The Importance of Vitamin D in Winter

As winter settles in and daylight hours shrink, there's one nutrient that often gets overlooked despite playing a starring role in your health and performance: vitamin D. Whether you're an athlete pushing for a new PB, a busy professional juggling work and training, or simply someone trying to stay healthy through the colder months, understanding vitamin D is essential.

In this post, we'll explore what vitamin D actually does in the body, why winter makes deficiency so common, the signs you might be running low, and how to optimise your levels through food, supplementation, and smart lifestyle choices. Let's dive in.

What is Vitamin D and Why Does It Matter?

Vitamin D is often called the "sunshine vitamin" because your skin produces it when exposed to sunlight. But calling it a vitamin undersells its importance. Vitamin D functions more like a hormone, influencing nearly every system in your body and brain (Caccamo, Ricca et al. 2018).

Here's what makes it so vital:

• Bone health: Vitamin D regulates calcium absorption in your gut (Carmeliet, Dermauw et al. 2015, Fleet 2022). Without adequate vitamin D, your body can't properly absorb calcium from food, which weakens bones over time and increases fracture risk (Wang, Chen et al. 2020). This matters to everyone, especially for us fitness enthusiasts putting load through the body.
• Immune function: Vitamin D plays a crucial role in activating your immune defences (Wei and Christakos 2015). Research shows that adequate vitamin D levels help your body fight off respiratory infections—something particularly relevant during winter when colds and flu are rampant (Prietl, Treiber et al. 2013, Carlberg 2019, Bishop, Ismailova et al. 2020).

In fact, vitamin D supports both your innate immune response (your first line of defence) and adaptive immunity (your targeted response to specific pathogens). This makes Vitamin D a vital nutrient in winter, when respiratory pathogens are circulating (Charoenngam and Holick 2020, Daryabor, Gholijani et al. 2023).

• Muscle function: You may not know this, but Vitamin D receptors are present in muscle tissue, where they influence protein synthesis, muscle strength, and recovery (Montenegro, Cruzat et al. 2019). Low vitamin D levels have been linked to reduced muscle performance and increased injury risk in athletes (Yoon, Kwon et al. 2021, Agoncillo, Yu et al. 2023, Carswell, O'Leary et al. 2023).

Mood and Mental Health

There's a reason Seasonal Affective Disorder (SAD) peaks in winter. Vitamin D influences neurotransmitter production and brain function, and low levels are associated with low mood, brain fog, and reduced cognitive performance (Spedding 2014, Farghali, Ruga et al. 2020, Pertile, Brigden et al. 2023).

Inflammation Control

Vitamin D helps regulate inflammatory responses in the body (Infante, Buoso et al. 2021, Mieszkowski, Borkowska et al. 2021, Ziada, Wishahe et al. 2025). This is vital because chronic low-grade inflammation impairs recovery, increases injury risk, and compromises long-term health (Fulop, Larbi et al. 2021). Think of vitamin D as the oil that keeps multiple engines running smoothly. When levels drop, small issues in your bones, muscles, immune system, and brain can compound.

The Winter Problem: Why We're All at Risk

Here's the challenge: most of our vitamin D comes from sunlight exposure. When UVB rays hit your skin, they trigger a chemical reaction that produces vitamin D3 (cholecalciferol), the active form your body uses.

But winter throws a spanner in the works. Between October and March in the UK, the sun's angle means UVB rays are too weak to stimulate vitamin D production, even on clear days (Prentice 2016).

Add in:

• Shorter daylight hours
• More time spent indoors
• Heavy clothing when outdoors
• Overcast skies

And you have a perfect storm for deficiency.

Research suggests that up to 50% of the global population is vitamin D deficient, and that figure climbs significantly during winter months in northern latitudes (Cosentino, Campodonico et al. 2021). If you're training indoors, working long hours, or simply not prioritising outdoor time, you're at even greater risk.

