A low-GI food has a glycaemic index of less than 55. You can buy a book that lists the GI value of foods or look it up on the internet. You only need to look at the GI for carbohydrate-rich foods.

The higher the number, the more it can increase your blood sugar. Foods with a number between 1 and 55 are considered "low GI," like beans, non-starchy vegetables, and bran cereal. From 56 to 69 ...

Low-glycemic foods, such as sweet potatoes, rolled oats, chickpeas, and milk, are slower to raise a person’s blood sugar levels than foods with moderate or high scores on the glycemic index (GI).