The short answer
- For bones and muscle when your level is low: works
- Vitamin D is needed to handle calcium. A shortage causes bone pain and soft bones, and topping up corrects it.
- For energy and mood: not proven
- There is no good evidence that extra vitamin D lifts energy or mood in women whose level is already fine.
- For weight: not proven
- Taking vitamin D supplements does not help with weight loss.
What it is
A vitamin your skin makes from summer sunlight, and that you get in smaller amounts from oily fish, eggs and fortified foods. It helps your body handle calcium and phosphate, which keeps bones, teeth and muscles working.
What the studies say
- From late March to the end of September most people in the UK can make what they need from sunlight. From October to March there is not enough sun for it.
- UK government advice is that everyone should consider a daily 10 microgram supplement during the autumn and winter.
- The US recommended amount is 15 micrograms, or 600 IU, a day for adults aged 19 to 70, and 20 micrograms, or 800 IU, from 71.
- A lack of vitamin D causes soft bones and bone pain in adults, a condition called osteomalacia.
- Studies do not show that vitamin D supplements help people lose weight.
How much people take in trials
The NHS says 10 micrograms, which is 400 IU, a day is enough for most people. Do not take more than 100 micrograms, or 4,000 IU, a day without medical supervision.
This is what the research used. It is not a dose for you.
What a label is allowed to say
UK law only lets a supplement say what is on the official list. For this one, that is:
- Vitamin D contributes to the maintenance of normal bones
- Vitamin D contributes to the maintenance of normal muscle function
- Vitamin D contributes to the normal function of the immune system
- Vitamin D contributes to normal absorption and use of calcium
There is no approved claim for energy, mood or weight.
In the US the rules are looser. A label may say how a nutrient supports the body if the company holds evidence for it. The FDA does not check the claim first, which is why US labels carry the line saying the FDA has not evaluated it.
Safety
- Too much over a long time raises calcium in the blood. That can weaken bones and harm the kidneys and heart.
- Very high blood levels can cause nausea, vomiting, confusion, thirst and kidney stones.
- High doses may make some statins work less well, and steroids can lower vitamin D levels.
- Orlistat, a weight loss medicine, reduces how much vitamin D you absorb.
Ask your doctor first if
- You take statins, steroids or orlistat.
- You have a kidney or parathyroid condition, or a history of kidney stones.
- You have been offered a high dose, or want a blood test before starting.
This is general information, not medical advice. Check with your doctor or pharmacist, especially if you take medication.
Sources
Checked against these sources on 2026-09-19.