Many people believe that excessive sugar intake is one of the main reasons for the “obesity epidemic.”
This is not because sugar is high in calories. Instead, evidence suggests that sugar may increase cravings and promote higher overall calorie intake.
Recently, a team of researchers examined the effects of sugar — glucose and fructose — on brain activity in lean and obese adolescents. Here is a detailed summary of their findings.
High intakes of sugar, especially sugar-sweetened beverages, have been associated with an increased risk of becoming overweight or obese.
The two most common forms of simple sugars (monosaccharides) in the diet are glucose and fructose. In foods, they often occur together or are combined in the form of sucrose (table sugar).
Although several studies support this idea, strong evidence is still lacking.
This study examined the effects of glucose and fructose on blood flow in the brain of lean and obese adolescents.
This observational study investigated the effects of eating glucose and fructose on brain blood flow and appetite hormones in lean and obese adolescents.
A total 38 adolescents were recruited — 14 lean and 24 obese. They were 13–19 years old and apparently healthy.
On separate occasions after an overnight fast, the participants consumed 75 grams of glucose or 75 grams of fructose, dissolved in 300 ml of cherry-flavored water.
Afterwards, the researchers measured the following:
- Brain blood flow: The researchers assessed cerebral blood flow (brain perfusion) in different brain regions, using pulsed arterial spin labeling (PASL) and functional MRI. The brain scans took an hour.
- Glucose: Every 10 minutes the researchers took blood samples to measure blood sugar (glucose).
- Fructose: Circulating levels of fructose were measured 20, 40 and 60 minutes after drinking the glucose and fructose beverages.
- Appetite hormones: Ghrelin, insulin, leptin and adiponectin were also measured in blood samples.
- Self-rated appetite: At the beginning and end of each session, the participants were asked to rate their feelings of hunger, satiety and fullness using a visual analog scale.
Bottom Line: This observational study examined the effects of eating glucose and fructose on brain activity in lean and obese adolescents.
Finding 1: Obese Adolescents’ Brains Responded Differently to Sugar
The researchers discovered that the brains of obese adolescents responded differently to sugar consumption, compared to those of lean adolescents.
In obese individuals, eating glucose or fructose reduced blood flow in the prefrontal cortex — a brain region involved with decision making and behavioral choices.
Glucose also increased blood flow in the hypothalamus — a region involved with appetite — whereas fructose increased blood flow in the ventral striatum — a brain region involved with food rewards, cravings and pleasure (3).
In contrast, when normal-weight individuals ate the same amount of glucose, blood flow increased in the prefrontal cortex, while remaining unchanged in the hypothalamus and ventral striatum.
Interpreting these findings, the authors speculated that sugar intake may reduce conscious control of sugar intake and increase brain activity involved with food reward processing. These effects might promote the overconsumption of sugar.
The results are supported by a previous study showing that obese adolescents had higher ratings of disinhibition (lack of restraint) and impulsivity (4).
Animal studies have also found that regular sugar intake may lead to binge drinking of sugar-sweetened beverages, sugar cravings and increased consumption of other foods when sugar is not available (1).
A previous study in lean adults showed that eating glucose reduced blood flow in the hypothalamus and ventral striatum, whereas fructose did not significantly affect blood flow (5).
Taken together, the results of the current and previous studies indicate that regular and excessive sugar intake might lead to sugar addiction in some people.
Bottom Line: In obese adolescents, sugar increased brain activity in regions involved with pleasure and cravings and reduced activity in regions involved with decision making.
Finding 2: Eating Sugar Increased Hunger in Obese Adolescents
Hunger ratings increased significantly after eating glucose and fructose in obese adolescents, whereas they remained unchanged in those who were lean.
However, ratings of fullness were higher among lean adolescents after drinking the fructose beverage.
These findings suggest that sugar intake may promote excessive calorie intake in obese individuals.
Bottom Line: Eating either glucose or fructose significantly increased self-rated feelings of hunger in obese adolescents. In contrast, glucose and fructose did not affect hunger ratings in lean participants.
Finding 3: Effects on Circulating Levels of Hormones and Sugar
Blood sugar (glucose) rose similarly in both obese and lean adolescents after consuming glucose. However, insulin levels were slightly higher in those who were obese.
A similar but insignificant increase in blood sugar was seen after eating the fructose.
Normally, glucose and fructose consumption reduces the levels of ghrelin — the hunger hormone. Compared to lean adolescents, this suppression of ghrelin levels was lower in obese adolescents.
These changes were associated with changes in blood flow in several brain regions — the hypothalamus, thalamus and hippocampus.
The authors speculated that changes in ghrelin and insulin may possibly contribute to the differences in brain activity. However, the role of insulin and ghrelin in obesity is still unclear.
Bottom Line: The hormones ghrelin and insulin might be involved with the glucose and fructose-related changes in brain activity. However, their exact role is still poorly understood.
The main limitation of the current study is its assessment of brain activity. It measured neuronal activity indirectly by assessing brain blood flow (brain perfusion), a marker of neuronal activity.
Second, the study didn’t include a control group and the data was observational. For this reason, we cannot rule out that factors other than sugar affected brain blood flow.
Third, the researchers didn’t measure eating behavior directly.
Finally, the simple sugars glucose and fructose are usually not eaten in isolation. Rather, they’re found in foods or consumed together as sucrose or high-fructose corn syrup.
Summary and Real-Life Application
In short, this study showed that sugar (fructose and glucose) has different effects on brain activity in obese and lean adolescents.
Specifically, sugar intake among obese individuals was associated with increased activity (blood flow) in brain regions involved with pleasure and food-reward processing and reduced activity in regions involved with decision making.
These findings indicate that obese adolescents are more likely to succumb to cravings and eat excessive amounts of sugar. However, further studies are needed to confirm these findings by measuring eating behavior directly.
Although it’s easier said than done, limiting your sugar intake is one of the most important things you can do to lose weight.