The glycaemic index (GI) is a measure of the rate of increase in plasma glucose following consumption of a carbohydrate food. Generally a reference food which is either white bread or glucose power is given a value of 100, and the food to be tested is rated in relation to this value. In practice, 50 grams of the test carbohydrate are compared to 50 grams of the reference standard under conditions of fasting blood glucose levels, and the area under the glucose curve used as a comparison. Higher values close to 100 represent faster rises in blood sugar and this is generally considered to be detrimental to health because this results in hypersecretion of insulin which may be a driver of insulin resistance and metabolic syndrome. The glycaemic index is widely used by nutritionist, the medical establishment and government agencies to determine the quality of carbohydrate choices. However, more recently the use of the glycaemic index has come under scrutiny as it might not be as useful as once believed.
One of the main problems with the glycaemic index is that it does not take into account the amount of a food normally consumed. In this regard the glycaemic load is a better measure of the effects of a particular carbohydrate in the real world because it corrects for the amount of carbohydrate in a particular food. For example, many vegetables are high on the glycaemic index and will elicit rapid rises in blood sugar levels when 50 grams of the food are tested against 50 grams of the reference standard. Based on this, the neophyte nutritionist might conclude that these foods are poor choices and should not be eaten. Parsnips, swede and beets for example are vegetables that have glycaemic indices of 97, 73 and 61, respectively. However, while the carbohydrate in the foods might be very rapidly absorbed to the blood, the foods are actually low in carbohydrate and so normal consumption does not produce rapid elevations in blood sugar levels. In this way the glycaemic index is misleading because it does not take into account serving size.
Another major problem of the glycaemic index is the fact that the values for many foods vary so widely that it becomes difficult to discern the true glycaemic index within a real world setting. Values for white rice for example can vary from 45 to 112 and spaghetti made from white durum-wheat semolina can vary from 46 to 65. This is largely due to the role of cooking in the determination of the glycaemic index values. Heat can change the carbohydrate structure within foods such as rice and spaghetti and therefore the degree of cooking will alter the availability to the carbohydrate to digestion and this will affect the absorption rate. Bananas can range on the glycaemic index from 30 to 70 and this relates to the ripening process of many fruits whereby starch within the fruit is converted to sugars as the fruit ripens. Measurements have also been made of processed and packaged breakfast cereals which might be assumed to show some consistency, and yet the same foods in the same packaging often vary wildly in their glycaemic index values.
It also becomes immediately apparent to anyone who has studied the glycaemic index table that fructose has a very low value of around 20. This might tempt suggestion that it is a wise choice of carbohydrate for those who wish to maintain better control of their blood sugar. However, this leads us to another major problem with the glycaemic index, and that is that it is only a measure of glucose response. Consumption of fructose is often recommended to individuals with diabetes because of its low glycaemic index value. However, while fructose is not problematic in terms of its blood sugar raising effect, it is a primary driver of insulin resistance. This is because fructose is converted in the liver to fatty acids through the de novo lipogenesis pathway, and these fatty acids are thought to be deposited in skeletal muscle and hepatic tissue where they lead to the development of peripheral and central insulin resistance, respectively. Insulin resistance then causes insulin hypersecretion in response to glucose, leading to further detrimental metabolic effects.
The pattern of hormone release seen with the ingestion of fructose is completely opposite to that of glucose, and this is also problematic. Fructose does not cause the release of insulin, it inhibits leptin release and it does not suppress ghrelin. Because insulin, leptin and ghrelin are important regulators of satiety with the hypothalamus fructose may have opposite effects on appetite compared to glucose. Both insulin and leptin decrease feeding behaviour, and in addition to this role, leptin increases energy expenditure and insulin suppresses the pleasure response from food. While glucose causes the release of leptin and insulin, fructose does not. Ghrelin rises during fasting and is a strong appetite stimulatory hormone. Glucose inhibits ghrelin following ingestion of carbohydrates, but fructose does not. Therefore carbohydrates that increase blood glucose levels switch off appetite and increase energy expenditure, but fructose does not do this. The glycaemic index is therefore not a good judge of the metabolic effects of fructose containing foods.
The glycaemic index is a useful research tool because it directly compares the blood sugar effects of 50 grams of glucose or white bread with 50 grams of a test food. In this respect, as long as it is used in content and in relation to other measures of carbohydrate rating, it can be useful. However, the use of the glycaemic index by the layman to make food choices is not recommended. Simplifying the index as a measure of the benefit of carbohydrates has done a disservice to the field of nutrition and provided erroneous and misleading information to the public. In particular, the inability of the glycaemic index values to highlight the detrimental metabolic effects of fructose are concerning. The road to hell is paved with good intentions and this could not be more true for the recommendation of carbohydrates based on solely on their plasma glucose response. Instead, a shift towards understanding the overall carbohydrate quality is required to improve diet quality, encompassing a holistic view of carbohydrates through multiple metabolic pathways.
RdB
