The Cooper Institute
 

Founded in 1970 by the "Father of Aerobics"
Kenneth H. Cooper MD, MPH

 
 
 

Blood Vitamin D Levels and the Risk of Dementia and Alzheimer’s Disease

Posted in
Live well

Thursday, Oct 09, 2014

As you might have heard, vitamin D is nicknamed ‘the sunshine vitamin’. Skin exposure to the sun’s ultraviolet rays triggers a metabolic pathway which ultimately results in production of the active form of vitamin D. In the past, it was thought that the only important function of vitamin D was to increase absorption of calcium and phosphorus from the gut into the blood. By increasing blood levels of these two minerals, they become more available to the bones. More recently, it has become more clear that improving bone health is just the tip of the iceberg with regard to the many functions of vitamin D.

One thing that has clued us into this has been the study of vitamin D receptors. Receptors are proteins that exist on the surface of cells; there are many types of receptors. Each receptor type binds to a certain substance in the blood and then carries that substance to the interior of the cell. Over the past decade or so, vitamin D receptors have been found in every type of cell within the human body. Because of this relatively new finding, we now know that vitamin D has an effect on every type of cell.

The prevalence of vitamin D deficiency worldwide is very high, and there is a broad consensus that these deficiencies are associated with a wide variety of adverse health conditions. As the U.S. population ages, the prevalence of dementia and Alzheimer’s disease has increased. Since brain cells and blood vessels within the brain contain vitamin D receptors, scientists have been trying to determine whether low blood levels of vitamin D are associated with an increased risk of dementia and Alzheimer’s disease. To help answer this question, Littlejohns and colleagues recently published an important study in the journal Neurology.1 A group of 1658 ambulatory and apparently healthy white and black men and women with an average age of 73.6 years participated. At baseline, blood levels of 25-hydroxyvitamin D were measured. The group was then followed for an average of 5.6 years.  During the follow-up period, 171 participants developed all-cause dementia, including 102 cases of Alzheimer’s disease. These cases were diagnosed by a committee of neurologists and psychiatrists. Among participants who had severely deficient vitamin D levels at baseline, the risk of developing both all-cause dementia and Alzheimer’s disease was about 2.5 times higher than those that had sufficient vitamin D levels. This finding persisted even after taking into account other factors that might have ‘muddied the waters’. The authors listed several possible biological mechanisms by which low vitamin D levels are linked to an increased risk of dementia/Alzheimer’s. They also acknowledged that one limitation of the study was that it did not include people of Hispanic or other ethnicities. Thus, their findings are only applicable to elderly black and white men and women.

Below are some of the factors related to having a low blood level of vitamin D:

  • Having dark skin
  • Obesity*
  • Living in the northern latitudes
  • Non-milk drinker
  • Low dietary intake of  fatty fish
  • Overuse of sunscreen
  • Staying indoors most of the time
  • Not taking a vitamin D supplement
*Since vitamin D is fat-soluble, it is stored in fat cells. The greater the number of fat cells you have and the bigger those cells are, the more vitamin D is stored there. When stored in fat cells, blood levels of vitamin D are low and it is not available to the other cells of the body.

So you may be wondering, “Should I be taking a vitamin D supplement?” The first thing that you need to do is have your vitamin D status measured with a simple blood test called the 25-hydroxyvitamin D test. Categories for blood levels of vitamin D are listed in Table 1. In the U.S. the unit of measure is ug/mL, while in other countries the unit of measure is nmol/L. If your levels indicate deficiency or insufficiency, then your doctor may recommend a supplement. In fact, it is not uncommon for patients with severe deficiency to be given a megadose (a very large dose) of vitamin D. However, if your values are normal, then your doctor may not necessarily recommend a supplement. It is unwise to take a megadose of any vitamin or mineral unless your primary health care provider has given you a good reason to do so.

To learn more about the role of vitamin D in maintaining good health, consider taking the Nutrition for Health and Fitness course (formerly called Providing Dietary Guidance) either online or here in Dallas. You need not be a health and fitness professional to take this course; the information can be applied to everyone’s life!

 

Table 1.  Categories for Blood Levels of 25-hydroxyvitamin D from the Vitamin D Council (note that the optimal level of vitamin D for general health remains controversial).

Category

Units of ug/mL

Units of nmol/L

Vitamin D deficient

<30

<75

Vitamin D insufficient

30-39.9

75-100

Sufficient vitamin D

40-80

100-200

 

Reference

1Littlejohns TJ, Henley WE, Lang IA, et al. (2014). Vitamin D and the risk of dementia and Alzheimer’s disease. Neurology, 83:920-928.