Alzheimer’s: be careful not to lack vitamin D

Alzheimer’s disease is the most common form of dementia worldwide. It is characterized by a progressive deterioration of memory, language and reasoning skills, to the point of profoundly affecting the autonomy of those affected.

In the absence of effective curative treatment, acting on modifiable risk factors remains the most promising strategy. And vitamin D could be one of these levers.

“The prevalence of Alzheimer’s disease and dementia will continue to increase steadily, » says Dr. Dale Bredesen, neurologist and author of The end of Alzheimer’s And The end of Alzheimer’s – The program. “ Given that no effective treatment can combat the disease, acting on modifiable risk factors represents a more promising strategy. »

Read: Alzheimer’s disease worries 9 out of 10 French people

Vitamin D, a nutrient often insufficient

Vitamin D is produced by the skin during exposure to the sun between the months of April and October in the northern hemisphere. It can also be obtained through food (oily fish) or supplements. In the northern hemisphere, the reserves built up in summer are quickly exhausted and as winter approaches, vitamin D deficiency is widespread in the population. Taking vitamin D supplements can be essential in winter.

Lack of vitamin D is notably associated with an increased risk of diabetes, depression and lower life expectancy.

A deficit associated with the risk of dementia

Low levels of vitamin D are linked to a higher risk of dementia. In a synthesis of scientific studies published in Nutrition Journalscientists reported that people with the least vitamin D have a greater risk of developing Alzheimer’s disease or dementia compared to people who do not have a deficiency in this vitamin.

Read: Alzheimer’s: 9 risk factors we can act on

In this study, researchers therefore evaluated the association between vitamin D deficiency and the risk of developing Alzheimer’s disease or dementia. To do this, they produced a synthesis of the scientific studies carried out on this subject.

The results showed that people with vitamin D deficiency – defined by a 25-hydroxy-vitamin D level below 50 nmol/L – had 21% more risk to develop Alzheimer’s disease compared to people with a 25-hydroxy-vitamin D level greater than 50 nmol/L. The risk of dementia is also increased in people who are deficient in vitamin D.

Taking supplements reduces risk

A study suggests that people who don’t have enough vitamin D have an increased risk of Alzheimer’s disease and dementia. “ These results, says Dr. Bredesenconfirm those of a recent meta-analysis which shows that in 5010 healthy people initially and after 17 years of follow-up, those who have a higher vitamin D concentration have a lower risk of developing dementia “.

The new study was conducted among more than 12,388 participants from the US National Alzheimer’s Coordinating Center, who had an average age of 71 years and did not have dementia when they enrolled. Of the group, 37% (4,637) took vitamin D supplements.

The results showed that taking vitamin D was associated with a longer dementia-free life; there was also 40% fewer dementia diagnoses in the group that took supplements. “This study suggests that earlier supplementation may be particularly beneficial, before cognitive decline begins” says Dr. Bredesen.

The effects of vitamin D were greater in women, and people who did not report mild cognitive impairment, as well as in those who did not carry the APOEe4 gene, known to have a higher risk of Alzheimer’s dementia.

Vitamin D helps eliminate amyloid plaque in the brain, the accumulation of which is one of the hallmarks of Alzheimer’s disease. It may also help protect the brain against the buildup of tau, another protein involved in the development of dementia.

The need to act early

A study published in April 2026 in Neurology® Open Accessthe official journal of the American Academy of Neurology, sheds new light: high levels of vitamin D in the blood in mid-adulthood are associated with lower levels of tau protein in the brain — a biomarker of dementia — measured on average 16 years later.

The study involved 793 people with an average age of 39 years, without dementia at baseline. Their vitamin D levels were measured at the start of the study, then their brains were scanned an average of 16 years later to assess the levels of tau and beta-amyloid proteins, two biomarkers of Alzheimer’s disease. A high vitamin D level was defined as greater than 30 ng/mL. In total, 34% of participants had a vitamin D deficiency (less than 30 ng/mL), and only 5% took supplements.

Regarding the results, higher levels of vitamin D were associated with lower levels of tau protein years later. In contrast, no link was observed between vitamin D and the amount of beta-amyloid protein in the brain.

“These results suggest that higher vitamin D levels in early adult life are associated with lower tau load on average 16 years later”said Dr Martin David Mulligan, of the University of Galway in Ireland. “Midlife is a time when changes in risk factors can have a greater impact. »

The study is observational and does not prove causality, but it suggests that correcting a vitamin D deficiency in midlife could constitute an avenue for prevention.

How to avoid vitamin D deficiency

Vitamin D supplements come in the form of capsules, oils or tablets. As vitamin D is fat soluble, fats must accompany it in supplements. For the vast majority of French people, this requires winter supplementation with vitamin D3, at an average of 1,000 IU per day, preferably in oily form.

Read: How much vitamin D? Optimal level controversy