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Researchers are learning how a diet high in fructose can be linked to impulsivity and aggression. Catherine Falls Commercial / Getty Images
  • High consumption of fructose may stimulate impulsivity and aggression, behaviors similar to the foraging response in animals.
  • Fructose is a source of energy. But in many animals, it also triggers a foraging response, similar to what occurs in starvation.
  • Many of the behaviors seen with the foraging response are similar to symptoms of ADHD and bipolar disorder.

Food has a long history of being used to alter how we feel.

Caffeine to boost our mental alertness. Foods like macaroni and cheese or deep-dish pizza can be a comforting meal after an emotionally draining day. And a fresh salad can be just the thing when we’re feeling sluggish.

The flip side is that what we eat can also negatively affect our mood — think post-party hangovers, pre-caffeine morning fog, late-day sugar crashes.

Chronically, diet may contribute to symptoms related to attention deficit hyperactivity disorder (ADHD), bipolar disorder, and other neurodevelopmental disorders.

While many studies have found links between diet — in particular, sugar — and the behaviors seen with these conditions, the reason for the connection is not clear.

A group of researchers from the University of Colorado suggests the answer may lie in our evolutionary past.

In a paper published October 16 in the journal Evolution and Human Behavior, the researchers outline a possible role for fructose, a type of sugar found in fruit and honey, in increasing the chance of these neurodevelopmental disorders.

Fructose is a source of energy. But in many animals, it also triggers a foraging response, similar to what occurs in starvation. This response is useful for animals building up energy stores before hibernation or long-distance migration.

Foraging involves behaviors that support seeking out new sources of food and water — risk taking, impulsivity, increased movement, rapid processing of information with less attention to details, and sometimes aggression.

The authors of the new paper write that many of the behaviors seen with the foraging response are similar to symptoms of ADHD, bipolar disorder, and other disorders.

They also point out that these conditions have increased among the population in parallel with the rate of obesity, which comes alongside a rise in the intake of sugars over the past century.

While fructose is relatively rare in nature, it’s very common in our modern food environment, showing up in many processed foods and beverages as refined sugars and high fructose corn syrup.

In 2010, Americans consumed almost 15 percent of their calories in the form of added sugars, according to one national survey. For some groups, added sugars made up 25 percent of their calorie intake.

Some research in mice has also found that a high-salt diet — another aspect of our modern diet — can stimulate the body to make its own fructose.

Dr. Shebani Sethi Dalai, a physician in obesity medicine and psychiatry at Stanford University, who was not involved in the new study, agrees that diet can impact our mental health.

“There’s a mismatch between our modern lifestyle and our ancestral potential, or our genes,” she said. “That’s why I think we’re seeing more illnesses today than before.”

While the parallel rise in processed food intake and certain neurodevelopmental disorders doesn’t prove that sugar is to blame, some research supports the idea that excess sugars may stimulate foraging-like behaviors.

In one study, people who scored higher on a test for ADHD tended to display more exploratory foraging patterns.

In adolescents, intake of sugary soft drinks has also been linked to aggressive behavior, as has high levels of urinary uric acid — a molecule produced when the body breaks down fructose.

Other research, though, found no link between sugar consumption in children and the development of ADHD.

The mixed research to date may be due to genetics and other factors affecting how different people respond to sugar.

Some researchers have also suggested that ADHD might be a chronic response to a high-sugar diet, so the effects may not show up in short-term studies.

Sethi Dalai says several mechanisms have been proposed for how sugar and ultra-processed foods might worsen symptoms of mood disorders and even psychosis, such as an increase in inflammation or oxidative stress in the brain.

“Some of the medications for [mood disorders] can lead to metabolic side effects,” she adds. “Those can include increasing your blood glucose, or putting somebody into a category of higher weight gain or a prediabetic state.”

More research is needed to fully understand the link between diet and ADHD, bipolar disorder, or aggressive behavior, says Dr. Richard Johnson, lead author of the Evolution and Human Behavior paper.

He would like to see randomized controlled trials in which people with symptoms of these conditions eat a diet low in sugar and high fructose corn syrup for at least 8 to 12 weeks.

They would be compared to a control group of people who ate their usual diet to see if eating less sugar improved patients’ symptoms.

However, “there is already sufficient evidence that reducing sugar intake is good for overall health as well as mental and behavioral health, especially for sugary beverages,” said Johnson.

Sethi Dalai agrees. She uses the ketogenic diet with many of her patients who have bipolar disorder.

“Clinically, I have seen it improve a lot of patients’ symptoms, and even reduce the medication dosage to some extent,” she said.

The ketogenic diet, or keto diet, is a low carb, high fat diet that can help people lose weight and is being investigated as a treatment for diabetes and other metabolic conditions.

It has been used for years as a way to help diminish symptoms of epilepsy.

But research on the benefits of this diet for bipolar disorder is just beginning.

Sethi Dalai is currently recruiting patients for a pilot clinical trial looking at whether a ketogenic diet can improve symptoms and metabolic measures in patients with bipolar disorder or schizophrenia.

Similar to how sugar affects people differently, eating healthier may also help people with mental health conditions to varying degrees. For many, it will be no substitute for medical treatments.

“It would be a huge leap to say that you could cure bipolar [disorder] with the ketogenic diet,” said Sethi Dalai.

“For some patients, [diet] might be something that can be used in the place of medication. But I think for the vast majority of patients, medications still play a role.”