The bright side of sun exposure

While we're generally encouraged to be cautious about sun exposure, the evidence shows that, on balance, it's good for us. One well-known mechanism is related to sunlight's capacity to generate vitamin D in the skin, something which appears to be good for a range of things including bone health, immunity and cancer protection.

Evidence published in 2023 also found that in a group of older adults, higher concentrations of vitamin D measured in brain tissue at autopsy were related to a 25-33% lower risk of dementia or its lesser forerunner, known as 'mild cognitive impairment' [1]. Interestingly, this same study found no significant link between brain vitamin D and the underlying neuropathology – for example, the build-up of the damaged protein that are characteristic of Alzheimer’s disease – which should remind us that these relationships are not straightforward.

So-called 'epidemiological' studies of this nature may be useful for finding associations and forming what are called 'hypotheses', but generally cannot be used to provide a 'causal' link between two things. So, higher vitamin D levels may actually reduce the risk of cognitive impairment, or may simply be associated with a reduced risk. In the latter case, maybe people who are less cognitively impaired are more likely to spend time outdoors, and thereby have higher vitamin D levels.

However, a recently-published study sheds some light (excuse the pun) on the mechanisms by which vitamin D may be protective for the brain [2]. The review study examined different lines of evidence and didn't just focus on cognitive impairment, but other neurological issues such as multiple sclerosis, depression, autism and ADHD. Across a range of conditions, lower vitamin D was found to be associated with increased risk. Again, 'correlation does not prove causation.'

This paper did point out, though, that brain cells in several parts of the brain have vitamin D receptors (VDRs). That doesn't prove a causal role, but it does establish the biological plausibility for one – the brain is clearly equipped to respond to vitamin D directly.

In addition, vitamin D has anti-inflammatory effects, and there is evidence that inflammation can be an underlying process in conditions including cognitive decline, depression and multiple sclerosis.

Vitamin D has also, in preclinical research, been shown to increase production of 'brain-derived neurotrophic factor' (BDNF) and 'nerve growth factor' (NGF) – two substances that support brain cell health. BDNF is closely tied to ‘synaptic plasticity’, the process underlying learning and memory; NGF plays more of a role in keeping neurons alive and supporting their growth. Whether this translates into measurable benefit in humans is not known.

All the evidence is not in. But what we do have, I think, is enough to suggest that vitamin D has broad benefits for the brain. Sunburn is to be avoided, but prudent sun exposure when the sun is high in the sky (the condition when vitamin D can be made in the skin) may be a low-friction way to help preserve our mental faculties as we age.

References:

1. Shea MK, et al. Brain vitamin D forms, cognitive decline, and neuropathology in community-dwelling older adults. Alzheimer's Dement. 2023; 19: 2389–2396.

2. Barakat G, et al. Vitamin D and brain health: A systematic review. Clinical Nutrition Open Science, 2026; 67.