Winter, the Sun, and Vitamin D

Vitamin D: Why deficiency quietly wrecks your resilience.

That’s the brand I take: Bronson Basics Vitamin K2 + D3 (amazon)

We are heading into the darkest stretch of the year. The winter solstice is around the corner, daylight is shrinking, and for most people that means one thing whether they realize it or not. Vitamin D levels are dropping.

Vitamin D deficiency is incredibly common in winter, especially for people who live above the southern states, work indoors, train early or late, or use sunscreen year-round. The problem is most people have no idea they are deficient until it starts showing up as frequent illness, low energy, poor recovery, or nagging inflammation.

Why vitamin D matters more than most people think

Vitamin D is not just a vitamin. It functions more like a hormone and plays a role in hundreds of processes in the body. When levels fall too low, several downstream effects can show up.

Low vitamin D may be able to impact:

Immune Function & Infections

• Vitamin D supports innate and adaptive immunity, helping produce antimicrobial peptides and regulate immune responses.

• Low levels are linked to greater risk of respiratory infections in observational studies.

Inflammation

• Vitamin D deficiency correlates with higher inflammatory markers and may contribute to chronic inflammatory conditions. Healthline

Muscle Strength

• People with low 25-hydroxyvitamin D have higher risk of losing muscle strength over time. health.harvard.edu

Bone Health

• Insufficient vitamin D impairs bone mineralization and increases fracture risk.

Mood & Seasonal Patterns

• Low vitamin D status is associated with higher rates of depressive symptoms and may be more pronounced in winter with reduced sunlight. The Recover

Metabolic Health

• Research links low vitamin D with insulin resistance and features of metabolic syndrome. PMC

This could be a large contributor as to why people feel more fragile in the winter. You get sick more easily. Training feels harder. Recovery takes longer. Resilience drops.

What your blood work should show

Vitamin D is typically measured as 25-hydroxyvitamin D in blood work and reported in ng/mL.

General clinical ranges:

• Below 20 ng/mL: deficient

• 20–30 ng/mL: insufficient

• 30–50 ng/mL: generally considered adequate

• 50–70 ng/mL: often considered optimal by many clinicians

If you are under 20 or even under 25, you are simply too low and likely feeling some of the effects whether you connect the dots or not.

Why sunlight still matters most

Sunlight exposure on bare skin is the most natural and efficient way to raise vitamin D. The issue in winter is that the sun angle is too low in many regions for meaningful D production, even if you are outside.

In other words, you can be outdoors and still not be making much vitamin D at all.

That is why winter supplementation makes sense for a lot of people.

Supplementing intelligently, not aggressively

I personally supplement vitamin D during the winter months. It is affordable, easy, and effective when done responsibly.

Important guardrails:

• Do not megadose blindly

• Do not chase extremely high levels

• Avoid pushing levels above roughly 70 ng/mL

Excessive vitamin D over time can lead to hypercalcemia, which means too much calcium in the blood. This can contribute to kidney stones, vascular calcification, nausea, fatigue, and heart rhythm issues. More is not better here.

A moderate daily dose, combined with occasional blood work, is the smart approach.

What about vitamin K2?

Many vitamin D supplements include vitamin K2, which helps guide calcium into bones rather than soft tissues. It can be a helpful pairing, especially if supplementing D long-term.

That said, many people already get adequate K2 from food, so it is not mandatory. Think of it as supportive, not essential.

Foods that contain vitamin K2 include:

• Aged cheeses

• Egg yolks

• Grass-fed butter

• Natto

• Fermented foods

Food sources of vitamin D

Food alone rarely gets most people to optimal levels, but it helps.

Foods with vitamin D include:

• Fatty fish like salmon, sardines, and mackerel

• Egg yolks

• Cod liver oil

• Fortified dairy or plant milks

The winter takeaway

Winter is a predictable stressor on your immune system and recovery. Vitamin D deficiency is one of the most overlooked contributors.

Get your levels checked if you can. Respect sunlight when available. Supplement conservatively if needed. Avoid extremes.

This is one of the simplest seasonal upgrades you can make for your health, training consistency, and overall resilience.

Stay strong through the dark months.