Some girls may have a 'sweet tooth' gene

"Scientists discover a gene which makes certain foods taste better for women," Mail Online reports. The story is based on a study of 150 four-year-olds that looked at whether there was an association between a specific gene variant (exon III seven-repeat allele (7R) of DRD4) and activity in the brain's dopamine pathways. 

These sections of the brain – known as the brain's reward centre – light up during activities that an individual finds pleasurable, which can range from eating chocolate to smoking crack. As you might suspect, these pathways are also associated with addiction.

The researchers wanted to see if the genetic variant meant that affected girls favoured some foods over others because they gave them more pleasure. The children were given a test snack that offered choices between different types of food, and their mothers filled out a food questionnaire about the kind of foods they usually ate.

Researchers found that in girls, carriers of the gene variation ate more fat and protein during the snack test than those without the gene. The food diaries also suggested that carriers of the gene variation ate more portions of ice cream and less vegetables, eggs, nuts and whole grain bread.

This small study does not prove that girls are genetically inclined to prefer sweet or fat-laden foods, as the Mail's headline suggests.

The study also did not look at the long-term outcomes of children with the "sweet tooth" gene, such as whether they had an increased risk of obesity in later life.

Children's eating behaviour is influenced by many factors, including environment, mood, other genetic differences, and, most importantly, upbringing. The most important influence on a child's eating habits is their parents.

Where did the story come from?

The study was carried out by researchers from a number of universities in Canada as well as the Universidade Federal do Rio Grande do Sul in Brazil, Brown University in the US, and The Agency for Science, Technology and Research, Singapore. It was funded by the Canadian Institutes of Health Research.

The study was published in the peer-reviewed journal Appetite.

The Mail Online's claim that the study showed why girls have a sweet tooth is far-fetched. The results only highlighted a potential association between the exon III seven-repeat allele (7R) of DRD4 variant and an inclination to prefer sweet and fatty foods. The study did not prove a direct cause and effect relationship.

What kind of research was this?

This research is part of an ongoing cohort study in Canada, where researchers are following pregnant women and their children from birth until the age of 10.

The authors say that one important contributor to disordered eating behaviour, such as binge eating, bulimia and obesity, seems to be an exaggerated sensitivity to the rewarding aspects of food. Some people may therefore find eating more rewarding than others. This could be associated with alterations in activity of the hormone dopamine in the brain, they suggest.

They point out that studies in adults suggest that variations in a gene called the dopamine-4 receptor gene (DRD4) increase eating and obesity, particularly in women. The variation is called the seven-repeat allele (7R).

What did the research involve?

The study sample included 150 four-year-old children who were recruited from a birth cohort study in Canada, 30% of whom came from low income families.

At the study laboratory, researchers collected saliva samples from the children, which were used to extract DNA for testing for the 7R variation. Children and mothers were then given a mid-morning 30-minute test meal, which included different types of foods in pre-weighed portions – Frosted Flakes, sliced apple, muffin with chocolate drops, 3.25% milk, baked beans, croissant, cooked egg, cheddar cheese, All Bran, white bread and orange juice. The foods were chosen with advice from a nutritionist to include familiar snacks and to have similar colours.

A table with two sets of plates was placed in the centre of the room, with chairs for mother and child on both sides facing each other. Mothers were instructed to offer a light breakfast to the children at home beforehand and not to share plates or influence the children's choices.

The researchers made a number of efforts to standardise this procedure. For example, they:

• booked all the lab visits at mid-morning to reduce any variations in whether the children were hungry
• made notes on the time and content of the last meal
• checked whether the child slept while driving to the laboratory or not
• asked the families to avoid booking the laboratory measure the day after large "food events" such as birthdays or parties

The laboratory visit was always booked to ensure that children were within a few weeks of being 48 months old at the time.

Based on the nutritional content of each food and the amount eaten, the researchers calculated the amount of fat, carbohydrates and protein the children consumed.

The mothers were also asked to fill in a food frequency questionnaire to evaluate their children's eating habits. Researchers used these to analyse the children's calorie and nutrient intake. They also calculated the children's body mass index (BMI).

Researchers then looked at the relationship between the presence of the 7R gene variation, the child's gender and their consumption of foods. They adjusted their findings for various factors that might affect the results, such as BMI.

What were the basic results?

Overall, the children's genetic make-up had no relationship with their total calorie intake, but sex did, with boys consuming more calories than girls.

• Among girls, carriers of the 7R gene variation ate more fat and protein than non-carriers at the test meal.
• Based on the food diaries, 7R carriers consumed more portions of ice cream and less vegetables, eggs, nuts and whole bread across both sexes.

How did the researchers interpret the results?

The researchers conclude that the 7R variation of DRD4 influences what children eat and their choice of food as early as the age of four.

They say the results suggest that prior associations between the 7R allele and adult overeating and obesity may originate in food choices observable in the preschool years.

"Longitudinal follow-up of these children will help establish the relevance of these findings for obesity risk and prevention," they say.

Conclusion

Scientists are investigating whether dopamine activity in the brain influences our behaviour towards food, so this study is of interest. However, it does not show that girls with a particular gene variation are more likely to consume more unhealthy foods or become obese. 

As the researchers point out, the study was limited by its small sample size. There are also many factors that influence eating behaviour, including mood, food environment, stress levels and upbringing, all of which might have had an effect on the children's food choices.

Of note, the study's findings do not indicate that there is a relationship between calorie intake and genetic make-up during the snack test, only between calorie intake and sex, as might be expected.

The results from the food diaries do not show that 7R carriers eat significantly more sweet foods generally, only more ice cream.

There are also many factors that might have influenced what the children chose to eat at the test meal – most notably, what their mothers ate.

As this is an ongoing cohort study, more information on the potential influence of the 7R allele on long-term health outcomes may come to light in the future.