"Do Vitamin D Supplements Help Prevent Diabetes, Cancer Mortality, and Overall Mortality?"
The VITAL study was a randomized trial
to test whether vitamin D supplementation
could prevent cardiovascular disease
or cancer.
It also tested fish oil and convincingly showed
that the use of omega-3s is not effective
in preventing cardiovascular disease,
but normal-weight study participants
randomized to vitamin D did end up
having a lower incidence of cancer.
Over a period of five years,
a 24 percent lower risk of getting cancer.
But overweight or obese individuals did not.
The dose used in the study, 2,000 units a day,
may not have led to similar results
in overweight and obese participants
due to the higher vitamin D requirements
of such individuals.
2,000 units a day may be enough to get normal weight people
up to target, but obese adults may need
at least two to three times more,
a dose closer to 3,000 to 6,000 units a day
to reach the same blood levels.
A similar result was found
in the largest ever vitamin D diabetes trial,
where pre-diabetics randomized to take 4,000 units a day
reduced their risk of slipping into full-blown diabetes
by 29 percent,
but it seemed to work only if they were not obese.
That's why, in the future perhaps,
vitamin D clinical trials should be based on
achieved vitamin D concentrations in the blood,
rather than one-size-fits-all dosing.
For example, if you look at vitamin D concentrations
achieved in breast cancer trials,
regardless of which group the subjects were randomized to,
there was a whopping 82 percent lower incidence rate
of breast cancer for women with vitamin D concentrations
greater than or equal to 60 ng/ml
versus less than 20,
and the higher their levels were,
the lower their breast cancer risk
seemed to fall.
But if you just lump everyone together,
it's no longer a randomized controlled trial,
and we get back to the issue of confounding,
like maybe the women only had higher levels
of the sunshine vitamin
because they were outside jogging every day,
and we know that physical activity alone
may decrease breast cancer risk.
The finding of benefit in certain subgroups
of study individuals, like less cancer
among those who are not overweight,
or less diabetes for those who are not obese,
are examples of secondary analyses,
where you go back after the trial's over
to see whether it may have worked
in some subset, even if the intervention
had flopped overall.
This isn't uncommon,
but such results must be interpreted cautiously,
because you can slice and dice practically any data set
and come up with some sort of spurious fluke
if you try hard enough.
But if you put all of the randomized controlled trials
on vitamin D supplementation for the prevention
of type 2 diabetes together,
daily supplementation with 1,000 or more units of vitamin D
does indeed reduce the risk of diabetes among pre-diabetics.
In the 3 large trials that were specifically designed
and conducted for the prevention of diabetes,
vitamin D supplementation reduced the risk
of developing diabetes by about 10 percent overall.
What about putting all the randomized controlled
vitamin D studies together for cancer prevention?
Those randomized to take vitamin D supplements
were 13 percent less likely to die from cancer
over the subsequent 3 to 10 years.
Now vitamin D supplementation wasn't able
to reduce the risk of getting cancer,
but it was shown to significantly reduce
the risk of dying from cancer.
I mean, it's a tiny benefit,
basically taking these people's chances of dying from cancer
over a span of a few years from around 24 in 1,000
to, like, 21 in 1,000,
but it's a beneficial effect
on lowering cancer mortality nonetheless,
based on a comprehensive analysis
of 10 randomized controlled trials
involving more than 80,000 participants.
Note that a 13 percent reduction in cancer mortality
doesn't necessarily mean you live any longer.
Since vitamin D supplements don't help heart disease,
killer number one, you would only expect
your lifespan to significantly benefit
if your heart disease risk is really low.
Only if you're protected from heart disease
would killer number two take on a more prominent role,
or if there was some other reason
you were at particularly high cancer risk.
For those getting inadequate sunshine,
I recommend 2,000 international units
a day of vitamin D3.
Daily dosing is important
since taking monthly or yearly mega-doses
doesn't seem to work for reducing cancer mortality.
And D3 may be preferable.
Historically, it's been suggested
there's no difference in terms of effectiveness
between the two types you can buy,
vitamins D2 and D3,
but it turns out that vitamin D3 may work better,
as it can increase D levels by about 75 percent,
compared to the same dose of D2,
only increasing levels by about a third.
So it looks like D3 is about twice as effective
at raising blood levels.
Does it matter if you get powder-filled capsules,
oil-based preparations,
or water-based preparations of vitamin D?
Nope, they all appear to raise your D levels the same.