In 1978 a study of 265 hyperactive children found that they all had abnormally low blood sugar levels. This might have been a sign that they had previously eaten too much sugar and were now ‘crashing’. But almost every study since then has failed to find any link. A meta-analysis in 1995 looked at 23 studies and found that there was no statistically significant effect of sugar on any of 14 different measures of behaviour.
Parents who see hyperactive children at birthday parties may blame the cake and sweets, when it is really just the excitement of the party itself. In fact parents who are told their child has been given a sugary drink are more likely to judge their behaviour as hyperactive, even if it was really a sugar-free drink that had been drunk.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.