THE G.I. FACTOR: WHAT GIVES ONE FOOD A HIGH G.I. FACTOR AND ANOTHER FOOD A LOW ONE?
Scientists have been studying what makes one food high and another low for more than fifteen years. There is a wealth of information that can easily confuse. We have summarised the results of their research in the following table which looks at the factors which influence the G.I. factor of a food.
The key message is that the physical state of the starch in the food is by far the most important factor influencing the G.I. value. That’s why the advances in food processing over the past two hundred years have had such a profound effect on the overall G.I. factor of the food we eat.
Particle size. Another factor that influences starch gelatinisation is the particle size of the food. Grinding or milling of cereals reduces the particle size and makes it easier for water to be absorbed and enzymes to attack. That is why cereal foods made from fine flours tend to have high G.I. factors. One of the most significant alterations to our food supply came with the introduction of steel roller mills in the mid-nineteenth century. Not only did they make it easier to remove the fibre from cereal grains, the particle size of the starch was smaller than ever before. Prior to the nineteenth century, stone grinding produced quite coarse flours that resulted in lower rates of digestion and absorption.
When starch is consumed in its natural packaging—whole intact grains that have been softened by soaking and cooking—the food will have a low G.I. factor. For example, cooked barley has a G.I. factor of only 25. Most cooked legumes have a G.I. factor between 30 and 40. Cooked whole wheat has a G.I. factor of 41.
The only whole (intact) grain food with a high G.I. factor is rice, specifically low amylose rice, such as Calrose rice at 83. These varieties of rice have starch which is very easily gelatinised during cooking and therefore easily broken down by digestive enzymes. This may help explain why we sometimes feel hungry not long after rice-based meals. However, some varieties of rice (Basmati, a long grain fragrant rice, and Doongara, a new Australian variety of rice) have lower G.I. factors because they have a higher amylose content than normal rice. Their G.I. factors are in the range of 54 to 64.
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