Ask any parent or teacher of school-aged children, and there's generally no argument: high sugar intake leads to a clear change in the behavior of kids — and usually not a good one! Some have even discouraged the eating of birthday cakes during school hours because of the "predictable" disruption in behavior, including poor concentration, hyperactivity and generalized chaos.
Is there any scientific evidence or physiologic explanation supporting this notion? Is the association between sugar and behavior so well-accepted that studying it is unnecessary? Could this concept be a myth?
It turns out that this concept has been well-studied, and as surprising as it may seem, overwhelming evidence suggests that there is no connection (or certainly no strong link) between sugar intake and the behavior of healthy children.
Does the Source of Sugar Matter?
It is reasonable to first ask if refined sugar has any different effect on the body than other sweet foods, such as apples or fruit juice. The term "sugar" actually refers to a number of molecules:
All of these sugars occur naturally in animals, fruits and vegetables. However, whether occurring in nature, refined or synthetic, the body handles them in similar ways — ultimately, they are broken down or metabolized to glucose as a source of energy. With respect to how the body handles different sources of sugar, there may be little difference between sprinkling table sugar over the top of your cereal or putting a few strawberries on top. Depending on one's taste, one could even be getting more sugar (in the form of fructose) with the strawberries. There are, of course, other differences to consider, such as other vitamins and fiber in fruit sources of sugar, but these have not been linked to behavioral problems.
What the Studies Show
Several studies of the association between sugar intake and behavior have found little or no connection between the two — that is, even when children reported by their parents to be hyperactive after sugar intake were studied on different diets, no clear correlation between low or high sugar intake and a number of behavioral assessments could be established. In one of the best studies (reported in the New England Journal of Medicine in 1994), healthy children aged 3 to 5 and children aged 6 to 10 who were reported to be sensitive to sugar were placed on diets that varied primarily in sucrose content over three-week periods. No correlation was found between the type of diet and performance on an array of psychiatric, behavioral and intelligence measures. Other studies with slightly different methods have come to similar conclusions.
Even so, the issue remains controversial: it is always possible that, in some cases, individual children do react in a negative way to sugar, even if no correlation could be demonstrated among groups of kids. Perhaps a different type or amount of sugar would have a more profound effect on behavior; and some studies have found a small effect on behavior after sugar intake. Most studies were careful to include only "normal" (that is, healthy) kids, not those diagnosed with attention deficit disorder or other conditions. When these studies are taken together, however, the assumption that sugar causes hyperactivity or other behavior problems is suspect. As one researcher wrote, "There is now considerable evidence that the concern about sugar consumption reflected by the media in the 1970s was misplaced." If the parents and teachers I know are a reflection of current thinking, this misplaced concern remains alive and well.
The Bottom Line
Although sugar's bad reputation regarding behavior isn't particularly well- supported by evidence, there are other good reasons to restrict sugar, including dental caries (cavities), obesity and the risk of nudging other more healthful foods out of the diet. Also, concentrated sugar, for instance in fruit juices, may cause diarrhea and reduce a child's appetite before a meal. For these reasons, moderation in sugar intake may be important.
Moderation (including sugar intake) within a balanced diet seems the most sensible course — just don't count on improved behavior as a result.
Robert H. Shmerling, M.D. is associate physician at Beth Israel Deaconess Medical Center and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program.