You might think of vitamin D as just another item on a routine blood test. Take a pill, check a box, move on. Yet many specialists now describe it as a hormone that quietly coordinates bones, muscles, mood, and even the gut.
For groups such as the Sociedad Española de Endocrinología y Nutrición and international endocrine experts, it makes more sense to treat vitamin D as a prohormone that the body activates and uses much like other steroid hormones.
A hormone hiding in plain sight
Specialists like Sara Marín remind patients that most vitamin D is not coming from a bottle at all. It starts in the skin when sunlight hits, then the liver and kidneys transform it into active forms that act on many tissues. Scientific reviews describe vitamin D as a prohormone, with D₂ and D₃ forms from food and supplements and D₃ also produced in skin under ultraviolet B light.
Because hormone D receptors are scattered across the body, its influence reaches the skeleton, pancreas, cardiovascular system, immune cells, and intestines.
That broad reach explains why Marín likes to compare vitamin D to a magnetic keycard. Once sunlight has activated it, that keycard can open four doors in her words, bone, muscle, mood, and immune plus gut function.
With age, that keycard tends to lose strength. Studies in older adults show that skin makes vitamin D less efficiently and that reduced outdoor time further lowers levels.
Four doors opened by sunlight
On the bone side, vitamin D helps the intestine absorb calcium and supports normal bone remodeling. Without enough, bones gradually become weaker, and fracture risk rises.
Guidance from the Office of Dietary Supplements at the National Institutes of Health notes that most adults between 19 and 70 years need about 600 international units of vitamin D each day, rising to 800 international units after age 70.
Muscles also respond. Research in older people links low vitamin D levels with poorer physical performance and slower walking, although not every study finds a simple cause and effect. For many readers, that translates to how steady you feel on the stairs or how fast you move across a crosswalk on a busy day.
The mood door is drawing more attention. Observational work has repeatedly found that low vitamin D status is associated with higher rates of depression and anxiety. A recent meta analysis suggests that vitamin D₃ supplements can reduce depressive symptoms in the short term, though experts stress that larger, longer trials are still needed.
Finally, the gut and immune system. Reviews show that vitamin D helps maintain intestinal barrier integrity, modulates the microbiome, and exerts anti-inflammatory effects that may be relevant in conditions such as inflammatory bowel disease. Marín also highlights magnesium as a necessary helper so the body can properly activate and use vitamin D.
Sun, food, and when supplements make sense
In practical terms, a short, daily exposure to midday sun can cover a large share of vitamin D needs for many people, while diet contributes the rest through foods such as fatty fish, egg yolks, and some mushrooms. That picture changes in winter, for people who work indoors behind glass, for those with darker skin, and for older adults, who often need testing and tailored advice.
At the same time, doctors are seeing more patients arrive with very high blood levels after months of unsupervised high dose drops or capsules.
Vitamin D is fat soluble, so the body stores it, and toxicity almost always comes from supplements rather than food or sunlight. When levels climb too high, calcium in the blood can rise, with symptoms such as nausea, vomiting, weakness, kidney injury, and heart rhythm problems.
To reduce that risk, the European Food Safety Authority recently confirmed a tolerable upper intake level of 100 micrograms per day, equal to 4,000 international units, for adolescents and adults, including during pregnancy and breastfeeding.
That ceiling is not a target for everyone, but rather a safety line for long term intake without close medical supervision.
At the end of the day, the message from endocrinologists is simple. Let sunlight and food do as much of the work as is safe for your skin, check your levels if you are in a risk group, and only use higher-dose supplements under professional guidance.
The scientific opinion was published in the EFSA Journal.
