Foods containing carbohydrates that are easily digested, absorbed, and metabolized have a high GI while low-GI foods have slowly digestible carbohydrates that elicit a reduced postprandial glucose response. 
Carbohydrates that are higher on the GI scale, given a value of 70 or higher, can contribute to a rapid rise in blood sugar levels, whereas foods lower on the GI scale can lead to a much slower response. Simple carbohydrates, such as table sugar, juice, sweetened soda, and candy, are generally higher on the GI scale, whereas most complex carbohydrates and high-fiber foods (such as whole grains, legumes, and vegetables) are much lower. Higher GI foods may be helpful for athletes during pre-training and post-training periods. Easily digestible carbohydrates provide a ready source of fuel when taken immediately prior to training, and in the post-training period, they provide immediate glucose for rapid glycogen replenishment.