Vitamin D isn’t just a winter supplement

Vitamin D is one of those nutrients most people think they understand.

We associate it with winter. We link it to immunity. We’re reminded to supplement when the days get shorter.

And then we stop thinking about it.

But vitamin D isn’t just a seasonal add-on or a box to tick during cold and flu season. It behaves more like a hormone than a vitamin, influencing systems across the body in ways that are often subtle and, therefore, easy to overlook.

That’s why so many people are unknowingly running low, while experiencing symptoms that don’t obviously point back to vitamin D at all.

Why is deficiency so common, even when you’re doing everything right?

In the UK, vitamin D deficiency is widespread, and not just in the depths of winter.

For much of the year, sunlight simply isn’t strong enough to stimulate adequate vitamin D production in the skin. Add modern indoor lifestyles, sunscreen use, covering up, and the fact that very few foods naturally contain meaningful amounts of vitamin D, and it becomes clear why levels quietly decline.

What’s less well known is that genetics also plays a role. Variations in certain genes (often called SNPs) can affect how efficiently someone produces or activates vitamin D from sunlight. This means that two people with the same sun exposure can end up with very different vitamin D levels.

What’s often missed is that vitamin D doesn’t just sit in the background until your bones need it. Once levels drop, multiple systems begin to feel the impact – often gradually, and potentially at the same time.

Vitamin D and blood sugar: a quieter influence

Vitamin D receptors are found throughout the body, including in the pancreas, muscles and fat tissue, and they play key roles in blood sugar regulation.

Adequate vitamin D helps insulin work more effectively, allowing glucose to move from the bloodstream into cells where it can be used for energy. When levels are low, this process can become less efficient. Blood sugar may rise after meals, dip more sharply later, and place greater strain on insulin over time.

This doesn’t mean vitamin D deficiency causes diabetes. But it can contribute to poorer blood sugar control, particularly when combined with stress, disrupted sleep, hormonal shifts or dietary changes… all common during busy phases of life.

Blood pressure, cholesterol and cardiovascular strain

Vitamin D also plays a regulatory role in the cardiovascular system.

It influences something called the renin–angiotensin system, which helps control blood pressure and supports the function of the lining of blood vessels. Low vitamin D levels have been associated with higher blood pressure and reduced vascular flexibility, both of which increase cardiovascular strain over time.

Cholesterol comes into play here, too. Vitamin D is synthesised in the skin from cholesterol, and low vitamin D status has been linked to changes in lipid metabolism and inflammatory markers. This doesn’t mean vitamin D is a one-stop-shop cholesterol treatment, but deficiency may make it harder for the body to maintain balance, particularly alongside metabolic stress.

The gut connection: an underestimated link

One of vitamin D’s most overlooked roles is in gut health.

Vitamin D helps maintain the integrity of the gut lining and supports appropriate immune responses within the digestive tract. When levels are low, the gut barrier can become more permeable, increasing the likelihood of inflammation and immune activation.

For people experiencing bloating, digestive discomfort or food sensitivities, vitamin D is rarely the only factor - but it can be a foundational one. A compromised gut also absorbs nutrients less efficiently, making it harder to correct deficiencies and reinforcing a cycle of imbalance.

Vitamin D and mental health resilience

Vitamin D receptors are found throughout the brain, and adequate levels support neurotransmitter function, including pathways involved in mood and motivation.

Low vitamin D has been associated with low mood, fatigue, and seasonal affective symptoms, particularly during the darker months, when daylight exposure is already limited. It also plays a role in circadian rhythm regulation, linking vitamin D status to sleep quality and emotional resilience.

Again, vitamin D isn’t a cure-all. But deficiency can quietly reduce the body’s ability to cope with stress, change and low-energy periods.

Bones, muscles and immunity - still essential

Of course, vitamin D’s role in bone health is well established. It supports calcium absorption and bone mineralisation (when available with K2, magnesium and boron), helping maintain skeletal strength over time.

Less talked about is its importance for muscle function. Adequate vitamin D supports muscle strength, balance, and recovery, reducing the risk of falls and injuries, which is even more important as we age or when we increase training demands.

And then there’s immunity. Vitamin D helps regulate immune responses, supporting the body’s ability to respond to infections without tipping into excessive inflammation. It’s not about “boosting” immunity, but about keeping it responsive and well-regulated.

Why vitamin D is underestimated

Vitamin D doesn’t usually announce itself with one dramatic symptom - that would be too easy! Instead, low levels tend to show up as a collection of smaller issues:

• low energy
• unstable blood sugar,
• gut disruption
• low mood
• more frequent illness

Because these symptoms overlap with stress, lifestyle, and seasonal changes, vitamin D is often overlooked, despite its role in all of them.

So, if you’ve been thinking of vitamin D as “just another supplement”, it may be time to widen the lens. Sometimes the most underestimated nutrients are the ones doing the most work. Check out Immunity + Vitality for your Vitamin D and immunity support.