Fruits are rightfully counted among the healthiest components of our diet, so the very question of whether they can negatively affect our mental state borders on the absurd. But as in many areas of nutrition, the real story is more complex than the headline. The problem, researchers suggest, is not the fruit itself, but rather the total amount of fructose in the diet and the way our body succeeds – or fails – to absorb it.
A new study published in the journal Brain, Behavior, and Immunity examined the link between fructose malabsorption, anxiety, and brain inflammation. Fructose is a simple sugar found naturally in fruits and vegetables, but today it also appears in high amounts in soft drinks, commercial juices, and processed foods. According to the researchers, for most of history, humans consumed less than five grams of fructose per day, whereas today in developed countries, daily consumption can sometimes reach 50 to 80 grams.
The body absorbs fructose using a dedicated transporter located in the wall of the small intestine, but this transporter has a limited capacity. When the amount of fructose is too high, or when a person struggles to absorb it well, some of the sugar moves on to the large intestine. There, gut bacteria break it down through a fermentation process, which can alter the composition of the gut microbiota and trigger an unexpected immune response.
When the stomach sends signals to the brain
To deeply examine this connection, researchers from the French National Research Institute for Agriculture, Food and Environment designed a two-part study: An observational study in humans and a controlled experiment in mice. The human component involved 55 healthy, young men with an average BMI, who were asked to log everything they ate for a week. Their average fructose consumption stood at about 30 grams per day, with nearly 40 percent of them exceeding recommendations for added sugar intake.
The participants subsequently underwent breath tests, which measure hydrogen and methane gases produced when gut bacteria ferment unabsorbed sugars. The prominent finding was that about 60 percent of the participants showed fructose malabsorption, even when their fructose intake was no higher than that of those who absorbed it well. In simple terms, two people can consume a similar amount of fructose, but their digestive systems will handle it very differently.
The researchers found that participants with fructose malabsorption scored higher on a questionnaire examining a baseline predisposition to anxiety. However, it is important to emphasize that the difference was not significant enough to indicate a clinical psychiatric disorder, but rather a higher stress level. Additionally, their blood tests showed higher levels of inflammatory proteins and bacterial toxins that managed to penetrate the bloodstream, and their stool tests revealed differences in the composition of gut bacteria.
Should we eat less fruit?
To test whether this was merely a statistical correlation or a possible biological mechanism, the researchers also conducted an experiment on mice lacking the primary intestinal fructose transporter. For four weeks, the mice were fed a diet containing five percent fructose. In the mice that could not absorb the sugar properly, the researchers observed more behaviors related to fear and depression, significant changes in the composition of gut bacteria, and an inflammatory response in microglia cells – which are the immune cells of the brain.
These findings may seem dramatic, but it is important to put them into perspective. First, the human component of the study was observational, and therefore it does not prove that fructose causes anxiety. Second, the study included only men, so it remains unclear whether the results would be similar in women. Furthermore, the researchers themselves note that clinical trials are needed to examine whether reducing fructose in individuals with malabsorption can indeed improve various metrics related to mental health.
It is crucial to emphasize that this conclusion should not prompt you to consume less fruit. Whole fruits come packed with fiber, vitamins, minerals, and antioxidants, making them very different from sugary drinks, strained juices, or processed foods that contain high amounts of readily available sugar. For most people, eating fruit as part of a balanced diet should not be an issue. However, for those suffering from bloating, abdominal pain, diarrhea, or discomfort after eating fructose-rich foods, as well as those dealing with stress or anxiety, it may be worth consulting a clinical dietitian or a physician.
The bottom line is not that sugar from fruit "causes anxiety," but rather that our intestines do not always handle fructose the same way. When a portion of it remains undigested, it can alter gut bacteria activity, trigger inflammation, and affect the brain in unexpected ways.