Vitamin D deficiency is increasingly recognised as a widespread public health concern in the UK and other countries with limited sunlight. Often referred to as the “sunshine vitamin”, vitamin D is essential for bone health, immune function, muscle strength and overall wellbeing. Yet, despite its importance, a large percentage of the population struggles to maintain adequate levels throughout the year.
In low-sun regions, deficiency is not limited to those with poor diets or unhealthy lifestyles. Even individuals who eat well, exercise regularly and spend time outdoors can still fall short. This is largely due to geographical location, seasonal changes and modern indoor living. Understanding why vitamin D deficiency is so common, what optimal levels really mean and how to support healthy intake is key to protecting long-term health.
Vitamin D challenges
UK’s geographical position means that for much of the year, particularly from October to March, sunlight is simply not strong enough to stimulate vitamin D production in the skin. During these months, the sun’s UVB rays do not reach the earth at the correct angle, making natural synthesis virtually impossible, even on bright winter days.
This seasonal limitation is compounded by lifestyle factors. Many people spend most daylight hours indoors due to work, education or caring responsibilities. When outdoors, cold weather encourages covering up, leaving little skin exposed. Sunscreen use, while important for skin cancer prevention, also significantly reduces vitamin D synthesis.
These challenges are not unique to the UK. Countries across Northern Europe, Scandinavia, Canada and parts of Northern Asia experience similar conditions. Even populations in sunnier regions can be deficient due to air pollution, cultural clothing practices or avoidance of sun exposure.
Certain groups face a higher risk, including people with darker skin tones (as higher melanin levels reduce vitamin D production), older adults, pregnant women and those with medical conditions affecting fat absorption. As vitamin D deficiency often develops gradually, many people remain unaware until symptoms become more pronounced.
Optimal levels
Vitamin D plays a central role in helping the body absorb calcium and phosphorus, making it essential for maintaining strong bones and teeth. Beyond skeletal health, it supports muscle function, immune regulation, inflammation control and cellular health. Research has also explored links between vitamin D status and mood, energy levels and resilience against infections.
Symptoms of deficiency can be subtle and easily overlooked. Common signs include persistent fatigue, frequent illness, muscle weakness, low mood and general aches or pains. In the long term, inadequate vitamin D levels may contribute to osteoporosis, increased fracture risk, muscle loss and impaired immune response, particularly in older adults.
When discussing “optimal” vitamin D levels, the goal extends beyond simply avoiding deficiency. Optimal levels aim to support overall health and reduce long-term risk factors. In recognition of widespread deficiency, UK health authorities recommend routine vitamin D supplementation for most people, especially during autumn and winter.
In terms of optimal vitamin D levels, the NHS generally recommends maintaining a blood concentration of at least 25 nanomoles per litre (nmol/L), which is equivalent to 10 nanograms per millilitre (ng/mL). Optimal levels are often considered to be above 50 nmol/L (20 ng/mL), as this range is associated with better bone health, immune function and overall wellbeing. It is important to note that levels below 30 nmol/L (12 ng/mL) are considered deficient, and supplementation is often recommended for those who fall into this category.
Vitamin D exists in two primary forms: vitamin D2 and vitamin D3. Vitamin D3 (cholecalciferol) is generally preferred, as it is the same form produced by the skin and is more effective at raising and maintaining blood levels. Individual requirements can vary depending on age, body weight, lifestyle and sun exposure, which is why consistent intake over time is more important than occasional high doses.
Supplementation
Obtaining sufficient vitamin D from food alone is challenging, particularly in low-sun countries. While diet can contribute, it rarely provides enough to maintain optimal levels year-round.
Natural food sources of vitamin D include oily fish such as salmon, mackerel, sardines and herring, as well as egg yolks, liver and cod liver oil. In the UK, several foods are fortified to help improve intake, including breakfast cereals, plant-based milk alternatives, some dairy products and spreads. These fortified foods can be helpful but usually contain modest amounts.
For this reason, supplementation is widely considered the most reliable way to maintain healthy vitamin D levels, especially during winter months. Vitamin D is fat-soluble, meaning it is best absorbed when taken with meals that include healthy fats. Regular, moderate supplementation is generally preferred over irregular or excessive dosing.
While vitamin D supplementation is considered safe for most people, balance is important. Extremely high intakes over long periods can cause imbalances, so supplementation should follow recognised guidelines or be tailored with professional advice where appropriate.
Vitamin K2
In recent years, vitamin K2 has gained attention as a complementary nutrient to vitamin D. The relationship between the two lies in calcium metabolism. Vitamin D increases calcium absorption from the gut, while vitamin K2 helps direct that calcium into bones and teeth, where it supports structural strength.
Without sufficient vitamin K2, some researchers suggest calcium may be more likely to accumulate in soft tissues rather than being efficiently used by the skeleton. This has led to growing interest in combined vitamin D and K2 supplementation, particularly for individuals taking vitamin D long-term or at higher doses.
Vitamin K2 is found naturally in fermented foods such as natto, certain cheeses and some animal products, though intake is often low in modern diets. However, vitamin K supplementation is not suitable for everyone. Individuals taking blood-thinning medications must seek medical advice before supplementing, as vitamin K can interfere with these treatments.
Long-term benefits
Vitamin D deficiency is a significant and often underestimated issue in the UK and other low-sun countries. Limited sunlight, indoor lifestyles and dietary constraints make it difficult for many people to maintain healthy levels throughout the year. While outdoor activity and vitamin D, rich foods can help, supplementation is often the most practical and effective solution, particularly during the winter months.
Understanding what optimal vitamin D levels look like, and whether supporting nutrients such as vitamin K2 are appropriate, can play an important role in long-term health. As individual needs vary, personalised guidance from a qualified healthcare professional remains the best approach for safe and effective supplementation.
References:
NHS UK – Vitamin D overview and supplementation guidance
https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/
European Food Safety Authority (EFSA) – Dietary reference values for vitamin D
https://www.efsa.europa.eu/en/topics/topic/dietary-reference-values
