Sunshine isn't slimming and can't halt diabetes
"Sunshine can make you thin," claims the Daily Mirror, while the Daily Express splashed on its front page that, "Sunlight is key to fighting diabetes". Both are strong contenders for the title of the day's most inaccurate health headline.
The news – reported more circumspectly by The Times and BBC News – is based on highly artificial laboratory experiments on mice.
The study found that long-term ultraviolet (UV) light exposure stopped male mice fed a high-fat diet gaining weight. UV also reduced glucose intolerance and insulin resistance and levels of insulin in the blood after fasting, as well as glucose and cholesterol.
In humans, these are signs associated with metabolic syndrome – a combination of diabetes, high blood pressure and obesity that puts you at greater risk of heart disease.
Human skin produces vitamin D when it is exposed to UV light, so the researchers tested whether the same benefits were seen if the mice were given a vitamin D supplement in their food.
But this did not produce the same effects. The researchers instead think nitric oxide, which is also produced when skin is exposed to UV light, may be responsible for the effects of UV.
Mice are nocturnal animals covered in fur, so their skin is not usually exposed to much sunlight. This means this research has no immediate implications for people.
Where did the story come from?
The study was carried out by researchers from the University of Western Australia, the University of Southampton and the University of Edinburgh.
It was funded by the BrightSpark Foundation and the Telethon Kids Institute.
The study was published in the peer-reviewed medical journal Diabetes.
The results of this study were well reported by BBC News and The Times, but the same cannot be said for the Mirror and Express.
An extremely kind critic might put the Mirror's claim that "Sunshine can make you thin" and the Express' claim that "Sunlight is key to fighting diabetes" down to wishful thinking and youthful high spirits.
Despite the headline, the Express did manage to rustle up comment from an independent expert from Diabetes UK, something the Mirror failed to do.
What kind of research was this?
This was an animal study. It looked at whether UV radiation and taking vitamin D affected the development of obesity and type 2 diabetes in mice eating a high-fat diet.
As the BBC reports, further research is required to see if sunshine has the same effect in people. Mice are nocturnal animals covered in fur, so their skin is not usually exposed to much sunlight.
What did the research involve?
The researchers fed 432 male mice either a low-fat diet containing added vitamin D, or a low-fat diet without added vitamin D, for four weeks.
This was done so the mice who had vitamin D supplementation would definitely have enough vitamin D and the mice on the standard diet would have a vitamin D deficiency.
The mice were continued on these diets, and some were switched from a diet low in fat to a diet high in fat. This meant there were four groups of mice:
- low-fat diet
- low-fat diet plus vitamin D
- high-fat diet
- high-fat diet plus vitamin D
In each group, mice further split into three groups, which either received no UV radiation or UV radiation at two different doses on a shaved patch on their backs.
One was a low dose that wasn't enough to make the skin go red and was given twice a week. The other dose was enough to make the skin go red and was given once a fortnight.
The mice were fed these diets and irradiated with these UV radiation doses for 12 weeks.
The researchers monitored:
- the mice's weight
- their glucose and insulin tolerance
- their blood levels of vitamin D, cholesterol, triglycerides (fats), glucose and insulin, and certain hormones (leptin and adiponectin) and signalling molecules
What were the basic results?
A high-fat diet significantly increased vitamin D levels in mice fed diets not specifically supplemented with vitamin D.
Mice fed either diet that was further supplemented with vitamin D had significantly higher vitamin D levels than those mice fed a diet that was not supplemented with vitamin D, although the effect of a high-fat diet and vitamin D supplementation was not additive.
UV exposure significantly increased vitamin D levels in mice fed the low-fat diet without vitamin D supplementation, but had no effect on vitamin D levels in mice fed the other diets.
Weight gain
Mice put on weight during the study. Long-term UV radiation at both doses significantly reduced weight gain in mice fed the high-fat diet without vitamin D supplementation.
Weight gain was similar between the mice that hadn't been irradiated and were fed the high-fat diet without supplementation, and mice fed the high-fat diet with vitamin D supplementation.
Similar results were seen for mice fed the low-fat diet.
Glucose intolerance and insulin resistance
Mice fed the high-fat diet developed glucose intolerance and insulin resistance. However, if mice were also given long-term UV irradiation, they showed reduced glucose intolerance and insulin resistance.
Glucose intolerance and insulin resistance were similar between un-irradiated mice fed the high-fat diet without supplementation and mice fed the high-fat diet with vitamin D supplementation.
Glucose intolerance was also significantly suppressed by long-term, low-dose UV radiation in mice fed the high-fat diet with added vitamin D.
Fasting glucose and insulin levels were also reduced by UV treatment in mice fed the high-fat diet without extra vitamin D.
Cholesterol
Levels of high-density lipoproteins, low-density lipoproteins and total cholesterol were also suppressed by the higher dose of UV irradiation in mice fed the high-fat diet that was not supplemented with vitamin D.
The researchers found vitamin D supplementation reduces the effect UV had on weight gain and markers of metabolic syndrome.
They did further experiments to determine how UV radiation might be having its effect. Their results suggest UV radiation of the skin causes the production of nitric oxide, and this might cause the effects seen.
How did the researchers interpret the results?
The researchers conclude that, "Long-term skin exposure to low-dose (suberythemal) and high-dose (erythemal) [UV radiation] suppresses the development of obesity and measures of [metabolic syndrome] in mice fed a high-fat diet. Vitamin D supplementation alone did not reproduce these effects.
"In addition, the suppressive effects of [UV radiation] on obesity and [metabolic syndrome] development were not observed to the same degree in mice that were further supplemented with vitamin D."
Conclusion
This study on mice has found long-term ultraviolet (UV) irradiation significantly suppressed weight gain and markers of metabolic syndrome, including glucose intolerance and insulin resistance, and blood levels of fasting insulin, glucose and cholesterol, in male mice fed a high-fat diet.
Many of the benefits of UV radiation were not reproduced by vitamin D supplementation alone. The researchers instead think that another chemical called nitric oxide, which is also produced when skin is exposed to UV light, may be responsible for the differences seen.
Mice are nocturnal animals covered in fur whose skin is not usually exposed to much sunlight. As the BBC reports, further research is required to see if sunshine has the same effect in people.
We can pretty confidently sat that the Express' front page splash saying, "Sunlight is key to fighting diabetes", and the Mirror's claim that "Sunshine can make you thin", are both nonsense.
