In the first study to provide clues as to how a low GI meal produces satiety, researchers studied the effects of a low versus high GI meal on levels of gut hormones, and found that eating a meal with a low GI (glycemic index) increases gut hormone production which leads to suppression of appetite and the feeling of fullness. 

GI is a ranking assigned to carbohydrates according to their effect on the body’s blood sugar levels. A low GI meal takes longer to digest and releases sugar into the bloodstream more slowly than a high GI meal.  High GI foods include white bread, croissants and cornflakes, whereas granary bread, milk and most fruit and vegetables are all classed as low GI foods.

A low GI diet is known to cause reduced appetite but the mechanisms behind this have so far remained unknown.  Dr Reza Norouzy and colleagues looked at the effects of a single low versus high GI meal on gut hormone levels in twelve healthy volunteers.  Each participant ate an identical medium GI meal for dinner, fasted overnight, and was given either a low (46) or high (66) GI meal for breakfast.  Blood samples were then taken every 30 minutes for 150 minutes, and levels of the gut hormone glucagon-like peptide 1 (GLP-1) and insulin measured.  GLP-1 is a hormone produced by the gut that has been shown to cause a feeling of fullness and suppression of appetite.

Volunteers who ate a low GI breakfast had 20% higher blood plasma levels of GLP-1 (area under curve = 4839±1831) and 38% lower levels of insulin (10088±4757), compared to those who had consumed a high GI breakfast (3865±1630 and 16245±7600 respectively).  These results show for the first time that eating a low GI meal increases GLP-1 production and suggest a physiological mechanism as to why a low GI meal makes you feel fuller than a high GI meal.

Dr Reza Norouzy said, “Our results show for the first time the direct effect of a single GI meal on gut hormone levels.  We already know that the hormone GLP-1 and a low GI meal independently lead to suppression of appetite.  This study builds on these findings by providing a physiological mechanism to explain how a low GI meal makes you feel fuller than a high GI meal.  GLP-1 is one of the most potent hormones for suppressing appetite.  Our results suggest that low GI meals lead to a feeling of fullness because of increased levels of GLP-1 in the bloodstream.  This is an exciting result which provides further clues about how our appetite is regulated, and offers an insight into how a low GI diet produces satiety.  This is a preliminary study that only involved a small number of people.   We now need expand these findings and look at the effects of low versus high GI meals in a larger cohort of people.”
References:
1. Reza Norouzy, et al. Society for Endocrinology.