Can sunshine reduce the risk of diabetes and heart disease?

Having a vitamin D deficiency can cause inflammation, which could in turn contribute to heart and circulatory disease, according to a study published in the International Journal of Epidemiology.

We already know from previous studies that low vitamin D levels are linked with high blood levels of C-reactive protein (CRP), a sign of inflammation. Researchers wanted to find out whether low levels of vitamin D caused high levels of CRP, or whether high levels of CRP caused low levels of vitamin D.

Using large-scale genetic analysis, the researchers were able to show that low levels of vitamin D are likely to cause high levels of CRP, not the other way round. However, vitamin D levels only make a difference to CRP when they are low enough to be classed as vitamin D deficiency.

To show which way round the effect happens, they used a statistical method (called bidirectional Mendelian randomisation), that involves comparing people in different groups who have similar gene variations, which is useful for understanding which way round an association happens.

The study included data from just under 300,000 people aged 37-73, from the UK Biobank, a biomedical database of participants from across the UK which used for long-term research. Blood samples were used to measure the levels of vitamin D and CRP in their blood. 

The BHF verdict

This study supports current NHS recommendations on the importance of getting enough vitamin D. While we can get some vitamin D from the food we eat, most of our vitamin D is made through our skin when we are outside in daylight.

From April to September in the UK, sunlight and a healthy balanced diet is enough to meet most people’s needs. But from October to March sunlight levels in the UK aren’t as strong and it’s hard to meet our full requirement through diet alone. This means a supplement of 10mcg a day is a helpful way to make sure we get what we need during this half of the year.

You should take a vitamin D supplement all year round if any of these apply to you:

• you have dark skin (for example, if you have an African, African-Caribbean, or south Asian background)
• you don’t spend much time outdoors
• you cover most of your skin when outdoors
• you are a child aged one to four.

How good was the research?

A strength of this study is that they used Mendelian randomisation, an analysing method that can help researchers account for other factors, including unknown factors, that could affect the results.

The study included a large number of people, and so included a statistically significant number of people with low levels of vitamin D, whereas past studies hadn’t included enough people with low levels of vitamin D to show a casual effect.

A major weakness of this study is that it only included people from a White- British background, when we know that people with darker skin (for example, if you have an African, African-Caribbean or south Asian background), are more likely to have a vitamin D deficiency.

How good was the media coverage?

While the Daily Mail article mostly accurately reflected what the study said, the headline (Sunshine really IS good for you: Getting the daily recommended dose of vitamin D from natural light can reduce the risk of diabetes and heart disease) used was misleading. While sunshine can be one way our bodies get vitamin D, this study did not look at the effects of different sources of vitamin D.

We know that at some times of year, sunlight levels in the UK aren’t enough to meet our vitamin D requirements and we should consider taking vitamin D supplements, while for some people, vitamin D supplements are recommended throughout the year. The Mail article doesn’t include any suggestion that we might need to take supplements. In fact, it says: “The easiest and most commonly recommended way to get more vitamin D is to just spend more time outdoors. While the amount of sunlight needed can very greatly depending on a person's age and race, sun-derived vitamin D is believed to be more durable than versions from food and supplements.”
However, with sunshine levels too low in the UK for half of the year to be sufficient, we need to rely on other sources as well as this.

Also, the Mail says: “A person who is suffering from a condition tied to the inflammation should make sure to get more vitamin D.” Actually, the study shows that if you are already getting enough vitamin D, getting more vitamin D won't make a difference to inflammation levels. We also know that taking too much vitamin D (at least in supplement form) can be harmful. 

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