Ever felt tired and sluggish after eating sugar or a startling number of ginormous meals? That’s the effects of a sugar crash (aka reactive hypoglycemia), which researchers at New Zealand have shown can impair cognitive performance – in a double-blind, placebo-controlled study.

“I am fascinated by how our senses influence our behaviour and affect our everyday lives,” said study author Mei Peng, a lecturer in sensory science at the University of Otago in a statement at Psypost. “In particular, how sugar consumption might change the way our brains work. In the case of sweetness perception, we have evolved to favour this taste.”

Glucose ingestion has been shown to improve memory performance, but studies examining the effect of glucose on other cognitive processes, such as attention, problem solving, learning, decision-making and face recognition, have led to mixed results.

For the study, the researchers recruited 49 individuals, each of whom was given drinks containing either glucose, sucrose (table sugar), fructose (fruit sugar), or sucralose (an artificial sweetener). The researchers then had them take part in three cognitive tests –  a simple response time task, a measure of arithmetic processing, and the Stroop task. They also measured the participants’ blood glucose levels during the tests.

They found that participants who consumed glucose or sucrose performed worse on cognitive tests – that is, they demonstrated a delay in completing cognitive tasks as in reduced attention and response times – compared those who consumed fructose or sucralose. There were also participants who researchers had instructed to fast for 10 hours prior to the test – and they performed even worse.

Our body breaks down sucrose into glucose and fructose. But unlike glucose, fructose does not cross the blood-brain barrier.

“Our study suggests that the ‘sugar coma’ – with regards to glucose – is indeed a real phenomenon, where levels of attention seem to decline after consumption of glucose-containing sugar,” Peng told PsyPost.

“While the sample size is relatively small, the effect we observe is substantial,” Peng told. “Future research should further quantify how different brain regions change after sugar consumption – by using neuroimaging techniques. This will help us better understand how attention deficits arise after glucose consumption.”

“As food is becoming increasingly diverse, accessible and delicious, it is important to conduct more research in this area to understand food choices and eating behaviours,” she added.

The study, entitled “The “sweet” effect: Comparative assessments of dietary sugars on cognitive performance” has been published in the journal Physiology & Behavior.