Category: Brain health

Vitamin D deficiency is common. Analysis of National Health and Nutrition Examination Survey data has put the prevalence of deficiency in US adults at around 41 percent, with substantially higher rates among people with darker skin and those who get little sun exposure — which includes a great many homebound older adults.

Vitamin D’s role in bone and muscle is well established. The open question is whether it does anything for the brain.

What the study found

Researchers analyzed data from the Health, Aging and Body Composition (Health ABC) study — 2,777 adults aged 70 to 79 — over four years, and published the results in the Journal of the American Geriatrics Society. Lower blood levels of 25-hydroxyvitamin D were associated with poorer cognitive performance over the follow-up period.

Why “associated with” is doing a lot of work

The researchers were explicit that no causal factor was identified. That caveat is not boilerplate. There are at least three ways this association could arise without vitamin D affecting the brain at all:

• Reverse causation. People in early cognitive decline go outside less, eat less well, and are less likely to take supplements — all of which lower vitamin D. The decline could be causing the low vitamin D rather than the other way round.
• Confounding. Low vitamin D tracks with frailty, illness, poverty and immobility, each of which independently predicts cognitive decline.
• It could be real. Vitamin D receptors do exist in the brain, so a genuine mechanism is plausible.

Distinguishing these requires randomized trials of supplementation, and those have so far not shown that giving vitamin D protects cognition. As with falls, the observational signal has not translated into a supplement that works.

Sources

• Forrest KYZ, Stuhldreher WL — Prevalence and correlates of vitamin D deficiency in US adults, Nutrition Research
• Journal of the American Geriatrics Society — Relationship between 25-hydroxyvitamin D and cognitive function in older adults: the Health, Aging and Body Composition study
• NIH Office of Dietary Supplements — Vitamin D fact sheet for health professionals

Dementia is not one disease. It is a group of symptoms — decline in memory, reasoning and the ability to manage daily life — produced by several different underlying conditions, of which Alzheimer’s disease and vascular dementia are the most common. Tens of millions of people live with it worldwide and the number is expected to keep climbing.

Which is why an unlikely finding out of Finland got so much attention.

What the study found

The Kuopio Ischaemic Heart Disease Risk Factor Study followed 2,315 men aged 42 to 60 for around 20 years. Men who reported taking a sauna four to seven times a week were about 66 percent less likely to be diagnosed with dementia than men who used a sauna once a week, and about 65 percent less likely to be diagnosed with Alzheimer’s disease. The results were published in Age and Ageing.

There was a dose-response pattern — more frequent sauna use, lower risk — which is one of the things that makes an association more credible. The analysis adjusted for age, blood pressure, alcohol, smoking, cholesterol and other conditions.

What it does not show

This is observational. Men who sauna four to seven times a week are different from men who sauna once a week in ways that are hard to fully adjust for — they are likely healthier, more social, more physically able, and more embedded in community life. Any of those could be doing the work.

The researchers’ own hypothesis is cardiovascular: sauna bathing raises heart rate and lowers blood pressure in ways resembling moderate exercise, and vascular health is strongly tied to brain health. That is plausible and unproven.

Sources

• Age and Ageing — Sauna bathing is inversely associated with dementia and Alzheimer’s disease in middle-aged Finnish men
• Alzheimer’s Disease International — Dementia statistics
• Alzheimer’s Association — What is dementia?