Vitamin D: Immunity, Bones and Mood

Vitamin D — What It Is and Why It Matters

Vitamin D is technically a hormone, not a vitamin. The body manufactures it in the skin in response to UVB rays from the sun, and its receptors are found in almost every tissue: the liver, kidneys, brain, immune cells and muscles. This means its effects reach far beyond bone health.

The form measured in a blood test is 25(OH)D — the main storage form, which best reflects the body's reserves.

Immunity: Why This Is Not Just Marketing

Vitamin D receptors are present on T and B lymphocytes — the foot soldiers of the immune system. Research shows that adequate vitamin D levels help regulate the immune response: too weak a response lets viruses spread unchecked, while too strong a response drives inflammation. Some studies link low vitamin D to more frequent respiratory tract infections (Vitamin D and immune function, 2012).

Bones: Calcium Cannot Work Without D

Vitamin D is essential for absorbing calcium in the intestine. Without it, calcium simply passes through. When vitamin D is deficient for a prolonged period, the body begins to "steal" calcium from bones — leading to osteoporosis in adults and rickets in children. In older adults, low vitamin D is associated with a higher risk of fractures.

Mood and Energy

Vitamin D receptors are found throughout the brain, particularly in regions linked to mood regulation. Several observational studies have identified a correlation between low vitamin D levels and depressive symptoms (Vitamin D and depression, 2011). It is important to note that correlation is not causation. Even so, investigating winter fatigue and low mood with a simple blood test costs little.

Who Is at Risk of Deficiency

Seasonal risk: Northern Europe sits at approximately 54–56° latitude. From October through April, UVB rays arrive at too oblique an angle for the skin to synthesize meaningful amounts of vitamin D — even on a sunny winter day.

Age: Older skin produces less vitamin D for the same sun exposure. Time spent outdoors also tends to decrease with age.

Skin color: Melanin — the body's natural sun protection — also reduces vitamin D synthesis. People with darker skin living at northern latitudes are at particular risk.

Breastfed infants: Breast milk typically contains very little vitamin D, which is why pediatricians often recommend supplements from the first days of life.

Obesity: Vitamin D is fat-soluble — a portion of it becomes sequestered in adipose tissue and is less available to the rest of the body.

Sun vs. Supplements: What to Know

20–30 minutes of direct sun exposure to the arms, legs and face during the summer months can provide a substantial share of the daily requirement at mid-latitudes. However, sunscreen with a high SPF significantly reduces synthesis — balancing vitamin D production against skin protection is an individual decision.

Food: Fatty fish (salmon, herring, mackerel), fish liver oils and some fortified products (certain dairy products, plant-based milks) are the main dietary sources. Unfortunately they are rarely sufficient to make up a winter shortfall in northern countries.

Supplements: Most doctors recommend vitamin D supplements during winter, especially for at-risk groups. The right dose should depend on a measured blood level. Vitamin D is fat-soluble — it accumulates in the body, and prolonged high-dose use can cause toxicity (hypercalcemia).

What to Do in Practice

If you have never had your vitamin D level checked, it is one of the simplest and most informative tests available — as a standalone or as part of a routine panel. If the result is low, discuss the next steps with your doctor. And in summer — spend more time outside.