Nine weeks of hidden sugar
If sugar made children hyper, it should be easy to see. So in the early 1990s, a team led by Mark Wolraich at Vanderbilt University set up a double-blind trial with two groups: 25 preschoolers aged 3 to 5, and 23 school children aged 6 to 10 whose parents described them as sensitive to sugar.
Each family spent three weeks on each of three diets. One was high in sugar. The other two were low in sugar and sweetened with aspartame or saccharin instead. Nobody, not the children, the parents or the researchers measuring behaviour, knew which diet was which. And the sugar diet wasn’t a token sprinkle: the preschoolers ate on average about 5.6 grams of sugar per kilogram of body weight a day, more than children typically eat.
The result? For the children described as sugar-sensitive, there were “no significant differences among the three diets in any of 39 behavioral and cognitive variables.” For the preschoolers, 4 of 31 measures differed, with “no consistent pattern”. The authors’ conclusion: even above normal intake, sugar did not affect the children’s behaviour or thinking.
What 23 studies add up to
One trial can be a fluke, so a year later Wolraich, Wilson and White pooled the evidence in a meta-analysis in JAMA. They only counted studies that gave children a known amount of sugar, compared it with a sweetener placebo, and kept the children, parents and staff blind. Sixteen reports with 23 studies made the cut.
Across 14 kinds of measurement, from direct observation to school tests, none showed an effect that was clearly different from zero. Their summary: sugar “does not affect the behavior or cognitive performance of children.” They were careful to add that a small effect, or an effect in some children, “cannot be ruled out”. That’s the honest version: not “impossible”, but nothing the careful studies could find.
So why does it look so real?
Because you usually know who had the cake. In a 1994 experiment, the psychologists Hoover and Milich studied 35 boys aged 5 to 7, all described by their mothers as “sugar sensitive”, together with their mothers. Half the mothers were told their son had just had a large dose of sugar. In fact, every boy got the same placebo, the sweetener aspartame.
The mothers who believed their son was on a sugar high rated him as significantly more hyperactive. On video, those mothers also stayed physically closer and showed trends to criticise, watch and talk to their sons more. Nothing the boys had swallowed was different. The expectation changed what the parents saw, and how they acted.
Add the places where children eat the most sugar, such as birthday parties, holidays and sleepovers, which are exciting with or without the cake, and you have a belief that confirms itself again and again. The JAMA authors put it this way: the strong belief of parents “may be due to expectancy and common association.”
Not even a rush
The “sugar rush” doesn’t hold up for grown-ups either. A 2019 meta-analysis of 31 studies and 1,259 participants found no positive effect of carbohydrates on any aspect of mood at any time point. Within the first hour, people who had the sugar actually reported more fatigue and less alertness than those given a placebo.
None of this makes sugar a health food. There are good reasons to limit it, starting with teeth: MedlinePlus, the US National Library of Medicine’s health encyclopedia, calls a diet high in sugar “a major cause of tooth decay”. It just isn’t a switch for wild behaviour.
You’re in very good company
People who spend all day with children believe a version of this too. In a survey published in 2012, 57% of the UK teachers and 55% of the Dutch teachers who took part agreed that “children are less attentive after consuming sugary drinks, and/or snacks.” The researchers counted it as one of the neuromyths in their list.
So if you picked “yes”, you picked the answer most of us grew up with. The fun part is the real lesson hidden inside it: sometimes the thing that changes isn’t the child at all. It’s what we expect to see.
