The importance of Vitamin D Supplementation
- Kate Organ
- Sep 10
- 3 min read
Why it matters.
Vitamin D helps your body absorb calcium and phosphate—the building blocks for strong bones and working muscles. When levels are low, people can get bone or muscle aches, cramps, and weakness. Low vitamin D has also been linked with lower mood and poorer sleep quality in research studies, so correcting a low level can be one helpful part of a wider plan for feeling and sleeping better.
Where vitamin D comes from
Sunlight (the main source in the UK). From late March/early April to the end of September, UVB light on bare skin helps your body make vitamin D. In winter, UK sunlight isn’t strong enough, so most people rely on diet and/or supplements.
Food. Oily fish (salmon, mackerel, sardines), eggs and fortified foods (some milks, spreads and cereals) contain vitamin D—but diet alone rarely meets daily needs.
Supplements. These are common and safe when used correctly.
Vitamin D3 (cholecalciferol) usually raises blood levels better than D2 (ergocalciferol).
Vegan D3 (from lichen) is available.
Checking your level
A simple blood test (25-hydroxyvitamin D) shows your status.
Deficient: below 25 nmol/L
Insufficient: below 50 nmol/L
Many clinicians aim for a level around 75–100 nmol/L for “good” status; your own target may vary based on your health history.
Too much vitamin D can be harmful, so avoid high-dose supplements unless you’ve had a test and personal advice.
Mood, sleep and energy: what’s the evidence?
Mood. Several high-quality reviews show a small average improvement in depressive symptoms with vitamin D supplements, especially in people who start out deficient. It’s best used alongside other supports (sleep, activity, therapy/medication when appropriate).
Sleep. Some studies suggest better sleep quality after correcting low vitamin D, but findings are mixed. Keep up good sleep habits too.
Energy/fatigue. Many people with deficiency feel better after repleting their levels, likely because muscles work better and overall health improves. If tiredness continues, ask about other causes.
Vitamin D Supplements Guidance
In the UK, public health advice is:
Adults and children ≥4 years: take 10 µg (400 IU) daily in autumn and winter.
Consider taking 10 µg (400 IU) all year round if you:
have darker skin
cover most of your skin when outdoors
rarely go outside
are pregnant or breastfeeding
live in a care home
If you’re deficient
Your clinician may prescribe a short “loading” course (higher dose for a few weeks) followed by a maintenance dose (often 10–25 µg / 400–1,000 IU daily, sometimes higher) to keep levels steady. Avoid self-prescribing high doses for long periods.
Safety and interactions
Vitamin D toxicity is rare and usually comes from very high doses for a long time. Too much can raise blood calcium (hypercalcaemia), causing nausea, thirst/peeing a lot, confusion, heart rhythm problems and kidney issues.
Do not exceed 100 µg (4,000 IU) daily long-term unless prescribed and monitored.
Speak to a pharmacist or clinician before taking vitamin D if you have kidney disease or stones, parathyroid problems, sarcoidosis, or if you take digoxin, thiazide diuretics, anticonvulsants, or orlistat, as you may need tailored advice.
Quick tips
Short, regular daylight exposure in spring/summer helps
Take supplements
Re-check blood levels if you’ve been deficient or are on higher doses.
Summary
Vitamin D supports bones, muscles, mood and possibly sleep. A simple supplement of 10 µg (400 IU) daily in autumn/winter (year-round for some people) is safe and recommended in the UK. If you think you’re low—or plan to take higher doses—get a blood test and personal advice.
