The study found that obese people with a defective MC4R gene ate almost double the amount of high fat food than lean individuals and 65% more than obese people.
Professor Sadaf Farooqi from the Wellcome Trust–Medical Research Council Institute of Metabolic Science at the University of Cambridge, who led the research team, says: “Our work shows that even if you tightly control the appearance and taste of food, our brains can detect the nutrient content. Most of the time we eat foods that are both high in fat and high in sugar.
“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.”
Researchers gave participants an all-you-can-eat buffet of chicken korma with three options manipulated to look and taste the same, but in which the fat content provided 20% (low), 40% (medium) and 60% (high) of the calories.
After taking a small taster of each meal, people were allowed to eat freely from the three kormas. They could not tell the difference between the foods and were unaware that the fat content varied. The researchers found that, although there was no overall difference in the amount of food eaten between the groups, individuals with defective MC4R ate almost double the amount of high fat korma than lean individuals ate (95% more) and 65% more than obese individuals.
In a second arm of the study, people were given Eton mess, a dessert that includes a mixture of strawberries, whipped cream and broken meringue. Again, there were three options from which participants could freely choose, with sugar content providing 8% (low), 26% (medium) and 54% (high) of calorific content, but with the fat content fixed. Participants could choose freely which ones to eat.
Lean and obese individuals said they liked the high sugar Eton mess more than the other two desserts. However, paradoxically, individuals with defective MC4R liked the high sugar dessert less than their lean and obese counterparts and in fact, ate significantly less of all three desserts compared to the other two groups.
One in 100 obese people have a defect in the MC4R gene which makes them more likely to put on weight. The researchers think that for these individuals, the fact that the MC4R pathway is not working may lead to them preferring high fat food without realising it and therefore contribute to their weight problem.
Professor Farooqi and colleagues think that humans and animals may have evolved pathways in the brain that modulate the preference for high fat food in order to cope with times of famine.
She added: “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.
“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.”