Shannon Clinic – Melbourne Chiropractic and Sports Care Blog

By: Dr. Nicholas Shannon | Sports & Exercise Medicine Chiropractor | Shannon Clinic, Melbourne CBD

Ask most people why they take a vitamin D supplement and the answer is usually something like: ‘I don’t get outside much’ or ‘my doctor said my levels were low.’ Both are reasonable starting points. But a growing body of research spanning randomised controlled trials, systematic reviews, and major cohort studies is raising a more interesting question: is a supplement actually doing the same thing as sunlight? The short answer, increasingly, appears to be no.

This post summarises what the evidence from the last five years actually shows about vitamin D deficiency, supplementation, and sun exposure across five health domains that matter most to the patients we see at Shannon Clinic: immune function, cancer risk, mood and depression, athletic performance, and muscle health with ageing. It’s a complicated picture, so we’ll work through it domain by domain and end with what we think the practical takeaways are for people living and training in Melbourne.

One important note before we get into it: this is a summary of the population-level research. Individual health decisions, including supplementation should always be made in consultation with your GP or a relevant specialist, not a blog post.

First: the gap that keeps appearing in the research

Across almost every health domain, there is a consistent and striking pattern in the vitamin D literature. Studies that measure sun exposure and health outcomes show strong, consistent benefits. Studies that give people vitamin D supplements and then measure the same outcomes frequently show much weaker effects and often no effect at all.

This gap has puzzled researchers for years. If low vitamin D causes the problem, shouldn’t fixing vitamin D fix the problem? The answer seems to be that sunlight does something that the vitamin D molecule taken as an oral supplement simply cannot replicate.

There’s More to Sunlight Than Vitamin D Synthesis

A 2025 expert review published in Photochemical & Photobiological Sciences, authored by 17 researchers from institutions across Europe, Australia, and the United States specifically addressed this question. The authors concluded that the health benefits of UV radiation extend far beyond vitamin D synthesis, and include nitric oxide release from the skin, activation of neuro-immune-endocrine pathways, and the production of a range of other mediators that oral supplements simply do not trigger. The review specifically noted: for the general population, ‘the marked difference between a strong observational relationship, but weak or non-existent interventional proof of benefit shows that non-vitamin D pathways must be important.’

Those non-vitamin D pathways include:

• Nitric oxide: (NO) is released from the skin on UVA exposure. NO is a vasodilator, anti-inflammatory agent, and neurotransmitter. It cannot be generated by a supplement because it requires direct UV photolysis of nitrite stored in the skin.
• Beta-endorphins: UVB exposure triggers release of these endogenous opioids from skin cells, producing analgesia, mild euphoria, and social bonding effects.
• Alpha-melanocyte stimulating hormone: (α-MSH) is released on UV exposure, with direct anti-inflammatory and immunomodulatory downstream effects.
• Circadian rhythm entrainment via both retinal and skin photoreception, regulating the daily timing of cortisol, testosterone, melatonin, and growth hormone. A capsule has zero effect on any of this.
• Near-infrared and infrared radiation is present in natural sunlight, these wavelengths stimulate mitochondrial function and cellular energy production in ways that a supplement cannot.

These pathways matter differently across different health domains. Let’s go through each one.

Immune function — supplements help, but sunlight does more

The clearest recent evidence for vitamin D supplementation on immune outcomes came from the VITAL trial, a large, well-designed US study following over 25,000 adults 55 years or older for five years.

The VITAL autoimmune study (Hahn et al., BMJ, 2022) found that 2,000 IU/day of vitamin D3 reduced the incidence of confirmed autoimmune diseases including rheumatoid arthritis, polymyalgia rheumatica, and autoimmune thyroid disease by 22% compared to placebo. If only the last 3 years of the study are considered the vitamin D supplementation group had a 39% reduction in confirmed auto-immune disease. This is one of the most meaningful positive findings from the vitamin D supplementation literature in recent years, and it’s clinically significant.

But when you look at the observational data for sun exposure and autoimmune disease, the picture is even more striking and is not fully explained by vitamin D alone. Multiple sclerosis, type 1 diabetes, rheumatoid arthritis, and psoriasis all show strong latitude gradients: they are significantly more common in populations at higher latitudes with lower UV exposure, and these associations hold even after adjusting for vitamin D status.

This is consistent with the mechanistic evidence. A 2021 review in Photochemistry and Photobiology found that UV radiation directly alters the composition of dendritic cells and T regulatory cells through nitric oxide synthase and heme oxygenase pathways, effects that may directly or indirectly mitigate disease progression and susceptibility independently of vitamin D synthesis.

