Vitamin D is measured with a blood test, and your result is usually reported as one of a few bands: deficient, insufficient, or sufficient. The exact cut-offs matter, since they determine whether you need treatment, a lifestyle change, or nothing at all — so it's worth getting this from a source you trust rather than relying on the first number you see online.
Vitamin D is technically a group of five related fat-soluble compounds, of which D2 and D3 matter most for human health. It's essential for calcium and phosphate balance, bone health, muscle and nerve function, and immune support. Unlike other vitamins, it's the only one your body can produce itself, made in the skin following sunlight exposure. In this article, we'll cover what counts as a healthy level, the risks of running low, and how to check where you stand.
What are healthy vitamin D levels?
[Placeholder — pending confirmation from our medical reviewer] UK blood tests report vitamin D in nmol/L, and the exact bands for deficient, insufficient, and sufficient levels need confirming against current NHS/SACN guidance before this section is published, to make sure the figures shown here are accurate and consistent with the rest of our content.
What are the risks of low vitamin D?
The most well-established risk is to bone health. Low vitamin D reduces how much calcium your body absorbs from food — in some cases, from around 70% down to as little as 15% — which is why bone softening is the primary concern with deficiency. In children, this can cause rickets; in adults, it can contribute to osteoporosis (weaker, more fracture-prone bones) and osteomalacia (bone thinning combined with muscle weakness), the latter particularly affecting women who have had several children.
Muscle weakness, pain, or tremors are also common with low vitamin D. Research has additionally linked low levels to a higher risk of:
- Dental problems, including periodontitis
- Certain cancers
- High blood pressure
- Autoimmune conditions, including type 1 diabetes, inflammatory bowel disease, and psoriasis
- Pre-eclampsia, a serious pregnancy complication involving high blood pressure
These associations don't necessarily mean low vitamin D directly causes each of these conditions, but they're part of why maintaining a healthy level matters beyond bone health alone.
How do you maintain healthy vitamin D levels?
Sunlight is the biggest source of vitamin D for most people. Dietary sources are more limited, but include oily fish (herring, mackerel, salmon, sardines), egg yolks, red meat, and liver, alongside fortified foods such as some breakfast cereals. For a fuller breakdown of food sources and supplement guidance, see our guide to improving your vitamin D levels.
How do you check your levels?
The only way to know for certain is a blood test measuring 25-hydroxyvitamin D. You can ask your GP, who may refer you to a nutritionist, or use an at-home Vitamin D test to get a result without waiting for an appointment. If your levels come back low, you'll be able to adjust your diet, sun exposure, or supplement routine accordingly, and retest to confirm progress.
FAQ
Can you have healthy vitamin D levels without a blood test?
Not reliably. Many people with low vitamin D have no noticeable symptoms, so a blood test is the only way to know your actual level rather than guessing.
How often should you check your vitamin D levels?
This depends on your individual risk factors and whether you've previously been treated for a deficiency. If you've started or adjusted a supplement, a follow-up test around 3 months later is a common approach to confirm progress.
Are vitamin D levels the same all year round?
No — levels typically dip over autumn and winter in the UK due to reduced sunlight, which is why a daily supplement is recommended during these months regardless of your levels in summer.
If you're unsure where your levels stand, our at-home Vitamin D test can give you a clear answer. For the full picture on symptoms and who's most at risk, see our guide to vitamin D deficiency: signs, symptoms and what to do.
