This is a way of measuring how the food that you eat affects the amount of sugar in your blood.  This is important for a number of reasons.

A food with a GI value of 55 or less is labelled “Low GI”.  The food will have a slower, longer impact on you, energy-wise.  High GI foods (those with a GI above 70) will be used by your body before any stored energy like body fat. And this may make weight loss more difficult.

French Baguette - 95

Bagel - 72

White Bread - 71

Croissant - 67

Wholemeal Rye - 62

Hamburger Bun - 61

Pitta - 57

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Sourdough Wheat - 54

Whole Wheat - 49

Sourdough Rye - 48

Pumpernickel - 46

Heavy Mixed Grain - 45

Soya and Linseed - 36

Source

http://www.the-gi-diet.org/lowgifoods/