The practical implication: supplementation has a meaningful, evidence-backed immune benefit particularly for autoimmune risk reduction. But it is not doing the full job that regular sun exposure does.

👉 Exercise is offers another pathway for improving immune function as Melbourne city chiropractor explains.

Cancer — a more complicated picture than you might expect

This is probably the area where the gap between sun exposure data and supplementation data is most stark and where the research is most frequently misunderstood.

The supplementation RCTs are largely negative for cancer prevention. Across well-designed trials, vitamin D supplementation does not appear to meaningfully reduce the risk of developing cancer. Where the picture gets more interesting is cancer mortality, a different question.

VITAL found that vitamin D did not significantly reduce total cancer incidence (HR 0.96), but showed a meaningful directional reduction in total cancer mortality (HR 0.83) which strengthened when the first two years of follow-up were excluded (HR 0.75), suggesting that longer exposure may be needed before a mortality benefit emerges. Specific analyses have found significant reductions in lung cancer and respiratory cancer mortality from supplementation.

Sun Exposure Lowers Cancer Incidence

The sun exposure data tells a different story. Across a large number of ecological, case-control, and cohort studies, higher sun exposure is consistently associated with lower incidence of colorectal, prostate, breast, and non-Hodgkin lymphoma cancers. These associations are stronger and more consistent than the supplementation RCT data suggesting again that something beyond vitamin D is at work.

The leading candidate mechanisms are UV-induced immune modulation and circadian rhythm regulation. A systematic review in the European Journal of Cancer specifically identified that in prostate cancer and non-Hodgkin lymphoma, ‘other sunlight-potentiated and vitamin D-independent pathways including modulation of the immune system and the circadian rhythm’ may play a meaningful role in reduced cancer risk.

The irreducible tension here is melanoma risk. UV radiation is the primary environmental cause of melanoma and keratinocyte skin cancers. This is not in dispute. But two important nuances are worth understanding. First, the overall mortality data from high-UV populations does not show the penalty that a simplistic skin cancer narrative would predict. Second, a 2024 updated Australian position statement on sun exposure specifically concluded that advice on sun exposure should be risk-stratified, that is, the right balance is not the same for everyone.

The revised Australian sun exposure position statement (Neale et al., Aust NZ J Public Health, 2024) — developed by leading Australian dermatologists, epidemiologists, and endocrinologists concluded that people at intermediate skin cancer risk can obtain sufficient sun exposure to maintain vitamin D status while still practising sensible sun protection. We’ll return to what that looks like practically at the end.

Mood and depression — sun exposure has a biological advantage

This is where the comparison between sun exposure and supplementation becomes most clinically interesting because the two interventions appear to be targeting genuinely different mechanisms, which means they are not interchangeable.

The vitamin D supplementation evidence for depression has strengthened considerably. A 2024 dose-response meta-analysis of 31 RCTs (Ghaemi et al., Psychological Medicine) found that each additional 1,000 IU/day of vitamin D3 produced a small but statistically significant reduction in depressive symptoms (SMD −0.32). In people who already had depressive symptoms at baseline, the effect was meaningfully larger (SMD −0.57). The effect appeared greatest at higher doses around 4,000–8,000 IU/day and was most consistent in short-to-medium term studies of 8 to 24 weeks. It should be noted, there was no effect on anxiety symptoms, which requires further research.

The mechanisms are reasonably well understood: vitamin D regulates serotonin synthesis through the tryptophan hydroxylase 2 gene, modulates the HPA axis and cortisol response, and has anti-inflammatory effects in the central nervous system. All of these are relevant to depression biology.

What Sun Exposure Does Differently

But sunlight operates through additional pathways that a capsule cannot replicate. Beta-endorphins released from the skin on UVB exposure produce direct mood-elevating effects. Serotonin release is also triggered by light reaching the retina, a completely separate pathway from vitamin D that is activated by light exposure to the eyes, not to the skin. And circadian rhythm entrainment, one of the most established biological drivers of mood disorders which is entirely dependent on light exposure, with no vitamin D component whatsoever.

The large UK Biobank studies covering 400,000+ adults found that higher daylight exposure was independently associated with lower rates of low mood, anhedonia, and antidepressant use even after controlling for vitamin D status. The light-mood relationship exists beyond what vitamin D alone explains.

