'Everyone should be doing that' - Dr Amir Khan issues important supplement advice for winter

It's officially that time of year again. We were spoilt for sunshine in the UK this year, but the start of October means autumn and winter are on the way. It's time to swap shorts for workout leggings and get your winter supplements out again.

Dr Amir Khan, NHS GP and woman&home's own resident doctor, took to Instagram this week with a reminder: it's also now the time to start taking vitamin D. "I know it's really sunny today, but we don't get enough vitamin D from sunlight between October and March, so start taking your supplement," he says.

400 IU (or 10 micrograms) is the NHS-recommended dose, he explains.

"Everyone should be doing that. You might need a higher dose if you're deficient or insufficient, but generally that is the dose," he says.

"Some people should also consider taking it all year round, including people with darker skin, people who spend very little time outdoors, are housebound or live in a care home, or usually cover most of their skin when outside," he adds.

Vitamin D is one of the most important nutrients, yet research suggests over 20% of women are deficient in the UK. We get it from sunlight, and as we all know, this is very limited for half of the year.

Taking a supplement will "really help keep you healthy through the winter", says the doctor. It's important for healthy bones, teeth and muscles, and helps to regulate calcium and phosphate in the body. These minerals help the body work normally and maintain its energy levels.

In winter, a vitamin D supplement can help maintain the immune system, ward off colds and flu viruses by protecting the barriers around your lungs, gut, and skin, and reduce harmful inflammation.

Any brand of vitamin supplement will work, the doctor told ITV's Lorraine programme last year. It doesn't have to be the most expensive, but it is worth knowing the difference between vitamin D2 and D3 before buying.

However, to properly absorb vitamin D, the body needs other nutrients. The most important one is magnesium. The doctor previously told us this was one of the "most underrated" supplements for women.

"Magnesium activates vitamin D. It turns it from its inactive form to an active form so it can do all of its jobs," he says.

You don't have to add a magnesium supplement to the list to do this, though. Simply eating more magnesium-rich foods, such as dark, leafy green vegetables, nuts, seeds, and wholegrains, is enough for most people.

Vitamin K2 is also important. "One of the jobs of vitamin D is to absorb calcium and take it to the bones and teeth and make them nice and strong. What you don't want is calcium building up in arteries and organs. Vitamin K2 directs it away from them into your bones and teeth," he explains.

"But again, get it from foods rather than supplements, so things like eggs, cheese, liver, and fermented foods have vitamin K2 in them," he says. That way, you get all the benefits of those foods as well as the vitamins.