Our obesity epidemic has been blamed on everything from inheriting a fat gene to a lack of available fruit and vegetables.

But now Scots have the perfect excuse for their expanding waistlines — it is all the fault of the dreary weather.

A team of scientists at Aberdeen University has found that Scotland’s dismal weather makes it more difficult for people to shed those extra pounds.

Their research suggests that overweight people tend to have lower levels of vitamin D in their blood than their slimmer counterparts.

The most natural source of vitamin D is from sunlight but, because the sun makes such a rare appearance in Scotland, fat people are less likely to benefit from this slimming aid.

But even when it is sunny, excess body fat absorbs and holds onto vitamin D, preventing it from entering into the bloodstream. The absence of enough vitamin D in the blood interferes with the functioning of a hormone called leptin, which signals to the brain when the stomach is full.

The research, published in the scientific journal, Bone, used questionnaires completed by 3,100 women living in northeast Scotland between 1998 and 2000. The data included estimates of how long the women had been exposed to sunlight over the previous year and the amount of vitamin D obtained from food sources such as eggs and oily fish. Researchers found that those with an average body mass index (BMI) of 34 produced 10% less vitamin D than people of average weight.

BMI is calculated by dividing a person’s height (in metres) squared by their weight. A BMI of 30 is a measure of clinical obesity, while 40 is a measure of morbid obesity.

The findings suggest that obese people in countries such as Scotland, where there is less sunlight, should venture outside more often for sustained periods to top up their vitamin D levels.

“Obese people had less vitamin D and the link between obesity and vitamin D deficiency was statistically significant,” said Dr Helen MacDonald, from Aberdeen University’s department of medicine and therapeutics, who led the study.

“It could be because people are not getting enough sunlight or that they do not go out much. There’s also the possibility that their vitamin D is locked into fat stores. We are carrying out more research.”

Tony Barnett, professor of medicine at Birmingham University, said: “I think it’s a link which needs further testing. There’s a lot of interest in vitamin D at the moment. We are still learning a lot about the relationship between it and human health.”

According to figures published by the World Health Organisation, in 2005, around 1.6 billion adults were overweight and 400m were obese.

In Scotland, more than a fifth of adults are obese. Obesity and associated illnesses cost the NHS about £170m a year.

But Michelle McManus, 28, the Glaswegian singer who won Pop Idol in 2003, is not convinced. “When I ballooned to 23 stone, it had nothing to do with lack of sunlight, it was because I went to the chippie too often,” she said.

“I’m not discrediting anyone’s research, I think it’s important that this work is carried out, but the obesity problem is because people eat too much junk food.

“Look at America. The sun is splitting trees over there but there’s still an awful lot of overweight people.”

Times Recommends

• Tower block death blaze ‘suspicious’
• Licence payers fund BBC chief's £8m pension
• Potter star falls victim to swine flu