O bese people pile on the pounds because they never lose their childhood sweet tooth, research suggests.
Normally, as young people reach adulthood, their preferences for sweet foods decline.
But a new study suggests for some people this desire for sugar does not drop away – meaning they continue to seek out sweet foods and drinks, putting on weight as a result. Experts at Washington University think that the brain’s “reward system” may operate differently among fat people.
They suspect this is because being overweight alters their hormone balance, meaning their brain perceives pleasure in a different way.
The team found that overweight people’s brains contained more dopamine receptors – which receive the sensation of a sweet flavour and convert it into a pleasurable feeling.
Researcher Professor Yanina Pepino said, “We believe we may have identified a new abnormality in the relationship between reward response to food and dopamine in the brains of individuals with obesity. In general, people grow less fond of sweet things as they move from adolescence into adulthood. Also, as we age, we have fewer dopamine receptors in a brain structure, called the striatum, that is critical to the reward system. We find that both younger age and fewer dopamine receptors are associated with a higher preference for sweets in those of normal weight. However, in people with obesity, that was not the case in our study.”
The team, whose work is published in the Diabetes medical journal, studied 20 people of a healthy weight and compared them with 24 people considered obese.
The participants – who were each aged between 20 and 40 – received drinks containing different levels of sugar to determine the degrees of sweetness each individual preferred. The researchers then conducted brain scans, to identify dopamine receptors linked to rewards in each person’s brain.
Among the group of normal weight, there was a strong link between the number of dopamine receptors and the preference for sweet drinks – and these both dropped as the volunteers grew older.
But in the obese group the number of dopamine receptors did not fall with age.
Co-author Professor Tamara Hershey said, “We found disparities in preference for sweets between individuals, and we also found individual variations in dopamine receptors – some people have high levels and some low. But when we looked at how those things go together, the general trend in people of normal weight was that having fewer dopamine receptors was associated with a higher preference for sweets.”
But that pattern was not seen among the obese volunteers.
The scientists suspect that insulin resistance – a change in hormones which is often a consequence of obesity, and is linked to diabetes – could be to blame for the change in brain structure.
None of the participants had diabetes – but some had higher blood glucose and insulin concentrations, and some were already becoming resistant to insulin.
The researchers believe those factors could have altered the brain’s response to sweet things.
Professor Hershey said, “There is a relationship between insulin resistance and the brain’s reward system, so that might have something to do with what we saw in obese subjects. What’s clear is that extra body fat can exert effects not only in how we metabolise food but how our brains perceive rewards when we eat that food, particularly when it’s something sweet.”
Britain has among the worst levels of obesity in Europe, with nearly two thirds of adults and one in three children classed as being overweight.
Some 4.2 percent of women – almost one in 20 – and 1.7 percent of men in the UK are morbidly obese, according to recent research. And new statistics revealed that childhood obesity is worse than ever before.
They showed that 533 people under the age of 19 in England and Wales now have type 2 diabetes – the form of the disease linked to obesity and usually only seen in people over the age of 40. Until 2000 there had never been a case of type 2 diabetes reported among children in the UK.