Scientists have held an all-you-can-eat buffet to test how a rare gene mutation affects people's appetites – and found that those who were born with this mutation were significantly more likely to crave fatty food, but less likely to eat sugary food.

While lots of us enjoy foods that are high in fat, the research shows that people with a mutation in their melanocortin–4 receptor (MCR4) gene seem to prefer it more than others – and tend to eat more high-fat foods, even when the fat content is totally hidden.

"Our work shows that even if you tightly control the appearance and taste of food, our brains can detect the nutrient content," says lead researcher Sadaf Farooqi from the University of Cambridge in the UK.

Farooqi and her team recruited 54 people to take part in their chicken korma buffet – a popular, creamy curry dish. Of this group, 20 participants were lean, 20 were obese, and 14 were obese and had a mutation in their MCR4 gene.

According to the researchers, about 1 in 100 obese people have this defect in the MC4R gene, which makes them more likely to put on weight. The defect disrupts the melanocortin–4 receptor, which means that satiety signals in the brain don't get processed properly.

In the experiment, the three groups of participants were each given a taste test of three different chicken korma preparations. The three curries were manipulated to look and taste the same, but were in fact low, medium, and high-fat versions of the same dish. In other words the fat content provided 20 percent, 40 percent, or 60 percent of calories respectively.

After sampling each curry, the participants were free to serve themselves and eat as much of any version of the dish as they liked.

While each group ended up eating approximately the same amount of food overall, the researchers found that the participants with the MC4R gene mutation ate almost twice as much of the high-fat korma as the lean participants (95 percent more), and 65 percent more than the obese group.

But although a defective MC4R gene seems to make us prefer fat more, it can also reduce our intake of another food type: sugar.

In a second experiment, the same three groups had to pick between three desserts that were identical in appearance. The dessert, called Eton mess, is made from strawberries, whipped cream, and meringue.

Participants were offered three versions of the sweet, each with varying sugar content: low (with sugar making up 8 percent of the energy content), medium (26 percent), and high (54 percent).

This time around, the lean and obese participants preferred the high-sugar dessert, but the MC4R group reported liking it less than the other two groups – and ended up eating less of all three desserts compared to the lean and obese participants.

While scientists already knew that a defect in MC4R made people more likely to be obese, the team says this is the first time that the mutation has been shown to specifically affect a person's appetite for carbohydrates and fat.

"Most of the time, we eat foods that are both high in fat and high in sugar," says Farooqi. "By carefully testing these nutrients separately in this study, and by testing a relatively rare group of people with the defective MC4R gene, we were able to show that specific brain pathways can modulate food preference."

While the MCR4 defect may seem like a fairly useless (and indeed detrimental) mutation for us to have evolved, the researchers think that – once upon a time, in our more primitive past – the drive to seek out and detect high-fat content could have been a useful survival mechanism.

"When there is not much food around, we need energy that can be stored and accessed when needed: fat delivers twice as many calories per gram as carbohydrates or protein and can be readily stored in our bodies," says Farooqi.

"As such, having a pathway that tells you to eat more fat at the expense of sugar, which we can only store to a limited extent in the body, would be a very useful way of defending against starvation."

We should bear in mind that this is a very small study looking at only a limited sample of participants – including those with a defective MC4R gene – which means we can't read too much into its findings just yet, or how they extend to the wider population.

But if the results can be replicated in broader research, it could help us understand more about the drivers behind obesity – and with reports that there are now more obese people than skinny people in the world, that's something we desperately need.

"This study is really nice in that it's shown in [a] carefully controlled scientific way that people who do have certain genetic variations are driven to eat fattier, more substantial energy-dense foods which tend to promote weight gain," nutritionist Amanda Salis from the University of Sydney in Australia, who wasn't part of the research, told Bianca Nogrady at the ABC.

"They constantly have this drive to eat and this study shows that it is a stronger drive to eat not everything, but specifically food that contains fat."

The findings are reported in Nature Communications.