Older adults with low vitamin D levels may lose their memories and thinking abilities faster than those with normal vitamin D levels, researchers from California say.
The research 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.
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, noted the research team.
In the new study, the researchers looked at blood levels of 25-hydroxyvitamin D (25-OHD), which reflects the amount of vitamin D you produce from sunlight in the skin and how much you consume from foods and supplements.
Current guidelines consider adequate levels to be in the range of 20 nanograms per milliliter of serum to 50 ng/mL (50 to 125 nmol/L). Insufficient is 12 to 20 ng/mL (30 to 50 nmol/L). Levels below 12 ng/mL (less than 30 nmol/L) 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 than those with mild cognitive impairment or whose memory was normal, according to the results.
Low vitamin D levels were also associated with significantly more difficulty with remembering general information, seeing the relationship between objects (and managing overall thinking processes (called executive function).
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.
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.
So far, there have been no careful studies to determine whether taking vitamin D could slow or prevent memory loss, but the researchers suggested measuring your vitamin D level to see whether you need more vitamin D.
How much vitamin D?
Daily recommended intakes (RDAs) for vitamin D are 600 IU for children aged one to adults aged 70, and 800 IU for adults over 70. The safe upper limit is 4,000 IU per day.
Because vitamin D is stored in fat cells, excess doses can build up to harmful levels, causing high blood calcium and damage to the heart, blood vessels and kidneys.
And while vitamin D toxicity is unlikely at daily intakes below 10,000 IU, evidence suggests that some people are more sensitive to the adverse effects of too much vitamin D, and that these adverse effects can occur at lower vitamin D blood levels than in less sensitive people.
Emerging evidence also suggests there may be adverse health risks associated with blood vitamin D levels that are higher than recommended, but not yet at the point where they would be considered toxic. For these reasons, the safe upper limit is set at 4,000 IU per day.
Source: JAMA Neurology, online September 14, 2015.
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