When the days are short and dark in the UK, it becomes much harder for us to make enough
vitamin D from sunlight. Yet vitamin D plays a role in almost every system in the body, not just
bones.

How common is Vitamin D deficiency?
UK data shows that around 1 in 6 adults are deficient, rising to over 1 in 4 during winter. In people
with darker skin, those who spend little time outdoors, or adults over 65, the numbers are even
higher.

Every cell listens to vitamin D
Vitamin D receptors are found on nearly every cell in the body. This means vitamin D behaves more
like a hormone than a vitamin, influencing immune balance, inflammation, metabolism, and
cellular repair.

Mood, energy and mental wellbeing
Vitamin D receptors are also found in the brain. Low levels have been linked with low mood,
seasonal depression (SAD), fatigue, and reduced motivation. Many people report improved mood,
clearer thinking, and better resilience during the winter months when Vitamin D levels are optimised.

Blood pressure and heart health
Low vitamin D is associated with higher blood pressure. It appears to help regulate the
renin-angiotensin system, supporting healthy blood vessel tone and cardiovascular function.

Hormones and metabolic health
Vitamin D supports healthy testosterone and oestrogen balance, fertility, and menstrual regularity. It
also improves insulin sensitivity, helping reduce the risk of insulin resistance and type 2 diabetes.

Cancer and immune surveillance
Vitamin D plays a role in regulating cell growth, encouraging damaged cells to self-destruct and
supporting immune surveillance. Lower levels have been associated with increased risk of several
cancers, particularly colorectal and breast cancer.

Cholesterol, friend or foe?
Vitamin D is made from cholesterol. When levels are low, the body may compensate by producing
more cholesterol. Optimising vitamin D can sometimes help normalise cholesterol patterns rather
than worsen them.

How much do we really need?
The NHS currently recommends 400 IU daily. However, researchers such as Professor Bruce Hollis
suggest that many adults may require much higher intakes (often 4,000–10,000 IU daily) to reach
optimal blood levels, especially in winter. Vitamin K2 is often recommended alongside
higher-dose vitamin D to help guide calcium to bones and away from soft tissues.

The bottom line about Vitamin D
Vitamin D is not just about bones — it supports mood, immunity, hormones, metabolism, heart
health, and long-term resilience. Testing and personalised supplementation can make a meaningful