Simply eating healthier may improve the behaviour of children with attention deficit hyperactivity disorder (ADHD) if therapy and medication fail, says a study published in the journal Paediatrics.
Researchers, however, said that their review of recent controlled scientific studies had shown conflicting evidence on the impact of supplements and restricted diets - in some cases they were no better than the placebo effect.
Nutritional interventions should therefore be considered an alternative or secondary approach to treating ADHD, not a first-line attack, said the review by doctors at Northwestern University Medical School in Chicago, published on Monday.
"Supplemental diet therapy is simple, relatively inexpensive, and more acceptable to patient and parent," than strict additive-free diets that have been popular in the past, said the study.
"Public education regarding a healthy diet pattern and lifestyle to prevent or control ADHD may have greater long-term success."
The study reviewed research published on the sugar-restricted, additive-free Feingold Diet, megavitamin therapies, omega-3 fatty acid supplementation and the suspected link between ADHD and a "Western-style" high-fat, low-fibre diet.
Three to five per cent of US schoolchildren, or nearly five million youths, are diagnosed with ADHD, which involves hyperactive behaviour, inability to pay attention, and impulsivity. It is often treated with stimulant medications such as the controversial Ritalin drug.
The precise causes of ADHD are unknown, although studies have pointed to hereditary factors as well as social and environmental influences. Eating high-sugar and high-fat foods may exacerbate symptoms, some research has shown.
But while proposed interventions such as giving iron supplements or cutting out additives and food dyes have soared in popularity in recent years, the Paediatrics article said there is little solid science to back up those claims.
For instance, the much-hyped Feingold Diet which advocates no red or orange colour dyes in food as well as no apples, grapes, deli, sausage or hot dogs, was highly touted in the 1970s and '80s for improving symptoms in more than half of ADHD children.
"Controlled studies failed to confirm the effectiveness of the diet to the extent claimed," said the Paediatrics review, also noting that the regimen was very difficult for many parents to follow.
Similarly, studies focused on getting rid of potential allergens in the diet such as wheat, eggs, chocolate, cheese and nuts, have shown limited success with some ADHD kids "but a placebo effect could not be excluded," said the study.
Even when it comes to sugar and diet soda, two elements which many parents believe can trigger hyperactivity in children, scientific studies have been unable to prove a definitive link.
"The majority of controlled studies fail to demonstrate a significant adverse effect of sucrose or aspartame," said the study.
The authors noted that avoiding high sugar foods in young children "may prevent diet-related exacerbations of ADHD."
But when parents restrict a child's sugar intake in order to ward off bad behaviour, their inherent belief that it will work is likely to blur any objective assessment of whether it works or not.
"In practice, the link between sugar and hyperactive behaviour is so universal in the opinion of parents of children with ADHD that no controlled study or physician counsel is likely to change this perception."
The suspected role of zinc and iron-deficiency deserves further study, while megavitamin therapy has not been proven to work and may even be dangerous in the long term, it said.
For many parents, simply paying more attention to feeding their kids a healthy diet, rich in fish, vegetables, fruit, legumes, and whole-grains, is likely to help.
"A greater attention to the education of parents and children in a healthy dietary pattern, omitting items shown to predispose to ADHD, is perhaps the most promising and practical complementary or alternative treatment of ADHD," said the study.
Andrew Adesman, chief of developmental and behavioural pediatrics at Steven and Alexandra Cohen Children's Medical Centre of New York, who was not part of the study, said more research is needed into dietary treatments for ADHD.
"We have more questions than answers," he said. "It is unfortunate that more research is not being done to examine the role of dietary interventions for the treatment of ADHD.
"Since some of these nutritional interventions cannot be patented, drug companies are not willing to underwrite the costs of the needed research."