Can you taste the difference between a calorie-laden sugar and a zero-calorie sweetener? Specialised cells in your gut can, according to experiments in mice. These cells tell the brain whether a sugar or sweetener has been ingested within milliseconds.
revealed that food doesn’t need to pass through the mouth to stimulate a response from the brain. Mice respond to food rewards when they are put directly into their stomachs, too. More recent studies have found that .
A couple of years ago, Diego Bohorquez at Duke University in North Carolina and his colleagues  different nutrients. These cells, called neuropod cells, were spotted in the intestines of mice, and rapidly communicate with the brain via the vagus nerve, informing the brain of the presence of sweets.
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To find out whether these cells can tell the difference between calorific sugar and zero-calorie sweeteners, the researchers infused nine sugars or sweeteners into the guts of mice. At the same time, the team used a device to measure the activity of the vagus nerve, which connects the gut directly to the brain.
Favour fruit?
All of the sugars triggered the neuropod cells to send a signal to the vagus nerve – except for fructose. The fact that fructose – a natural sugar found in fruit – doesn’t seem to trigger a rapid signal to the brain may explain why fruit doesn’t give our brains the same reward as chocolate, says Elisa Hill at RMIT University in Australia, who wasn’t involved in the work. “It would be great to know if these systems can be trained to favour fruit over chocolate,” she says.
In other experiments, Bohorquez and his colleagues studied organoids – lumps of living tissue grown from cells taken from the intestines of mice or humans. The team found that only sugars with calories triggered the release of a compound called glutamate as a signal to the vagus nerve. Zero-calorie sugars had a different effect – they delivered an entirely separate signal to the vagus nerve, via a compound called ATP.
Importance of energy
The findings suggest that the gut senses the sweet taste and calorie content of sugars separately, and sends both signals to the brain within milliseconds, say the study authors. This sense might have evolved to enable animals to find energy-rich foods, says Jonathan Swann at Imperial College London. “Energy is really important for the host, and so determining what’s energy-dense and what’s energy-scarce would be very important in seeking out foods,” he says.
It is likely that similar cells are doing the same thing in people’s guts, says Hill. If so, tinkering with the way the cells work may influence the way our brains respond to sweet foods. In the future, the cells could be targeted in treatments to modify eating behaviours and for some people with obesity, she says.
Neuropod cells might also provide a way for gut bacteria to communicate directly, and instantly, with the brain, suggests Swann. Disruptions to the gut microbiome have been linked to a range of neurological disorders, including Parkinson’s disease and mood disorders. And gastrointestinal symptoms are common in mental health disorders, such as anxiety and depression.
