Why a low GI meal makes you feel full

Eating a meal with a low GI (glycaemic index) increases gut hormone production which leads to suppression of appetite and the feeling of fullness. This is the finding of new research being presented at the annual Society for Endocrinology BES meeting in Harrogate. Researchers from King’s College London studied the effects of a low versus high GI meal on levels of gut hormones. This is the first study to provide clues as to how a low GI meal produces satiety.

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. To address this, Dr Reza Norouzy and colleagues at King’s College London 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(2).

Volunteers who ate a low GI breakfast had 20% higher blood plasma levels of GLP-1 (area under curve = 4,839±1,831) and 38% lower levels of insulin (10,088±4,757), compared to those who had consumed a high GI breakfast (3,865±1,630 and 1,6245±7,600 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.

Researcher 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."

(Source: The Society for Endocrinology: Annual Society for Endocrinology BES meeting, Harrogate: April 2009)