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No, sunbathing is NOT a ‘great new way to lose weight’

by Sarah Williams | Analysis

24 October 2014

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The sun

Research published today about sun, weight and diabetes has made for some worrying, or even misleading, headlines.  While we all need some sun to make vitamin D, we also need to bear in mind the risk of skin cancer – especially for fairer skinned people and those who burn more easily.

This new research from an international team, and published in the journal Diabetes, investigated whether ultraviolet (UV) light could affect weight gain in mice.

In our opinion The Times (paywalled) got its headline about right: Sun can stop weight gain (if you’re a mouse)

The ‘mouse’ part is important.

But as well as being carried out in mice, there are other problems with this study too, and we’re not convinced by the way the findings have been interpreted.

Read on for our take on the study, its short-comings and our (unaltered) advice on enjoying the sun safely and the best way to keep a healthy weight.

The research

The study involved 4 groups of mice, fed different diets:

  • High-fat diet, supplemented with vitamin D
  • High-fat diet, no vitamin D
  • Low-fat diet, supplemented with vitamin D
  • Low-fat diet, no vitamin D

They exposed some of the mice in each group to a UV dose to see whether the UV affected how much weight the mice subsequently gained as they ate the different diets. The researchers also measured levels of molecules linked to diabetes, such as insulin levels (known as ‘markers’), in the mice’s blood.

The team’s main finding was that UV exposure led the mice fed a high-fat diet, with no vitamin D supplement to gain less weight and produce lower levels of the diabetes-markers compared to the same mice who didn’t receive UV.

On the face of it, this seemed like it might be good news. But it’s important to remember the UV-exposed mice only gained less weight – they didn’t actually lose any. So headlines about weight loss are completely inappropriate.

And as we’ve said, there are other problems with this study – particularly whether it even applies to people.

The mice

As we’ve noted – but we’ll stress again – the study was in mice.  Yes, the breeds they chose are the type most commonly used for human health research around the world – but as the researchers themselves note, mice are not ideal for studying the effects of sunlight since they are “nocturnal animals covered in fur and not usually exposed to much sunlight,”

We can’t be sure that responses we see in mice are what would happen in people. On top of that, this breed of mice respond very differently to UV light compared with humans – the males (which this study used) don’t produce vitamin D.

It’s also worth noting that the UV light they used was very different to sunlight. They used lamps with over half (65%) of the UV made up by UVB – the proportion is less than 5% of natural sunlight.

Crucially – the effect only showed up in mice who had no vitamin D

The mice who did get fed vitamin D gained the same amount of weight whether they were exposed to UV or not.

If sunlight only helps reduce weight gain if you’re vitamin D deficient, and UV light exposure causes our bodies to produce vitamin D….  we can’t actually see how this idea could ever work in practice – let alone the fact that it’s only in mice.

The importance of being balanced

In their press release, the researchers called for more balance in the way the harms and benefits of sunlight are discussed. We whole-heartedly agree that people need to know about both sides of the coin – and the world is already awash with information on weight loss that varies from poor to downright harmful– so we were disappointed at today’s coverage which generally failed to give people an accurate overall picture.

And ‘balance’ is also a word that crops up a lot in our health advice.

When it comes to sun, people need to find a balance between getting some sun for vitamin D while not putting themselves at risk of sunburn or skin cancer. That’s going to mean different things for different people – especially depending on how easily you burn and your skin tone. The best thing to do is to get to know your skin, and at times when you are at risk of burning use shade, clothing and sunscreen to help protect your skin.

At this time of year, in the UK, UV levels are very low so most of us should be fine with no protection. But if you’re going abroad skiing or for some winter sun, check out our SunSmart pages for more tips and information.

And the best ways to keep to a healthy weight are through a healthy balanced diet (there’s that word again) and regular activity. That means plenty of fibre, fruit and vegetables, eat less red and processed meat, salt and high calorie foods. And activity can be anything that gets you a bit warmer and out of breath. Outdoors or inside, just 10 minutes at a time can count. Make small changes that fit into your everyday life, that way they’re more likely to become a new healthy habit.



  • Geldenhuys, S et al Ultraviolet Radiation Suppresses Obesity and Symptoms of Metabolic Syndrome Independently of Vitamin D in Mice Fed a High-Fat Diet, Diabetes (2014) doi: 10.2337/db13-1675