Famously slender style guru Trinny Woodall was shocked to discover she has the so-called fat gene FTO, while her co-presenter Susannah Constantine does not.

The girls took a DNA test for their new ITV series The Great British Body, along with over hundred other volunteers as part of an investigation in to the gene. It is believed to increase a person’s propensity to put on weight if they are a carrier, and may increase their weight by up to half a stone compared to those who don’t have the gene.

Susannah said: “I was astonished, and over the moon. I’m not going to pretend I’m going to be magnanimous in victory…I know it’s down to genes and no one can help it [the result], but I’m quite elated! And meanwhile my fat friend here [to Trinny] I don’t know what you’re going to do?!”

Trinny: “I’m probably going to have my croissant and cappuccino!”

Later Trinny said: “I expected to be told I didn’t have the fat gene, so it was quite a surprise. But upon reflection I have realised that my father has a propensity to a bit of a tummy and it has probably come from his side of the family so it makes sense.”

Trinny adds: “I am a healthy eater, and feel it makes no difference to how I will maintain my figure from now on as I am happy with it already.”

While the girls’ results caused some jokes between the two of them, ‘the results from wider testing conducted for the series’ uncovered that a person with the gene is more likely to be overweight or obese.

In the first experiment of its kind, conducted in a mobile laboratory in approximately five to six hours the DNA test for the so-called fat gene FTO discovered that the majority of carriers of the gene had a BMI of over 25. The gene, the first related to a person’s weight to be discovered, is believed to increase a person’s propensity to gain weight, and can potentially add almost half a stone in weight.

Initially the experiment hoped to analyse the geographical spread of the gene across the UK with volunteers representative of counties across the country invited to take part. But the results showed a more surprising outcome relating to percentage body fat.

Each person can inherit no copies, one copy or two copies of the gene from their parents, and of those tested who had two copies, 64 per cent had a BMI of over 25, as did 54 per cent of people with one copy of the gene, compared with only 44 per cent of individuals with no copies of the gene.

Those with the fat gene also had a higher percentage of body fat, even if they had a healthy BMI. The people with no fat gene had on average 30.4 percent body fat, the people with one copy of the fat gene had 31.9 percent body fat, and those with two copies of the gene had an average 35.5 percent body fat – five percent more than those with no copies of the gene.

The study was carried out by population geneticist Dr Jim Wilson of Edinburgh University and EthnoAncestry. He was surprised by the affect the fat gene appears to have on a person’s body fat level, even if they are a healthy weight.

He says: “One of the main results when we looked at percentage body fat, and this is something you can’t see from the outside, people who had no copies of this gene on average had 30 percent body fat but people who had 2 copies of the so-called fat gene had over 35 percent of body fat on average. So it’s quite a bit difference, quite a strong effect that we’re seeing from this one gene.

“We could also see the effect of the gene on people with a BMI below 25, which is the healthy range. People in this group with two copies of the gene had on average seven percent more body fat than those with no copies of the gene.”

Of all the people tested 37 percent of people had no copies of the fat gene (TT), with 49 percent carrying one copy of the gene (AT) and 14 percentof people carrying two copies of the gene (AA).

Volunteers from counties across the country were invited to take part in the experiment at roadshows for the series in Gateshead, Brighton and Birmingham. The experiment was carried out in a mobile laboratory in approximately five hours, when the test usually takes at least two weeks two complete.

All those who took part were asked to complete a questionnaire about their diet and lifestyle, in order to analyse their results more accurately.

They were divided into three lifestyle groups – healthy diet and good exercise, average diet and some exercise, and bad diet and low levels of exercise. These risk factors are already well known to affect a person’s weight, body fat and health.

Alongside diet and lifestyle, the effects of a third risk factor – the gene – still need further study to determine how the gene actually behaves. But this study has shown that when comparing results from volunteers with no fat gene and lifestyle group one (no risk factors) against people with two copies of the fat gene and lifestyle groups two and three (all the risk factors), there is a visible difference of over a stone in weight.

Dr Wilson says: “Every way we looked at the results – percentage body fat, BMI, the numbers overweight – we saw the same pattern that those with two copies (AA) had more body fat, higher BMI or were more likely to be overweight than those with no copies (TT), while those with one copy (AT) tended to be in between.

“When we factored in diet and exercise things were even clearer – those with two copies and an unhealthy lifestyle tended to be the fattest.

“What is important is that within each group (TT, AT, AA) there was still a lot of diversity – there are fat people with no copies of this gene, and thin people with two copies – it’s all about a propensity to be fat, not a definitive prediction,” says Dr Wilson.

The results are an early indicator that there is potentially a link between genes and a person’s likelihood of becoming obese, as signs suggest that people with two copies of the FTO gene may have to work harder to maintain a healthy weight, especially as they age or if they are women.

Dr Wilson explains: “Sex is also an important factor, with women being on average 10% fatter than men, we also get fatter as we age. There will also be other genes involved, indeed one was found less than a month ago, so we still don’t have the full picture, so it was all the more interesting that the FTO gene’s effects could be clearly picked up.”

The experiment can be seen in action as part of The Great British Body which transmits on Monday 2nd, Tuesday 3rd and Wednesday 4th June at 9pm on ITV1.

The Great British Body transmits 2, 3, 4 June at 9pm on ITV1.