In a sunny country, it’s easy to assume vitamin D takes care of itself. Step outside, feel the warmth, done. Yet plenty of people in bright places still run low, often without knowing it. Long days indoors, winter mornings, covered skin and the simple fact of getting older all play a part.

This week’s Supplement Spotlight is about the vitamin your skin makes from sunlight, and why it’s one where more is not better.

What vitamin D3 is

Vitamin D is a fat-soluble vitamin that behaves a little like a hormone. Its best-known job is helping your body absorb calcium and phosphorus from food, which keeps bones and teeth strong. It also plays a part in how your muscles and immune system work.

D3 is the form your skin makes, and the form found in animal foods. There’s also a plant and mushroom form, and research suggests D3 tends to raise levels in the body a little more effectively.

Because it’s fat-soluble, vitamin D is stored in the body rather than washed out each day. It builds up over time. That’s useful when you’re low, and it’s also the reason to treat supplements with respect.

Where it comes from naturally

Sunlight on bare skin is the main source. When certain rays reach your skin, it begins making vitamin D. How much depends on the season, the time of day, how much skin is uncovered, your age and your skin tone, since darker skin makes it more slowly. Glass blocks the rays that matter, so sitting in a sunny window doesn’t count.

Food helps a little. Oily fish, egg yolks and liver all carry some, and certain milks, cereals and spreads have it added. Mushrooms left in sunlight make the plant form. Even so, food alone rarely provides a great deal, which is why sunlight matters so much. For more on food sources, Fat-soluble vitamins: What are the best dietary sources of vitamins A, D, E and K? covers this family of vitamins.

What the research suggests

Bones and muscles. This is where the evidence is strongest. Getting enough vitamin D, together with calcium, matters for healthy bones, and very low levels can leave bones soft and weak. For people who are low, topping up clearly helps.

Falls and fractures in later life. Here the research is mixed. Some studies suggest a benefit when vitamin D is combined with calcium in older adults who were low to begin with. For people who already have enough, adding more doesn’t seem to help much.

Immunity and mood. Both have been studied a great deal, and the results are mixed. Some research suggests people who are low may benefit, but it’s hard to say the same for everyone. It’s too soon to think of vitamin D as a shield.

Who it may help

Older adults. Skin makes less vitamin D as we age, and many of us spend more time indoors.

People with darker skin. More melanin means skin needs more sunlight to make the same amount.

People who cover most of their skin, whether for faith, culture, work or comfort.

Anyone who spends most days indoors, including night-shift workers and people who are housebound or recovering.

Plant-based eaters. Few plant foods carry vitamin D. Most D3 comes from sheep’s wool, but vegan D3 made from lichen is available.

Using it simply

Take it with a meal that contains some fat, since that helps your body absorb it. Most products are a small daily amount; some are taken weekly. Either way, follow the label.

Time outdoors is still worth enjoying, in sensible amounts. A short walk in the morning or late afternoon light is lovely for its own sake. There’s no need to burn in the name of vitamin D.

A few myths, gently set aside

“More is better.” Because vitamin D builds up, extra doesn’t leave quickly. Over time, too much can raise calcium in the blood, which is something to avoid.

“Sunscreen will leave you deficient.” Research suggests everyday sunscreen use has little effect on vitamin D for most people. Protecting your skin still matters.

“A sunny window counts.” Glass filters out the rays your skin needs, so the warmth through a window is pleasant, but it doesn’t make vitamin D.

Looking after yourself

Keep to the dose on the label. With vitamin D, more really isn’t better, because it builds up in the body.

Watch for doubling up. Multivitamins, cod liver oil, calcium tablets and some fortified drinks can all contain vitamin D. Add them together before you add another.

Signs of too much can include unusual thirst, nausea, needing the toilet more often and feeling unusually tired. If you notice these, stop taking it.

If you live with a kidney condition, or you’re on regular treatment of any kind, keep to that treatment and let your existing care know before adding vitamin D.

Choose a reputable product with a clearly stated amount per dose.

Small and steady

Vitamin D is a quiet helper, not a fix for everything. A little daylight, a few good foods and, if you need it, a modest daily supplement taken as the label says. That’s the whole of it.

If anything feels off, set the bottle aside and please get it looked at. You deserve that gentleness.

For more in the same spirit, visit the Guides page.