* One reason for the increased incidence of ADHD may be environmental. Lead, mercury and polychlorinated biphenyls (PCBs) - all common pollutants - are toxic to children at levels that are insignificant to adults. A child's developing brain is particularly vulnerable to such toxins, resulting in a host of neurological problems, including hyperactivity (Environ Health Perspect, 2001; 109 [Suppl 6]: 813-6).
* Other potentially toxic substances are even more widespread because they are deliberately put into our food. Food colourings and preservatives derived from coal tar were first linked to hyperactivity by Dr Benjamin Feingold in 1973. He found that simply withdrawing foods containing these additives could reduce or totally abolish hyperactive behaviour. Inevitably, such commercially damaging findings were challenged by the food industry and, at first, the weight of evidence swung against Feingold as study after study failed to replicate his findings (J Am Diet Assoc, 1983; 83: 132-4). The turning point came in 1985 with a landmark experiment carried out by doctors at Great Ormond Street Hospital in London, involving 76 hyperactive children. All were put on an additive-free diet and, within a few weeks, more than 80 per cent of them began to show significant improvements in behaviour. Food additives were then reintroduced back into their diet in a double-blind manner and, sure enough, the behavioural problems returned (Lancet, 1985; i: 540-5).
* Most recently, the UK Food Standards Agency has reported the results of a major double-blind trial from the Isle of Wight. Three-year-olds were given drinks laced with a cocktail of additives (sunset yellow, tartrazine, carmoisine, ponceau 4R and sodium benzoate) or a placebo drink during random weeks. The results were striking. Parents consistently noted that their children's behaviour got worse after taking the additive cocktail and improved with the placebo drink.
What makes this study stand out from previous ones is that the children were not a selected group of known hyperactives. They were simply a random selection of preschool toddlers - who all became hyperactive after consuming the additives. So, it is clear that such additives adversely affect all children, not just the subgroup who may have hypersensitivity reactions to these chemicals (Food Standards Agency Report T07004, 4 November 2002).
* Additives are not the whole story, as revealed by the Great Ormond Street doctors also in 1985. In a parallel study to the one described above, the effects of foods of all kinds, not just those containing additives, were investigated. To their surprise, the researchers found that all of their ADHD children were intolerant ('allergic') to at least one foodstuff that caused not only their behaviour to deteriorate, but also induced other symptoms such as stomach pain, bloating and headaches. Altogether, 48 different foods were implicated, chief among which were wheat, dairy products, chocolate and oranges.