The clinical implication: for mood and depression, the combination of correcting vitamin D deficiency and ensuring adequate daily light exposure — particularly morning light — appears to be more effective than either intervention alone. These are not the same thing.

Athletic performance — what matters for active Melburnians

For the athletes and active patients we work with at Shannon Clinic, from recreational runners and cyclists to competitive tennis players and team sport athletes, the vitamin D and sun exposure literature has some direct practical relevance.

The first thing worth knowing: indoor athletes consistently have lower vitamin D levels than outdoor athletes, though the magnitude of this difference is influenced by season, latitude, and skin type. A 2023 systematic review and meta-analysis (Bârsan et al., International Journal of Molecular Sciences) found that outdoor athletes had higher serum 25(OH)D levels than indoor athletes, with the difference most pronounced in Asian athletes and in winter months, consistent with sun exposure being the primary driver of the gap. The authors do note, that outdoor training alone is not sufficient athletes in increasing vitamin D levels, and supplementation decisions should not be based on training type alone.

Performance Benefits Depend on Vitamin D Levels.

The evidence for supplementation on performance is now reasonably clear: a 2024 systematic review in the Orthopaedic Journal of Sports Medicine (Wyatt et al.) concluded that the greatest benefit of vitamin D supplementation in elite athletes was in improving aerobic endurance, anaerobic power, and strength, but specifically in athletes who started with deficient or insufficient baseline levels. In athletes who were already vitamin D replete, supplementation produced no meaningful performance benefit.

This is the consistent finding across the performance literature: supplementation corrects a deficit but does not produce a performance enhancement beyond normal. It’s a floor, not a ceiling.

Where sun exposure appears to do more than a supplement is through circadian entrainment. Training timing relative to the body’s natural circadian rhythm has a measurable effect on performance. Peak neuromuscular output, hormonal profiles including testosterone and cortisol, and cardiovascular efficiency all vary predictably across the day in response to light-entrained rhythms. Outdoor training in daylight actively reinforces this circadian signal. Indoor training under artificial light does not. A supplement adds none of this.

For tennis players specifically and we see a lot of them at Shannon Clinic given Dr. Shannon’s nine years at the Australian Open there is the additional consideration that vitamin D levels are associated with stress fracture risk and soft tissue injury rates. Low serum 25(OH)D (below 30–50 nmol/L) is consistently associated with increased bone stress injury risk in athletes, and this is particularly relevant during winter training periods when sun exposure naturally falls.

Muscle health and ageing — the most nuanced finding

If you have read that vitamin D is good for muscles and sarcopenia prevention, you are not wrong about the biology however the clinical trial evidence for supplementation as a standalone treatment is consistently disappointing.

A meta-analysis of ten randomised controlled trials in older adults found that vitamin D supplementation monotherapy produced no meaningful improvement in handgrip strength, timed up-and-go performance, appendicular lean mass, or overall physical performance. This is a robust null finding that has been replicated across multiple well-designed studies.

This does not mean vitamin D is unimportant for muscle health. The biology is real, vitamin D receptors are expressed in skeletal muscle, and vitamin D deficiency is genuinely associated with accelerated muscle loss in older adults. The issue appears to be that correcting deficiency with a supplement alone without the co-stimulation of exercise, adequate protein, and the broader biological environment created by outdoor physical activity is insufficient to reverse established muscle loss.

Older adults who exercise outdoors consistently show better muscle preservation than those exercising indoors, even when both groups are vitamin D replete. Isolating exactly how much of this is attributable to sun exposure specifically versus exercise, social engagement, and other confounds is methodologically difficult but the pattern is consistent enough to be clinically meaningful.

👉 Here is how nutrition and exercise can fight against sarcopenia.

The big picture: what sun avoidance data tells us

The most striking piece of evidence in this whole literature and the one that most directly challenges the ‘just take a supplement’ framing comes from long-term cohort studies of sun exposure habits and mortality.

The Melanoma in Southern Sweden (MISS) study followed nearly 30,000 women over 20 years. Lindqvist et al. (Journal of Internal Medicine, 2014) found that women who actively avoided sun exposure had approximately double the all-cause mortality rate of those with the highest habitual sun exposure. A follow-up analysis in 2016 confirmed that the survival advantage in sun-seeking women was driven by lower rates of cardiovascular disease and non-cancer/non-CVD causes of death, not by cancer outcomes.

The authors explicitly acknowledged that they could not determine how much of this was mediated by vitamin D versus other UV-related mechanisms and noted that ‘vitamin D levels might simply be a marker of sun exposure rather than a causal agent.’ This is the fundamental uncertainty at the heart of the field.

