Vitamin D deficiency is probably more common than you think.

According to the National Institute for Healthcare and Excellence, around one in five adults and around one in six children may have low vitamin D status in the UK.

So how do you know if you are deficient in the nutrient?

“The only real way to know for sure is to have a blood test,” says Shona Wilkinson, a nutritionist at SuperfoodUK. “This will give you a measure of your Vitamin D levels.”

It’s Vitamin D Awareness Week, and experts have set out all the things you need to look out for:

1. Aching bones

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Vitamin D is essential for supporting bones and their structure. The nutrient helps regulate calcium and phosphorus – the minerals essential for maintaining strong bones – within the body.

When you’re low on vitamin D, your bones can weaken, increasing your risk of stress fractures.

“Perhaps the most crucial role that vitamin D plays within the body is to aid in absorbing and retaining calcium which is vital for bone density and strength,” says Dr Alexandra Phelan, GP and member of the Pharmacy2U Online Doctor Service team.

“Too little (and indeed too much) vitamin D can lead to weakness of the bones and the development of conditions such as osteoporosis (brittle bones) and rickets (soft bones).”

2. Weakness in muscles

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As well as bones, Vitamin D supports normal muscle function.

“It is also thought that Vitamin D deficiency could be linked to muscle pain,” says Wilkinson.

A study published in the British Medical Journal states: “Poor muscle strength and weakness may be associated with vitamin D deficiency, which is common among elderly people because the capacity of the skin to synthesise the provitamin calcidiol (25-hydroxycholecalciferol) decreases with age.”

3. Lack of sleep

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“A recent study from America has identified a possible link between low levels of vitamin D and poor sleep,” says Phelan.

“In the study, based on a group of older men, those with lower levels of this vital vitamin appeared to report not only less sleep but also poor quality sleep.”

[Read more: The vitamins your body needs to help you sleep better]

However, Phelan points out the study has only been conducted on a small group in America and needs further research.

“This research does seem to have some backing,” she adds. “Patients with hypoparathyroidism (people who have low levels of Vitamin D in their body as a result of a problem with the parathyroid glands) do feel fatigued, even after a good night’s sleep.”

4. Low mood or depression

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“Vitamin D receptors can be found on a number of cells located in the regions of the brain that are linked with depression,” says Geeta Sidhu-Robb, CEO and founder of Nosh Detox.

“A 2008 study in Norway found that people with low levels of vitamin D were more likely to be depressed. And taking vitamin D improved the symptoms.

[Read more: High fish intake linked to reduced risk of depression]

“Vitamin D is most effectively absorbed from exposure to sunlight, so during the darker months of the year, the depressive symptoms of SAD (Seasonal Affective Disorder) begin to emerge.

“These lower levels of vitamin D are connected with affecting the serotonin concentration in the brain.”

5. Weight gain

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“Studies have shown that vitamin D isn’t beneficial in weight loss, unless it is given to those who are very low calcium consumers,” says Phelan.

“However, a vitamin D deficiency has been linked to weight gain, so even though taking more won’t help you lose weight, getting less vitamin D than you need could make you gain.”

A five-year study by the Centre for Health Research in Portland, Oregon found that women with low vitamin D status may be prone to weight gain.

After examining the vitamin D status and weight of more than 4,000 women over the age of 65, researchers found those with insufficient levels weighed more than the women with adequate vitamin D levels.

6. Constantly coming down with coughs and colds

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Suffering from a never-ending and vicious cycle of cold? There is a chance lack of Vitamin D may have something to do with it.

“Vitamin D receptors are found all over the body, including the immune cells,” says nutritionist Sarah West.

[Read more: Losing hair? 5 key vitamins for thick, healthy locks]

“Research has suggested that vitamin D is crucial to activating our immune defences, and that without sufficient intake of the vitamin our immune cells are unable to react appropriately.

“A 2010 study supports this, demonstrating that flu incidence decreased when schoolchildren were given Vitamin D3 supplements through the winter.”

7. Trouble with your gut

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In its milder form, a vitamin D deficiency can both cause and be caused by gastrointestinal problems. The nutrient is fat soluble, which means having a gastrointestinal condition that affects your ability to absorb fat also affects your absorption of vitamin D.

Recent research, conducted by the University of Sheffield, suggests a large proportion of patients suffering from irritable bowel syndrome (IBS) are vitamin D deficient.

Lead scientist Dr Bernard Corfe states in his study: “Our data provide a potential new insight into the condition and importantly a new way to try to manage it.

“It was clear from our findings that many people with IBS should have their vitamin D levels tested, and the data suggests that they may benefit from supplementation with vitamin D.”

8. Feeling tired all the time

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“This is a common sign of mild deficiency that can often go unnoticed,” says Zoe Martin, nutritionist at Discount Supplements. “Fatigue is a tricky one to spot, since it can be attributed to so many causes.

“First, it’s important to establish whether there are probable reasons for feeling lacklustre, such as poor sleep patterns, stress and so on. Otherwise, this could be something to take note of – especially if it’s coupled with a feeling of general malaise.”

[Read more: 5 vitamins and minerals you need more of as you age]

A study published in the North American Journal of Medical Sciences found that vitamin D deficiency was common among patients suffering from fatigue. They found that taking vitamin D helped improve their symptoms.

The authors of the journal wrote: “Prevalence of low vitamin D was 77.2% in patients who presented with fatigue. After normalisation of vitamin D levels fatigue symptom scores improved significantly in all five subscale categories of fatigue assessment questionnaires.”

What you can do?

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Experts say only 10% of our daily vitamin D intake comes from our diet, while 90% is derived from sunlight. Guidelines from the Scientific Advisory Committee on Nutrition suggest we should all be getting about 10 micrograms (400 IU) of the nutrient per day.

Dr Sally Norton, NHS weight loss consultant surgeon and founder of VavistaLife, says: “We can get vitamin D from foods such as egg yolk, oily fish and red meat, wild mushrooms and vitamin D fortified foods (some margarines and breakfast cereals for example).”

But given the lack of sun in the UK (especially in the winter) and our need to use sunscreen to protect our skin from the sun’s ultraviolet rays (which can reduce vitamin D synthesis), experts recommend trying out supplements.

As Wilkinson explains: “It can be helpful for both adults and children to take a vitamin D supplement over winter – or year-round for those who don’t have much sun exposure. Ideally, you should ask your doctor for a blood test for vitamin D levels to determine whether you need a supplement.”

For more information, check out the NHS’s guidelines on vitamin D.