Parents and teachers commonly attribute excessive motor activity and other disruptive behaviour to children eating too many sweets - and the lay press constantly reports adverse behavioural responses when children eat any kind of confectionery. But such anecdotal observations need to be tested scientifically, and the criteria for interpreting diet behaviour studies must be rigorous.
Ingredients in non-chocolate sweets (sugar, artificial colours), components in chocolate (sugar, artificial colours in coatings, caffeine), and chocolate itself have all been investigated. The belief that additives (artificial colours and flavours) and natural salicylates (salts) cause hyperactivity in children - and that their elimination from foods would dramatically improve behaviour - has become widely-held, promulgated by the popular press. But many double-blind studies have shown that this elimination diet has no benefit beyond a placebo effect.
Although sugar is widely believed to cause hyperactive behaviour in children, this has not been scientifically substantiated. However, caffeine is the one substance that should cause concern, as studies have shown it is associated with disturbed night sleep and increased daytime sleep in children.
Regardless of whether caffeine causes the disturbed sleep or is consumed to counteract the daytime effect of interrupted sleep, caffeinated beverages - such as soft drinks - have detectable pharmacologic effects in children.
The evidence suggests the availability of caffeine to the young should be limited. The irony is that it's the soft drinks machine in the school corridor that could be causing the disruptive behaviour in class.
Specialists suggest children should consume less than the equivalent of 2.5mg of caffeine per kilogram body weight per day.
Intriguingly, there is one small group of children where higher caffeine consumption might be beneficial. Based on the theory that ADHD children - who are overactive, disruptive and suffer poor concentration - benefit from stimulant drugs such as Ritalin, stimulants such as caffeine might also be good for them.
A recent grouped analysis of caffeine, based on nine studies using 193 children (96 with ADHD), showed an extraordinarily beneficial effect - using parental ratings - in "externalising behaviours" (aggressive or disruptive behaviours). Scientists, however, urge caution over this finding, given the difficulty in establishing the diagnosis of ADHD definitively, plus the lack of understanding how stimulants such as Ritalin can ease the condition. But the area of work is intriguing because it does demonstrate that caffeine influences child behaviour profoundly.
Perhaps the truly holistic teacher should now consider whether diet could be a reason for a child's poor behaviour in the classroom.
Dr Raj Persaud is a consultant psychiatrist at the Maudsley hospital and senior lecturer at the Institute of Psychiatry in London. He is a fellow of University College London, and author of From the Edge of the Couch published by Bantam Press, pound;12.99. Email: email@example.com