What we can say with confidence is this: the health benefits associated with habitual sun exposure in population data are consistently larger and broader than the health benefits produced by vitamin D supplementation in clinical trials. This gap is best explained by the non-vitamin D pathways described above.

Practical takeaways — what to actually do

Given everything above, here is our evidence-informed summary of what makes sense for most active adults living in Melbourne.

On sun exposure

Habitual, moderate, non-burning sun exposure is the primary goal and it appears to do more for long-term health than supplementation alone. For most fair-to-medium-skinned Melburnians:

• In summer (UV Index typically 3 or above from around 9am to 5pm), a 10-20 minus of sun exposure to arms and legs on most days is sufficient for vitamin D.
• Specifically in latitudes like Melbourne, the winter sun (UV index 2–4) is too weak for meaningful vitamin D synthesis but UVA rays still penetrate the atmosphere sufficiently to trigger nitric oxide release from the skin, lowering blood pressure and supporting cardiovascular health. Avoiding the winter sun costs you both pathways. Longer exposure times are therefore needed and supplementation becomes more justified for those who cannot achieve adequate outdoor time.
• Morning light exposure (first 30–60 minutes after waking, outdoors or near a bright window) is particularly valuable for circadian rhythm entrainment, independent of UV index. This is relevant for sleep, mood, and hormonal function year-round.
• Outdoor exercise in natural light, walking, running, cycling, outdoor training sessions, provides UV exposure for vitamin D synthesis during the right seasons, circadian entrainment year-round, and the mood-enhancing beta-endorphin and serotonin effects of both light and physical activity.
• Avoid peak UV exposure and ad hoc exposure when the UV index exceeds 10 (common in Melbourne in December and January) without appropriate sun protection, particularly for fair-skinned individuals and those with personal or family history of skin cancer.

On supplementation

Supplementation makes most sense as an adjunct when adequate sun exposure is not achievable, not as a permanent substitute for it.

• Get your 25(OH)D tested first. The evidence for benefit from supplementation is most consistent in people who are actually deficient (below 50 nmol/L) or insufficient (50–75 nmol/L). Supplementing a person with normal levels adds little.
• If supplementing, use vitamin D3 (cholecalciferol) as it raises serum 25(OH)D more effectively than D2. Daily dosing outperforms monthly bolus doses across multiple outcome domains.
• Standard doses of 1,000–2,000 IU/day are sufficient for bone health. For immune, mood, and inflammatory outcomes, the evidence suggests that higher targets may be needed, which may require 2,000–4,000 IU/day depending on baseline status.
• Do not treat supplementation as a substitute for sun exposure. The non-vitamin D pathways mentioned like nitric oxide, beta-endorphins, circadian entrainment are not activated by a capsule. Supplementation corrects one biochemical deficit; it does not replicate the full biological environment created by natural light.

On skin cancer risk

The goal is not maximum sun exposure. It is habitual, moderate, non-burning exposure, the pattern humans evolved with. The evidence from the MISS cohort and related studies suggests that chronic sun avoidance carries its own health risks. The appropriate response is not recklessness but balance, which looks like:

• Frequent, short, unprotected exposures, particularly outside peak UV periods, rather than infrequent, prolonged, burning episodes.
• Protecting the face year-round (sun damage is cumulative) while allowing more exposure on limbs and trunk.
• Regular skin checks, particularly important for fair-skinned Australians. See your GP if you notice any new or changing lesions.
• Remembering that sunscreen use during planned outdoor activities does not meaningfully reduce vitamin D synthesis when used appropriately.

A note on where this fits for athletes and active patients

At Shannon Clinic, most of the active patients we assess from recreational tennis players to elite athletes are not thinking about sun exposure as a health intervention. They’re focused on training load, injury prevention, and recovery. But the evidence above suggests that for athletes training indoors or early in the morning before UV rises, particularly through the Melbourne winter, vitamin D levels are worth monitoring as part of routine health management. And wherever possible, incorporating outdoor training in natural light, particularly morning sessions when they fit the schedule appears to confer benefits that no supplement can fully replicate.

This is one of the reasons we include training load monitoring and broader lifestyle assessment in the clinical picture at Shannon Clinic, rather than treating injury in isolation. Recovery capacity, immune function, mood, and physical performance are all part of the same system and as this evidence makes clear, something as simple as where and when you train can influence all of them. If you’d like help building that into your broader health and performance picture book an appointment today.