It Doesn’t Make Sense at First
Over the last couple of years, I’ve noticed something that doesn’t quite add up. I’ve been getting ill during the summer.
Last year, I was in Egypt, in the sun all day, and still ended up with a chest infection. This year, there’s a virus doing the rounds through my friendship group, and I’ve felt that same dip again. Nothing dramatic, but enough to feel like a pattern rather than bad luck.
It doesn’t make much sense on the surface. Summer is supposed to be the season where your body catches up. You are outside more, you are getting more light, and things generally feel easier. If anything, this is when your immune system should be at its strongest.
That is the part that does not quite hold up.
The Assumption That Summer Fixes It
Vitamin D sits at the centre of this assumption. It is one of the few nutrients the body can produce on its own, triggered by sunlight. More sun should mean more vitamin D. It is simple, intuitive, and widely accepted.
It is also incomplete.
In the UK, around 1 in 6 adults are estimated to have low vitamin D levels, and that does not fully resolve in summer for everyone. The issue is not a lack of sunlight. It is the gap between how vitamin D is made and how most of us actually live.
The body does not respond to the idea of “being outside.” It responds to a very specific type of exposure.
What Actually Counts as Sunlight
Vitamin D production depends on ultraviolet B rays making direct contact with the skin. In the UK, those rays are only strong enough for meaningful production during a relatively narrow window around midday, typically between April and September.
Light outside of those hours still matters for mood and sleep, but it contributes very little to vitamin D synthesis.
This is where the assumption breaks down. A morning coffee in the sun, a late afternoon walk, or sitting near a bright window all feel like meaningful exposure. Physiologically, they are not doing the same job. Glass blocks the UVB radiation needed for vitamin D production, and indirect or brief exposure rarely accumulates into anything significant.
You can spend most of the day in daylight and still produce very little vitamin D.
A Lifestyle That Includes Sunlight, But Avoids It
Summer feels like an outdoor season, but most days are still structured indoors. Work happens inside, and time outside is often broken up, shaded, or secondary to everything else that needs to get done.
Even when people are outside, exposure is rarely consistent. Skin is covered, time in direct sun is limited, and movement between environments is constant. None of this is inherently wrong. It is simply not aligned with the conditions required for sustained vitamin D production.
We have, effectively, built a lifestyle that technically includes sunlight, but functionally avoids it.
The Trade-Off We Don’t Talk About
There has also been a shift in how we think about sun exposure. Increased awareness of skin damage and cancer risk has led to more consistent use of sunscreen and a general tendency to limit prolonged exposure. This is a rational and necessary change.
As someone approaching my 40s, I am using SPF on my face daily, even in winter. That is not unusual anymore. It is standard advice now, and it makes sense.
What is less often acknowledged is the trade-off. Vitamin D production relies on the same UVB exposure that we are now more careful to manage. In practice, most people are not blocking it entirely, but they are reducing it enough that consistent production becomes less likely.
This is not a mistake. It is simply a different balance, and the body responds to it.
Why Vitamin D Still Matters
Vitamin D is often framed as a bone health issue, which makes it easy to overlook. In reality, it plays a wider role in how the body maintains stability, particularly in the immune system.
When levels are low, the immune response tends to be less efficient. You do not necessarily get dramatically sicker, but you may find you get run down more easily, pick things up more often, or take longer to recover. It shows up as a pattern rather than a single event.
Vitamin D is also involved in inflammation, muscle function, and energy regulation over time. None of this is immediate, which is why it is easy to miss. But over months, the difference becomes noticeable.
This is less about fixing a problem and more about maintaining a baseline your body can rely on.
What This Actually Means in Practice
In theory, summer sunlight should be enough to maintain that baseline. In reality, most people are balancing indoor routines, inconsistent exposure, and sensible sun protection in a way that makes that unlikely.
For some people, summer will be enough. For many, it only looks like it is. And if levels are already low, a few weeks of better weather is unlikely to fully close the gap.
That is where supplementation starts to make sense. Not as a shortcut, and not as a replacement for being outside, but as a way to support a consistent baseline when lifestyle and environment do not quite line up with what the body expects.
The shift is simple. Instead of assuming summer will take care of it, you treat vitamin D as something that needs to be maintained.
Summer makes you feel better, but it doesn’t guarantee that your body is.
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