Vitamin D: An Evidence-Based Guide

04/08/2026

Vitamin D: An Evidence-Based Guide

What Is Vitamin D?

Vitamin D is a fat-soluble nutrient that plays an essential role in many processes throughout the body. Although commonly referred to as a vitamin, it actually functions more like a hormone once activated.

Its primary role is helping the body absorb calcium and phosphorus, making it essential for healthy bones and teeth. It also contributes to normal muscle function and supports the immune system.

Unlike most vitamins, your body can produce vitamin D when your skin is exposed to ultraviolet B (UVB) rays from sunlight, which is why it is often called the "sunshine vitamin."

Benefits

Vitamin D contributes to:

  • Healthy bones and teeth
  • Calcium and phosphorus absorption
  • Normal muscle function
  • Normal immune function
  • Maintaining normal blood calcium levels

If you are deficient, restoring vitamin D levels can help return these functions to normal.

What Happens If You're Deficient?

Vitamin D deficiency often develops gradually, meaning many people have no obvious symptoms until levels become quite low.

Possible signs and symptoms include:

  • Frequent coughs, colds or infections
  • Fatigue
  • Muscle weakness
  • Muscle aches or bone pain
  • Back pain
  • Poor recovery from exercise
  • Increased risk of fractures over time

These symptoms are not unique to vitamin D deficiency and can have many different causes. The only reliable way to know your vitamin D status is through a blood test measuring 25-hydroxyvitamin D (25(OH)D).

Who Is Most at Risk?

You are more likely to have low vitamin D levels if you:

  • Live in countries with limited sunlight, such as the UK
  • Spend little time outdoors
  • Have darker skin
  • Are overweight or obese
  • Are an older adult
  • Have a condition that affects nutrient absorption

Where Do We Get Vitamin D?

Around 80–90% of vitamin D is produced when your skin is exposed to sunlight.

The remaining amount comes from food, although very few foods naturally contain significant amounts.

Good dietary sources include:

  • Oily fish (such as salmon, mackerel and sardines)
  • Egg yolks
  • Fortified dairy products (prefer plain, minimally processed options)
  • Some fortified breakfast cereals (many are ultra-processed and high in added sugar, so they shouldn't be relied upon as a healthy source of vitamin D)

For many people—particularly in the UK—diet alone is unlikely to provide enough vitamin D throughout the year.

Should You Take a Supplement?

Many people can maintain healthy vitamin D levels during the summer through sensible sun exposure.

However, during autumn and winter, sunlight in the UK is often insufficient to produce adequate amounts.

Supplementation may therefore be beneficial, particularly for people at greater risk of deficiency.

If you're unsure whether you need to supplement, a blood test is the most accurate way to assess your vitamin D status.

NCC Practical Tips

  • Choose Vitamin D3 (cholecalciferol) rather than Vitamin D2.
  • Take it with a meal containing some fat to improve absorption.
  • Ensure adequate magnesium intake, as magnesium helps activate vitamin D.
  • Don't rely on fortified breakfast cereals as your main source of vitamin D—many are ultra-processed and high in added sugar.
  • If supplementing long term or taking higher doses, consider periodic blood testing.

NCC Thoughts

Vitamin D is one of the few supplements I believe deserves serious attention, particularly if you live in the UK. It's inexpensive, well researched and deficiency is far more common than many people realise. Rather than guessing whether you need it, my advice is simple: get a blood test, understand your levels and supplement appropriately if needed. At No Comfort Culture, we believe your decisions should be guided by evidence—not marketing claims.