Researchers unpack how sweetness perceptions are formed by past diets

14 Apr 2020 --- Taste perceptions around sweetness are based on previous experiences, with researchers at the University of Sydney, Australia, now unpacking the science of how this works using fruit flies. This response to sweetness uses the same functions that the brain uses to learn, the study finds. Additionally, molecular pathways that can extend lifespan are involved in enhancing taste perception, while diets in fruit flies that promote long life were also found to enhance taste perception. Ultimately, this research could be used as a first step to address human nutrition issues, including eating disorders.

“We know that diet can also change taste perception in humans. If we can better understand how to control this, we could construct diets that would make food more palatable for people with anorexia, or diets that make food less rewarding to help reduce caloric intake. However, this is a while off and we first need to understand how it works,” Professor Greg Neely, of the Charles Perkins Centre and School of Life and Environmental Sciences, tells NutritionInsight.

Neely believes that if there is a better understanding of how different diet compositions impact the taste system, researchers could be able to maximize the rewarding aspects of food, but minimize the calories consumed. “There is probably a ‘sweet spot’ where we can help people reduce caloric intake without confusing their sensory systems,” he notes.

Dopamine, a neurotransmitter, was central to the fruit fly’s taste sensors learning things in the same way its brain does. These are also the same chemical pathways that humans use to learn and remember a variety of things. Neely states that this highlights how learning is a whole-body phenomenon, which was a surprise. 

He was also interested to see how flexible taste perception was and how lots of different diet interventions had an impact on sweet taste sensitivity. “Changes in taste based on experience are probably the norm, not the exception,” he adds.

Altering the diet
It became clear that changing the diet of the fruit fly drastically altered how well the fly could taste subsequent sugar after a few days. This involved measures such as increasing sugar, removing the taste of sugar, increasing protein and changing sugar for complex carbohydrates. For example, when flies consumed unsweetened food, sugary food tasted much more intense. 

Professor Qiaoping Wang, leader of the study and based at Sun Yat-Sen University, China, was surprised to find that a protein-restricted diet that makes an animal live much longer also turns up the intensity of sucrose perception for that animal. This is also dependent on the same learning and longevity pathways.

“The response was also really specific. For example, when we fed flies food that had no sweetness, the animals’ sweet taste perception was enhanced, but only for glucose, not for fructose. We have no idea why they specifically focus just on one kind of sugar when they perceive them both as sweet,” Wang continues.

The researchers also found that eating high amounts of sugar suppressed sweet taste perception, making sugar seem less sweet. “This finding, which occurs through a different mechanism, matched nicely with recent results from our colleague Monica Dus at the University of Michigan, US, who is a world expert in this area,” adds Neely. 

Eating high amounts of sugar suppressed sweet taste perception, making sugar seem less sweet.Follow-up studies on the way
This study is a follow up to Neely’s prior work testing the effects of artificial sweeteners in humans. That study had found that artificial sweeteners activate a neuronal starvation pathway and end up promoting increased food intake, especially when combined with a low-carb diet. 

Explaining the inspiration behind the latest study, Neely states: “We started by looking at artificial sweeteners to see what impact they had on the nervous system. From that, we noticed that taste sensitivity was really flexible, but not much was known about how that worked. Therefore, we thought that was something fundamental that would be good to know more about.”

Looking ahead, Neely notes that although it is known that humans have similar pathways to those displayed in the fruit flies, the researchers still need to see if the system found in flies is also used to regulate taste perception in us. “We have a few more follow up studies ongoing, looking more at how this is controlled at the level of the brain.”

Sugar has been in the spotlight for some time, with a UK study recently finding that a tax on high sugar snacks, such as cookies, cakes, chocolates and candy, might be more effective at reducing obesity levels than a sugary drink tax. Meanwhile, the Mediterranean diet could prevent overeating, with a study finding that animals on a Western diet – rich in processed foods – ate far more than they needed and gained weight.

Edited by Katherine Durrell