Researchers say people with low vitamin D levels may be at greater risk for Alzheimer's.
In the US, deficiency and insufficiency of vitamin D - also known as 25-hydroxyvitamin D (25-OHD) - are prevalent, especially in older people. "Insufficiency" is classified as 25-OHD blood levels between 12-20 ng/mL, and "deficiency" is less than 12 ng/mL.
The National Health and Nutrition Examination Survey (NHANES) previously found that 42% of the general adult population were either deficient or insufficient in vitamin D, with the percentage rising to over 50% after age 65. The prevalence was even higher among Hispanic (69%) and African-American (82%) individuals.
The three main causes for low vitamin D are:
- Not eating the recommended levels of vitamin D in food - for example, when following a strict vegan diet, because most dietary sources are animal-based. Good sources include fish and fish oils, egg yolks, cheese, fortified milk and beef liver
- Limited exposure to sunlight, since the body creates vitamin D when the skin is exposed to sunlight. This can occur in northern latitudes, where people are home-bound, and in hot countries, where people avoid the sun, or in cultures where modesty requires the body to be fully covered
- Darker skin color. While the pigment melanin in darker skin offers more protection against skin cancer, it also reduces the skin's ability to make vitamin D when exposed to sunlight.
Awareness of how vitamin D affects bone strength is well established, and it is recognized that it helps in the fight against other diseases. But studies are starting to show that all parts of the body may be affected by it, since the vitamin D receptor and the enzyme that converts 25-OHD to the active form of the vitamin are expressed in all human organs, including the brain.
In recent research, Joshua W. Miller, PhD, of Rutgers University in New Brunswick, NJ, and co-authors from the University of California-Davis, examined vitamin D levels and change in cognitive function among 382 people, enrolled from an outpatient clinic in California between February 2000 and August 2010.
The group was ethnically diverse, with both genders represented. However, all participants were older adults, with an average age of 75.5 years. Almost 62% of participants were female, 41.4% were white, 29.6% Hispanic and 29.6% African-American. Of these, 17.5% had dementia, 32.7% had mild cognitive impairment (MCI) and 49.5% were cognitively normal.
Low vitamin D reflects decline in cognitive ability
In the current study, blood tests showed the average 25-OHD level of participants to be 19.2 ng/mL, and therefore below the healthy level for 61.3% of participants. Around 26.2% were vitamin D deficient and 35.1% were vitamin D insufficient.
African-American and Hispanic participants were found to have lower Vitamin D levels (average 17.2 ng/mL) than their white counterparts (average 21.7 ng/mL).
Vitamin D in the group diagnosed with dementia was lower overall, with an average of 16.2 ng/mL.
The participants were followed up annually over nearly 5 years, and tests were administered periodically to probe four cognitive domains: episodic memory, semantic memory, visual perception and executive function.
Results showed that participants who had been deficient in vitamin D experienced a greater decline in cognitive ability and episodic memory.
On the other hand, vitamin D status did not appear to be reflected in semantic memory or visuospatial ability.
Overall, the results supported previous findings that older adults lack vitamin D, and also confirmed that Hispanic and African-American individuals are more likely to do so. They also illustrated an apparent link between lack of vitamin D and cognitive impairment.
The authors explain:
"Vitamin D insufficiency was associated with significantly faster declines in both episodic memory and executive function performance, which may correspond to elevated risk for incident AD [Alzheimer's disease] dementia."
They suggest in their report that African-American and Hispanic groups, having overall lower vitamin D levels, could be invited to participate in "well-designed clinical trials" for vitamin D replacement treatment as a safeguard against dementia.
However, while lack of vitamin D appears to be linked to lower cognitive function, the benefit of giving supplements is not yet established. The authors point out that "it remains to be determined whether vitamin D supplementation slows cognitive decline."
Limitations to the study included not directly measuring dairy intake, sun exposure or exercise - each of which can influence vitamin D levels.
In a recent article, Medical News Today reported on research linking low vitamin D levels to multiple sclerosis.