While data are still being collected, the study, titled “Vitamin D and brain health: an observational and Mendelian randomization study”, was published in April 2022 to evaluate the relationship between a vitamin D deficiency and cognitive function in 294,514 people (mostly women aged 60 to 73) from the UK. After 11 years all participants had regular blood tests, and the brain images of 23,901 were analysed. 

A blood vitamin D deficiency is generally defined as 25(OH)D levels being less than 25 nanomoles per litre (nmol/L), compared with the normal 50 nmol/L. The serum 25(OH)D is produced in the liver as a result of vitamin D interacting with an enzyme. 

Globally, vitamin D insufficiency (different from deficiency) affects 30% to 50% of the population. Maintaining adequate levels of vitamin D is important for bone health, brain functioning and cardiovascular and endocrine systems, according to a 2020 study published in the Journal of Ageing Research.

“There is strong evidence to suggest that a diet deficient in vitamin D combined with inadequate sun exposure has potential to trigger many conditions, not only the well-known such as rickets and osteomalacia, but also neurocognitive disorder, Amyotrophic Lateral Sclerosis (ALS), Alzheimer’s disease (AD) and Parkinson’s Disease (PD),” the authors of a 2021 study in the journal Frontiers of Bioscience write. 

Rachel Hayes (L) and Gaby Harari relax on the beach on 21 December 2006 in Miami Beach, Florida. A study by the Harvard School of Public Health linked Vitamin D to a lower risk of developing multiple sclerosis. Most people do not get enough vitamin D from their diet, so exposure to sunlight is the most common source of the vitamin. Image: Joe Raedle / Getty Images

Defining dementia

Alzheimer’s disease is the most prevalent form of dementia, accounting for 60% to 70% of cases, writes Maverick Life’s Malibongwe Tyilo in “Vascular dementia – what is it and how to reduce your risk”. Dementia diagnoses vary across a spectrum of neurocognitive disorders and it is “suggested to qualify as a ‘silent epidemic’”, granting that a new case of dementia is diagnosed every four seconds globally, according to the 2021 study. 

Dementia is defined by the World Health Organization as “a syndrome – usually of a chronic or progressive nature – that leads to deterioration in cognitive function (the ability to process thought) beyond what might be expected from the usual consequences of biological ageing”, which affects “memory, thinking, orientation, comprehension, calculation, learning capacity, language, and judgement”. The Alzheimer’s Society also terms dementia as “a set of symptoms that over time can affect memory, problem-solving, language and behaviour”. Dementia is more common in people (particularly women) over the age of 65, states the society’s website. However, “atypical dementia” can be diagnosed as early as 40 years, according to the 2021 study. 

“The currently available treatment strategies work only as symptomatic and till now there is no ‘cure’ for dementia, hence various alternative and complementary ‘medicines’ are often used to help people with dementia,” the authors of the 2021 study write. Consequently, a recent interest in the role of vitamins and their efficacy in preventing or treating dementia has come to light. 

The role of vitamin D in preventing dementia

Given vitamin D’s crucial role in maintaining bone health, receptors responsible for absorbing vitamin D are present throughout the body – including in the brain. 

The 2020 study notes that vitamin D’s “neuroprotective, anti-inflammatory, and antioxidant effect on neurons promotes brain health… (It) promotes the production of neurotrophic factors such as nerve growth factor (NGF)”, and “prevents neuronal death”. NGF is a gene product that is mainly present in the hippocampus and neocortex areas of the brain – it “enhances neurotransmission and has been suggested to be critically involved in memory and executive functioning”, thus influencing both cognitive and behavioural aspects which is affected in those with dementia. 

The most recent study concurs that higher levels of 25(OH)D have a protective effect on brain health. The widespread distribution of vitamin D receptors in the brain suggests a need for vitamin D in different areas of the brain. It is necessary for the development of neurons, reducing the risk of blood clots that may cause blockages in veins or arteries that can lead to strokes, regulating blood pressure, and protecting nerve cells by limiting the amount of damage inflammation exerts on the protective outer layer of nerve fibres. The damaging of nerve cells due to inflammation is commonly observed in Alzheimer’s disease. 

An excess does not decrease the odds

People with low levels of vitamin D are 54% more at risk for developing dementia than those with normal levels. Additionally, improving vitamin D intake to achieve a normal 25(OH)D presence of 50 nmol/L could prevent 17% of dementia diagnoses. The 2022 study acknowledges that increasing vitamin D intake, whether through supplements or a conscious effort to spend more time in the sun, only helps to prevent dementia if a vitamin D deficiency is present. Having a 25(OH)D level of more than 50 nmol/L may limit further benefit and does not decrease the odds of dementia. 

The researchers’ findings demonstrate a causal relationship between the risk for developing dementia and vitamin D deficiency, thus highlighting the importance of maintaining a healthy vitamin D intake. Foremost, this causal relationship also provides a preventative opportunity for dementia diagnosis. DM/ML

Also read: “Looking after family members with dementia: Alzheimer’s disease”