“We were not particularly surprised by our findings because there is
a recent and growing literature on the associations between vitamin
D status and risk of Alzheimer's disease/dementia, cognitive
decline, and brain atrophy,” Dr. Joshua W. Miller from Rutgers
University, New Brunswick, New Jersey told Reuters Health by email
Miller and a team at University of California, Davis’s Alzheimer’s
Disease Center looked at the association between blood levels of
vitamin D and changes in memory and thinking ability in 318 adults
over an average of five years.
Participants were an average of 76 years old, and included 158
whites, 113 African Americans and 96 Hispanics.
Past research has shown that fully half of the U.S. population over
age 65 has insufficient or deficient levels of vitamin D, and that
non-whites are more likely to have levels that are too low, the
authors point out.
Previous studies have also linked low vitamin D levels to higher
risk of dementia. Among people with cognitive impairment, an
estimated 70 percent to 90 percent are insufficient in vitamin D,
Miller’s team writes in JAMA Neurology.
In the new study, the researchers looked at blood levels of
25-hydroxyvitamin D (25-OHD), which is the form generated when the
body converts vitamin D made in the skin in response to sunlight and
consumed in foods like eggs, oily fish and milk.
Circulating 25-OHD levels can only be measured by blood test, and
current guidelines consider adequate levels to be in the range of 20
nanograms per milliliter of serum to 50 ng/mL. Insufficient is 12 to
less than 20 ng/mL, and levels below 12 ng/mL are considered
deficient.
The study team found that more than 60 percent of the participants
had low vitamin D levels, including more than a quarter who had
vitamin D deficiency. African Americans were more than three times
as likely and Hispanics were more than twice as likely as whites to
have low vitamin D levels.
Individuals with dementia had lower vitamin D levels (about 16.2 ng/mL)
than those with mild cognitive impairment (average 20 ng/mL) or
whose memory was normal (19.7 ng/mL), according to the results.
Low vitamin D levels were also associated with significantly more
difficulty with remembering general information (semantic memory),
seeing the relationship between objects (visuospatial ability) and
managing overall thinking processes (executive function).
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Over a period of just under five years, individuals with low vitamin
D levels showed a more rapid decline in executive function and in
the ability to remember their own past personal experiences
(episodic memory).
These rates of decline were similar for individuals who had normal
brain function at the beginning of the study and for those who
already had dementia or mild cognitive impairment.
When the researchers adjusted for other cognitive risk factors, such
as vascular disease, obesity and the presence of a gene variant
associated with Alzheimer’s disease risk, the link between low
vitamin D and cognitive impairment remained.
“Low vitamin D status is very common in older adults and is
associated with accelerated rates of cognitive decline,” Miller
said.
So far, he added, there have been no careful studies to determine
whether taking vitamin D could slow or prevent memory loss, but he
suggested measuring your vitamin D level to see whether you need
more vitamin D.
“There is a potential danger in consuming too much vitamin D as a
supplement,” Miller cautioned. “The upper tolerable level of daily
intake (UL) for vitamin D established by the Institute for Medicine
is 4000 IU/day for individuals age 9 years and older. Above this
level, the risk of adverse health effects increases, with very high
doses (10,000 IU/day or more) potentially causing damage to kidneys
and other tissues. The recommended dietary allowance (RDA) is 600 IU/day.”
SOURCE: http://bit.ly/1iqnHzl JAMA Neurology, online September 14,
2015.
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