemingly independent correlations that strongly suggest causality. A recent study of vitamin D deficiency and health illustrates this danger.
Researchers had earlier found that lower levels of vitamin D correlated with a wide variety of non-skeletal disorders such as heart disease, weight gain, mood disorders, multiple sclerosis, and metabolic disorders. This seemed to strongly suggested correlation and that supplementing with vitamin D might stave off those ill effects.
But a report on a new study by Philippe Autier and others that was published in the journal The Lancet Diabetes & Endocrinology finds that rather than lower levels of vitamin D causing these problems, people who had these conditions had lower vitamin D levels. Autier says that “The absence of an effect of vitamin D supplementation on disease occurrence, severity, and clinical course leads to the hypothesis that variations [in vitamin D levels] would essentially be a result, and not a cause, of ill health.”
Another study found that supplementing diets with vitamin D did not help and may even have negative effects, such as increasing the risk of hip fractures. It added to studies that showed that it made no difference in preventing osteoporosis in most healthy adults.
This is important since many older people are told by their doctors to take vitamin D supplements. I too was also told this many years ago but ignored it because in general I am wary of taking medications as a preventative measure.
And I would expect doctors will continue to recommend Vitamin D to their patients regardless of this study. The British Medical Journal debunked the benefits of Omega-3 fatty acids about 8 years ago now, but doctors (not just commercials) continue to push that nonsense to this day.
In my personal experience, fish oil (but not flax seed oil) improves my skin immensely. Skipping the oil for a few days in a row makes my skin painfully dry, renewed taking of fish oil reverses the damage within a couple of days. No idea if it does anything else.
There is one other option as well—the two quantities may be causes of a common effect which you’re conditioning on.
It is not surprising that vitamin D deficiency has been thought to cause all sorts of maladies. A friend of mine does basic research on vitamin D biology. Every tissue in the body expresses vitamin D receptor, and there are vitamin D receptor binding sites all over the genome. Vitamin D signaling is involved in just about every bodily process in some way. So a deficiency in vitamin D would cause systemic effects everywhere.
The question is not whether vitamin D deficiency causes health problems, nor whether it can be remedied by vitamin D supplementation. The question is how many people, given the current western diet and lifestyle, actually are suffering from a deficiency. That’s something that’s been very difficult to answer.
There’s no question that fortifying milk with vitamin D reduced the incidence of rickets. It has also been shown, as the first paper in the OP mentions, that vitamin D supplementation is beneficial for older people, especially women. The link between vitamin D and bone density is largely due to the fact that calcium uptake in the intestines is a vitamin D-dependent process, and so vitamin D is often recommended in addition to calcium supplementation for bone health. This recommendation is a valid one in the elderly and certain at-risk populations.
But the trick is separating the at-risk populations from those populations that would receive no benefit. As this meta-study from 2007 finds, few studies have been done on younger or ethnically diverse populations. Exacerbating the issue is that accurate and consistent measurement of serum vitamin D has been elusive, though I seem to recall some significant progress has been made on this front.
Fortunately, there seems to be little or no harm in taking vitamin D supplements even when they are not needed. I suspect that Mano’s third link is a statistical anomaly, given that many studies have tested the same parameters and found no harm from vitamin D (see the link above).