A vitamin D deficiency can be serious, so it is crucial to fix low levels of vitamin D before they turn into a deficiency. However, doing so can be tricky. “You tend to only get clear cut signs when you are severely lacking in vitamin D,” explains Shah. “But there might be general signs that your body isn’t working optimally because of low vitamin levels.”

These signs include:

Low immune system (being more susceptible to cold and flu)
Not recovering well from exercise
Struggling to sleep
Feeling tired

If you aren’t exposed to a lot of sunlight, you’re probably right to assume you’ll be low in vitamin D. “In the UK and in northern latitudes, especially during winter, we’re almost all insufficient,” says Shah.

She also says that while it’s important to wear suncream when we’re outside, it also blocks the absorption of sunlight and limits the amount of vitamin D we can make.

How to tell if you've got a vitamin D deficiency (and what to do about it)