Do you have enough vitamin D, and should you take a supplement?
For healthy adults under 75, the evidence for supplementation is weak, but for people aged 75 and older, those with pre-diabetes, or those with an infant at home, there are good reasons to supplement.
Guidelines focused on bone health use a target blood level of 20 ng/mL. Guidelines that take a broader look at effects on the body aim for 30 ng/mL. Sunlight and food are generally not enough throughout the year to reach an optimal level. On top of that, the American dermatology association classifies ultraviolet radiation as a known cause of skin cancer, which makes deliberately seeking out the sun a less attractive strategy.
Whether you should supplement depends strongly on your situation. In healthy adults between the ages of 19 and 74, no clear effect of supplementation was found on the health outcomes studied. In people aged 75 and older, however, there is strong evidence: supplementation reduces the risk of death, although the reduction is very small. In people with pre-diabetes, supplementation slows the progression to type 2 diabetes, though that evidence is moderate. And for respiratory infections in children and young people up to the age of 18, there are signs of a protective effect, but the evidence is still limited.
Vitamin D3 is more effective than the other main form of vitamin D at raising your blood level, particularly when taken as a single large dose at one time. When taken daily, that difference is less clear. If you are supplementing daily, D3 is the better choice. Taking large irregular doses all at once is also inadvisable: compared with a low daily dose, it increases the risk of falls.
Calcium and vitamin D together reduce the overall fracture risk by 15% and the risk of a hip fracture by 30%. This applies to adults in general. For infants who are breastfed, the American academy of paediatrics recommends 400 IU of vitamin D per day from the first days of life, because breast milk contains too little vitamin D. Of children who developed rickets, 96% were breastfed.
Getting tested for a deficiency when you have no risk factors is not recommended by guidelines. There is no research showing that population-wide screening produces any benefit. If you do have risk factors, such as spending little time outdoors, having darker skin, or an increased risk of falls at an older age, it is worth discussing this with your doctor. Side effects from supplementation are rare and only occur with extremely high doses taken over many years.
This answer draws on a large international guideline from 2024 that brought together dozens of analyses, supplemented by a comparison of the two main forms of vitamin D, an analysis of calcium plus vitamin D and bone fractures, and an overview article on vitamin D deficiency. The researchers behind the 2024 guideline rated the majority of the analyses as low or very low quality. Only the finding on risk of death in people aged 75 and older was rated as high certainty. The analyses on fractures and the comparison of the two forms of vitamin D are of moderate certainty.
Which supplements are worthwhile after forty?
A few supplements have reasonably solid backing after forty: calcium plus vitamin D for bone protection, B vitamins and a daily multivitamin for cognitive maintenance, and adequate protein for muscle preservation. Collagen is worthwhile for skin.
Does taking vitamin E have any proven benefit?
For the most widely studied application, the heart, vitamin E demonstrably does not work; for skin and fertility the evidence is too thin to say it helps, so there is no clear reason to take it.
What does a NAD supplement do, and does it work in people?
NAD supplements demonstrably raise NAD levels in your blood, but whether you actually benefit from that remains uncertain. If you want to try them, keep your expectations realistic and choose a dose with the best available evidence behind it.
Can you take too much magnesium?
Yes, too much magnesium can be dangerous, although serious toxicity through ordinary supplements is rare in people with healthy kidneys. With kidney problems or the use of intravenous magnesium, medical supervision is genuinely necessary.
Can you get too much vitamin D?
Too much vitamin D is genuinely harmful, but that risk only arises at blood levels far above the recommended values. Stick to the dose on the packaging, and if you are unsure about your level, have it measured by your GP.
How much vitamin D should you take per day?
For healthy adults under 75, extra vitamin D is probably not necessary. For children, adults aged 75 and over, pregnant women, and people at high risk of diabetes, there are indications that supplementation helps, but discuss this with your doctor.