This is why sitting in the sun indoors won't boost your vitamin D, according to experts

On a sunny morning, sitting by the window with a cup of coffee can feel like the perfect way to enjoy the sunshine without stepping outside. You can feel the warmth on your face, and the room fills with light.

But there's a catch: sunlight coming through an ordinary window won't help your skin produce vitamin D. And as autumn turns into winter, getting enough of this important vitamin can become more difficult, particularly for people who spend much of their time indoors.

Why sunlight through a window doesn't count

Your skin produces vitamin D when it is exposed to a particular type of ultraviolet light called UVB. Ordinary window glass blocks these rays, so sitting in a sunny living room or car won't provide the exposure your skin needs to make vitamin D.

The US National Institutes of Health confirms that the skin cannot make vitamin D from sunlight passing through a window.

There's another detail worth knowing. Glass can let some UVA rays through, and these contribute to premature skin ageing. The American Academy of Dermatology explains that UVA rays can pass through window glass, while UVB rays are blocked.

So while your sunny window seat might feel lovely, it isn't a substitute for getting vitamin D from appropriate sunlight exposure, food or supplements.

Why October makes a difference

The changing seasons matter, too. In the UK, the sun is too low in the sky for the skin to make enough vitamin D from sunlight between October and early March. Similar seasonal limitations affect the Netherlands.

That's why the NHS recommends that adults and children over four consider taking a daily supplement containing 10 micrograms (400 IU) of vitamin D during autumn and winter. People who get very little sunlight, or who cover most of their skin outdoors, may need a supplement throughout the year.

Advice differs in the United States. The NIH recommends a daily vitamin D intake of 600 IU for adults aged 19–70 and 800 IU for adults over 70, from food and supplements combined. These are general intake recommendations, not a specific winter-only supplement rule.

What can you do to support your vitamin D levels?

You don't need to give up your favourite sunny spot. Just remember that warmth through glass doesn't tell you whether you're getting enough vitamin D.

A few simple steps can help:

• Check the guidance for your country. Supplement recommendations vary by age and individual circumstances.
• Include vitamin D-rich foods. Oily fish, egg yolks and fortified foods can contribute to your intake.
• Get outdoors when you can. A walk is still valuable for movement and general wellbeing, even when winter sunlight is too weak for meaningful vitamin D production.
• Don't take extra supplements just in case. More isn't always better. Check the dose on the label and ask a pharmacist or doctor if you're unsure.

Vitamin D helps your body absorb calcium, which is essential for healthy bones. Looking after your vitamin D intake is one part of maintaining bone health as you get older.

With World Osteoporosis Day falling on 20 October, it's a timely reminder to pay attention to your bones. Enjoy the sunshine streaming through your window, by all means just don't rely on it to supply your vitamin D.