A lack of vitamin D can increase the risk of depression by up to 75 percent in older people, according to a new study that involved four years of follow-up research – findings that are worrying considering how many people don't get enough vitamin D later in life.

With late life depression having a knock-on effect on quality of life, how soon people go into residential care, and even the chance of an early death, boosting vitamin D levels could offer new opportunities for tackling these problems in older people.

Vitamin D supplements aren't difficult to come by and are relatively safe to take, so the researchers behind the study are calling for updated guidelines on recommended vitamin D intake – guidelines that could have a real impact on the older population.

"This is the largest representative and most comprehensive study of depression risk and vitamin D status in older adults ever conducted in Ireland," says one of the researchers, Robert Briggs from Trinity College Dublin in Ireland.

"Our findings will provide useful information to help inform public health policy – particularly regarding the proposition of the usefulness of vitamin D treatment/supplementation for depression."

The study covered 3,965 people aged over 50 taking part in the The Irish Longitudinal Study on Ageing (TILDA). After initial assessments, researchers checked back in on the participants after two years and after four years.

By the end of that four year period, 400 people had developed depression. The participants with a vitamin D deficiency showed a 75 percent higher risk of depression, even after accounting for factors like depressive symptoms, chronic diseases, level of physical activity, and cardiovascular disease.

Vitamin D levels have been linked to depression before, but the large sample and the four-year period used in this latest study makes it one of the most compelling pieces of evidence yet that the two are related.

While it's long been established that vitamin D is important for healthy bones, teeth and muscles, this link to our mental wellbeing is relatively new – and considering one in four older people in Ireland are estimated to be not getting enough vitamin D in the winter months, the problem is not a minor one.

The researchers think that vitamin D may be able to protect the brain against some of the wear and tear associated with ageing, but we'll need further studies to find out for sure if that's the case.

Should vitamin D supplements be given out on a regular basis or even added to food to try and stave off this very real risk of depression? It's something that could be on the table, according to the researchers behind the study.

"Given that vitamin D is safe in the recommended intakes and is relatively cheap, this study adds to the growing evidence on the benefits of vitamin D for health," says one of the team, Eamon Laird from Trinity College Dublin.

That said, research over the years has also put a dent in the shiny reputation of vitamin D supplements, with some doctors saying there's no consensus over what deficiency even is, and that it's certainly possible to take too much of it.

So, it's probably best to just speak to your doctor about all this; but these results do show it might be a good idea to make sure older people get enough sunshine (but not too much) and at least receive enough vitamin D from their diet.

The research has been published in the Journal of Post-Acute and Long-Term Care Medicine.