Signs You Might Be Low in Vitamin D

Vitamin D deficiency doesn't always announce itself with obvious symptoms, but there are warning signs worth noting:

• Persistent fatigue despite adequate sleep
• Frequent colds or infections
• Low mood, irritability, or feeling flat
• Muscle weakness or slower recovery from training
• Joint pain or stiffness
• Poor bone health or stress fractures (in severe cases)
• Brain fog or difficulty concentrating

If any of these symptoms sound familiar, it's worth investigating your vitamin D status. For active adults and athletes, many sports‑medicine experts aim for serum 25(OH)D of at least 75 nmol/L, with some suggesting 80–100 nmol/L as a preferred range, even though most general health guidelines define ≥50 nmol/L as adequate (Yoon, Kwon et al. 2021, Ip, Fu et al. 2022).

How to Optimise Vitamin D in Winter

The good news? You're not powerless. Here are the most effective strategies to maintain healthy vitamin D levels through winter:

1. Get Outside When You Can

Even though winter sunlight won't produce vitamin D in the UK, outdoor exposure still matters. Natural light helps regulate your circadian rhythm, supports mood, and encourages physical activity. Aim for at least 20-30 minutes of outdoor time daily, ideally in the morning.

If you're travelling to sunnier climates during winter, make the most of it. Brief periods of sun exposure (10-30 minutes, depending on skin tone) on the arms and legs can significantly boost vitamin D production (Webb, Kazantzidis et al. 2018).

2. Prioritise Vitamin D-Rich Foods

While it's difficult to meet all your vitamin D needs solely through diet, every bit helps. The best dietary sources include:

• Fatty fish (salmon, mackerel, sardines, trout)
• Egg yolks (from pasture-raised eggs ideally)
• Fortified foods (some dairy products, plant milks, cereals)
• Mushrooms exposed to UV light
• Cod liver oil

A 100g serving of salmon provides roughly 500-600 IU of vitamin D, while an egg yolk contains about 40 IU. For context, many experts recommend 1000-2000 IU daily during winter, so food alone won't get you there, but it contributes.

3. Supplement Strategically

For most people living in the UK, winter supplementation isn't optional—it's essential. Here's what to consider:

• Form: Choose vitamin D3 (cholecalciferol) over D2 (ergocalciferol). D3 is more effective at raising and maintaining blood levels (van den Heuvel, Lips et al. 2024).
• Dosage: A good starting point is 1000-2000 IU daily for maintenance. Many individuals need higher doses (3000-6000 IU for certain periods of time) depending on baseline levels, body weight, and activity levels (Yoon, Kwon et al. 2021). If you're unsure, get tested and work with a healthcare professional to determine your optimal dose.
• Take with fat: Vitamin D is fat-soluble, meaning it's better absorbed when taken with a meal containing dietary fat.
• Timing: There's no magic time, but taking vitamin D in the morning may align better with natural circadian patterns. Consistency matters more than timing.

4. Monitor Your Levels

If you're serious about maximising performance and health, consider checking your vitamin D levels twice a year—once at the end of summer (when levels are at their peak) and once in winter (when they are at their lowest). This provides a clear understanding of whether your supplementation plan is effective.

Special Considerations for Athletes

If you're training hard, vitamin D becomes even more important. Research suggests that correcting low vitamin D levels help with:

• Better muscle function and strength (Han, Xiang et al. 2024, Wyatt, Reiter et al. 2024)
• Reduced inflammation and faster recovery (Giustina, Bilezikian et al. 2024)
• Lower injury rates (Yoon, Kwon et al. 2021)
• Improved immune resilience during heavy training blocks (Jung, Seo et al. 2018)

Winter is already challenging for athletes—shorter days, colder weather, and a higher risk of illness. Please don't let low vitamin D make it more complicated. Treat supplementation as part of your training plan, not an afterthought.

The Bottom Line

Vitamin D is a foundational nutrient that influences bone health, immune function, muscle performance, mood, and inflammation control.

Winter makes deficiency almost inevitable for those living in northern climates, but the solution is straightforward: get outside when possible, eat vitamin D-rich foods, and supplement strategically with D3.

If you're training hard, dealing with frequent illness, or experiencing low energy and mood, consider testing your levels and adjusting your approach accordingly.

Kinetica Sports's supplement range includes high-quality, WADA-compliant vitamin D3 at 3000 IU per spray. This will support your health and performance goals throughout winter. By prioritising vitamin D, you're investing in your resilience, recovery, and long-term wellbeing.