Different Sugars Exert Different Effects on Brain

A number of previous studies observe that increases in fructose consumption parallel an increasing prevalence of obesity, as high-fructose diets are thought to promote weight gain and insulin resistance. Whereas fructose ingestion produces smaller increases in circulating satiety hormones as compared with glucose ingestion, in lab animal models glucose promotes satiety.  A Yale University School of Medicine (Connecticut, USA) team studied neurophysiological factors that might underlie associations between fructose consumption and weight gain.  Robert Sherwin and colleagues conducted functional MRIs (fMRIs) in 20 normal-weight, healthy adults who were given 75 grams of either glucose or fructose in a cherry-flavored drink, and then crossed over to a drink with the other sweetener.  Participants rated their feelings of hunger, satiety, and fullness before and after the scan, and the researchers took blood to assess circulating hormone levels. Overall, the researchers found that glucose significantly reduced cerebral blood flow in the hypothalamus, while fructose did not. Specifically, blood flow fell 5.45 mL/g per minute from baseline with glucose, compared with an increase of 2.84 mL/g per minute with fructose, for a mean difference of 8.3 ml/g per minute. As well, the team found that glucose reduced cerebral blood flow in the thalamus, insula, anterior cingulate, and striatum – brain regions associated with regulation of appetite and reward.   In contrast, fructose reduced blood flow in the hippocampus, posterior cingulate cortex, fusiform, and visual cortex — but also in the thalamus.   They also found that glucose, but not fructose, had effects on circulating “hunger” hormone levels, and that glucose elevated levels of insulin and glucagon-like polypeptide 1 (GLP-1), as compared with fructose.