Most healthy adults who go outdoors regularly and eat a varied diet do not need a vitamin D supplement. It makes sense to take one when real sun exposure is low, when there is a condition making fat absorption difficult, or when a blood test has shown a deficiency. Outside those scenarios the supplement adds little and, wrongly dosed, can do harm.
It’s worth understanding why. Vitamin D is not exactly a vitamin: it behaves as the precursor of a steroid hormone. It is made in the skin on receiving ultraviolet B radiation, passes through the liver and then the kidney, and its active form regulates how much calcium you absorb in the intestine. Without enough of it, bone does not mineralise properly — rickets in childhood, osteomalacia in adults.
How much you need a day
The reference amounts were set assuming minimal sun exposure, that is, the worst case. They are expressed in micrograms (mcg) and international units (IU): 1 mcg equals 40 IU.
| Age | Recommended daily amount |
|---|---|
| 1-13 years | 15 mcg (600 IU) |
| 14-18 years | 15 mcg (600 IU) |
| 19-50 years | 15 mcg (600 IU) |
| 51-70 years | 15 mcg (600 IU) |
| Over 70 years | 20 mcg (800 IU) |
| Pregnancy and breastfeeding | 15 mcg (600 IU) |
Source: the US National Institutes of Health (NIH) review of the Food and Nutrition Board reference intakes.
Why food rarely gets you there
Very few foods contain vitamin D naturally, and those that do aren’t usually eaten daily:
- Cod liver oil: about 34 mcg (1,360 IU) per tablespoon
- Cooked salmon, about 85 g: 14.2 mcg (570 IU)
- White mushrooms treated with ultraviolet light, half a cup: 9.2 mcg (366 IU)
- Fortified milk, one cup: about 2.9 mcg (120 IU)
- Fortified plant drinks, one cup: 2.5-3.6 mcg (100-144 IU)
Egg yolk, beef liver and cheese provide small amounts. In countries where dairy is not systematically fortified, covering 600 IU from diet alone is difficult unless you eat oily fish very frequently.
Sun makes up that difference, but with conditions. UVB radiation does not pass through glass, so sitting by the window does nothing. More pigmented skin produces less vitamin D from the same exposure, and the ability to synthesise it declines with age. At the same time, ultraviolet radiation is a carcinogen and the most avoidable cause of skin cancer: the advice to protect yourself is not suspended in order to make vitamin D.
Who most often falls short
The groups most likely to have insufficient vitamin D status are fairly recognisable:
- Breastfed infants, whose supplementation is always set by the paediatrician
- Older people, through reduced skin synthesis and less time outdoors
- People with little sun exposure: indoor work, reduced mobility, clothing covering almost all the skin
- People with dark skin living at latitudes with low radiation
- People with fat malabsorption: coeliac disease, Crohn’s disease, ulcerative colitis, cystic fibrosis, some liver diseases
- People with obesity or previous bariatric surgery
If you are in none of those groups, the probability that a supplement will change anything for you is low.
How to take it so it’s absorbed
Vitamin D is fat-soluble: its absorption depends on the intestine absorbing fat. Taking the supplement with a meal containing some fat increases absorption. Capsules are swallowed whole, with water, without chewing.
Cholecalciferol (D3) and ergocalciferol (D2) are activated by the same metabolic route and, according to the data sheet authorised in Spain, have the same qualitative and quantitative effects. D3 is stored and eliminated slowly: the apparent half-life of serum 25(OH)D3 is around 50 days, which is why an excess takes weeks to correct.
The upper limit, and why it genuinely matters
The tolerable upper intake level for adults is 100 mcg (4,000 IU) a day. Above that, the margin narrows. The toxicity threshold in adults with normal parathyroid function sits between 40,000 and 100,000 IU daily for one or two months; infants and small children react to far lower concentrations.
This is not theoretical. In 2019 the AEMPS published a safety notice over serious cases of hypercalcaemia from overdosing. There were two patterns: newborns and infants given doses far above those recommended — in some cases with an adult presentation, not authorised in paediatrics — and adults who took daily a calcifediol that should have been given once a month.
Three practical rules follow:
- Read on the pack whether the regimen is daily, weekly or monthly, and confirm it before starting.
- Don’t use adult presentations in children. The concentrations are not comparable.
- If you already take a multivitamin, count the vitamin D it contains. Stacking products without realising is the most common way to overshoot.
Symptoms of hypercalcaemia from excess include tiredness, headache, muscle and joint pain, nausea, vomiting, loss of appetite, weight loss, intense thirst and increased urination.
Medicines that interfere
| Medicine | What happens |
|---|---|
| Orlistat | Reduces absorption; separate by at least 2 hours |
| Colestyramine, liquid paraffin | Reduce intestinal absorption |
| Prolonged systemic corticosteroids | Inhibit calcium absorption and counteract the effect |
| Phenytoin, barbiturates, rifampicin, isoniazid | Reduce the effectiveness of vitamin D |
| Thiazide diuretics | Less calcium elimination: more risk of hypercalcaemia |
| Digoxin and cardiac glycosides | Vitamin D may increase the risk of arrhythmia |
| Magnesium-containing antacids | Risk of hypermagnesaemia |
| Statins | May compete with vitamin D for the same metabolic route |
Vitamin D is contraindicated where there is hypercalcaemia, hypercalciuria, calcium kidney stones, nephrocalcinosis or severe renal failure.
Should it be measured in a blood test?
The indicator is serum 25-hydroxyvitamin D. Values below 30 nmol/l (12 ng/ml) indicate a risk of deficiency; between 30 and 50 nmol/l there may be insufficiency; 50 nmol/l (20 ng/ml) or more is sufficient for most people. Above 125 nmol/l (50 ng/ml) adverse effects start to be described.
That said, there is no consensus on the optimal level, and the Endocrine Society does not recommend routinely measuring 25(OH)D in healthy people. Requesting the test “just in case” generates more unnecessary treatment than solved problems.
When to seek advice
- You are taking more than 4,000 IU daily without a professional having told you to
- Intense thirst, urinating far more than normal, persistent nausea, confusion or marked weakness while taking vitamin D: these can be signs of hypercalcaemia
- Diffuse bone pain, muscle weakness in the thighs and hips, or difficulty getting up from a chair
- You have kidney disease, sarcoidosis, hyperparathyroidism or a history of kidney stones and a supplement has been suggested to you
- You take digoxin, thiazide diuretics or corticosteroids long term
- It concerns an infant or a child: the regimen is always set by the paediatrician
- You are unsure whether your presentation is for daily, weekly or monthly administration
This article is general information and does not replace assessment by a healthcare professional. Sources: Vitamin D — NIH Office of Dietary Supplements, AEMPS: Vitamin D, serious cases of hypercalcaemia from overdosing (MUH FV 2/2019) and cholecalciferol technical data sheet at CIMA (AEMPS).