Most people associate sun exposure with vitamin D — and while that’s not entirely wrong, it’s only one small part of a much bigger story.
People tend to treat vitamin D like a magic bullet — if your blood levels are in range, you’re “good.”
But the truth is more nuanced: vitamin D is best understood as a biomarker of sun exposure, not a fix-all supplement.
When your body is exposed to sunlight — specifically UVB rays — it produces vitamin D in the skin.
But this natural production happens alongside a complex network of other light-triggered molecules and physiological responses.
So if your vitamin D levels are optimal because you’ve been out in the sun regularly, it likely reflects broader benefits: healthy circadian rhythms, improved mitochondrial function, increased endorphins, and more.
If you get to the same blood level using a supplement, you’re skipping all of those synergistic pathways.
This distinction became especially clear in the VITAL study, a large, randomized controlled trial conducted in the U.S. It involved 25,871 healthy adults (men over 50, women over 55), and ran for over 5 years.
Participants were given either:
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2,000 IU of vitamin D3 daily
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1 gram of omega-3 fatty acids
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Both
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Or placebo
Researchers wanted to know if these supplements would prevent major diseases like cancer or cardiovascular events.
The results?
Vitamin D supplementation showed no significant reduction in cancer incidence or heart disease.
Even in people who started with low vitamin D levels, supplementation didn’t meaningfully change health outcomes.
So what does this mean?
Despite vitamin D being associated (in observational studies) with lower disease risk, raising vitamin D levels through supplements doesn’t seem to cause those benefits.
Instead, it suggests that people with high natural vitamin D levels tend to have healthier lifestyles — especially more regular sun exposure — and that’s likely what's driving better outcomes.
In essence, optimal vitamin D levels are a marker that you’re spending time outdoors, syncing with your environment, and activating a host of light-dependent health processes.
Supplements can raise the number on a lab test, but they don’t replicate the complex symphony of effects that sunlight initiates.
When UVB light from the sun hits your skin, it does so much more than just initiate the process of vitamin D synthesis.
It triggers a cascade of biological responses that involve many other molecules — some of which we’re still learning about.
For example, in addition to producing vitamin D, sunlight also activates the production of proopiomelanocortin (POMC), a large precursor molecule. POMC gets broken down into several important compounds, including:
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Alpha-MSH (melanocyte-stimulating hormone), which helps regulate melanin production and also influences appetite suppression and energy expenditure — both important for body composition and metabolic health.
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Beta-endorphins, which are natural opioids that improve mood and reduce pain.
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Melanin, which helps protect your skin from UV damage and can convert light into usable energy — almost like a biological solar panel.
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Other signaling molecules that play roles in immune regulation and inflammation control.
On top of that, the skin — when exposed to UVB — doesn’t just make one form of vitamin D. It produces a family of vitamin D–like molecules, which have diverse effects in the body beyond just calcium regulation. These include molecules that may influence cellular repair, immune function, and even brain health.
So, when you take a vitamin D supplement, you’re getting one molecule (usually cholecalciferol). But when you get sun exposure — especially midday sunlight with a full spectrum of wavelengths — you’re activating dozens of biochemical pathways simultaneously.
This is why vitamin D levels are often described as a proxy marker for overall sun exposure — not the main benefit itself.
It's not just about raising a number on your blood test.
It's about stimulating systems in your body that evolved to respond to light.
In short: sunlight is not just a source of vitamin D. It’s a multi-signal input that your body relies on for mood, metabolism, immunity, circadian rhythm regulation, and much more.
