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The much-maligned vegetable broccoli may hold promise as a possible treatment for autism.

Nearly half the participants in a recent study of 40 men aged 13 to 27 with moderate to severe autism spectrum disorder (ASD) showed significant improvement after taking sulforaphane, an oral supplement derived from broccoli sprouts, for 18 weeks.

“The substantial improvements of individual ASD patients’ trajectories were conspicuous and suggest that further investigation of sulforaphane in ASD is promising,” the researchers wrote in a study titled “Sulforaphane treatment of autism spectrum disorder (ASD)” published in the Proceedings of the National Academy of Sciences.

In the United States, autism affects about one in 68 children, mostly males. Symptoms include impaired social interactions, abnormal behavior and difficulty with verbal communication.

Researchers led by Kanwaljit Singh of Massachusetts General Hospital hypothesized daily sulforaphane treatment might reduce the severity of such behaviors because it’s known to reverse other abnormalities associated with autism, including inflammation, oxidative stress and DNA-damage.

Those symptoms improved for nearly half of 29 men who took a sulforaphane oral supplement for 18 weeks, and then reappeared after the treatment stopped. The 15 men who got a placebo pill showed minimal change.

Participants who got sulforaphane, a chemical also present in other cruciferous vegetables such as kale and bok choy, showed “significantly greater improvement” after 18 weeks in irritability, lethargy, hyperactivity, awareness, communication, motivation and other behavioral measures. A control group showed no improvement.

Paul Wang, senior vice president for medical research of the advocacy organization Autism Speaks, pointed out the results don’t mean autism can be cured by eating broccoli for every meal. “The amount of sulforaphane that was administered in the study is many times higher than you can reasonably get through food,” he wrote in a Q&A about the study on the organization’s Web site. “Even sulforaphane-rich foods like brussels sprouts, broccoli and broccoli sprouts don’t have enough of the chemical to get you close. So eating these vegetables can’t be expected to improve autism symptoms.”

He also cautioned against taking sulforaphane until more is known about possible side effects. Some study participants showed a small increase in liver enzymes, a sign sulforaphane might cause liver inflammation. He noted the lack of safety data and less-rigorous testing of nutritional supplements. The supplement used in the study isn’t available over-the-counter.

An unexpected side effect among those who took the supplement was significant weight gain. Less surprisingly, they also experienced more flatulence.

The researchers called for more study of links between how sulforphane and fevers affect people with autism. “Widespread anecdotal reports have suggested that fever can dramatically but temporarily ameliorate the disturbed behavior of many autistic patients,” the researchers wrote. They theorized sulforaphane triggers a biological reaction similar to having a fever, which causes a “heat-shock response” that reduces inflammation in cells.

Two participants with a history of seizures who took the supplement had seizures during or shortly after the study. Researchers said they couldn’t rule out the possibility seizures are a side effect of treating autistic patients, who are generally more prone to seizures, with sulforaphane. Researchers also cautioned against reading too much into a single-site study of just 40 participants, all with similar symptoms.