The Highs and Lows of carbohydrates

How do foods affect blood sugar levels? The old myth that a sugary snack will cause a dramatic rise, and then a fall, in blood sugar levels has been overturned as researchers find out more about the physiological effects of foods on blood sugar levels. New ways to measure just how our blood sugar will react to various foods has provided insights into the prevention and control of conditions such as obesity, diabetes and heart disease and has also revolutionised diet in sport and the foods eaten in the hours before physical exertion in sport.

The Glycaemic index (GI)
The glycaemic index (GI) is a way of assessing measuring the effects of food on blood sugar levels. After a carbohydrate food is eaten there is a rise, and subsequent decrease, in blood sugar (glucose). This phenomenon is level known as the glycaemic response. The glycaemic response to the ingestion of carbohydrate-containing foods can be compared to the response to a standard food, such as white bread or glucose. This resulting value measurement is called the glycaemic index of that food.

A ranking on a scale up to 100 rates the increase in blood sugar levels after eating a portion of food providing 50 grams of carbohydrate. A 50-gram portion of pure glucose is the standard that is now accepted for use as the basis of comparison. Foods that break down quickly during digestion, for example rice, have the highest glycaemic indices. They raise blood sugar levels higher and more quickly than foods with low G.I.s. Low G.I. foods, such as beans, break down more slowly, releasing glucose gradually into the bloodstream.

A number of factors influence the rate and duration of the glycaemic response. For example, it depends on the type of sugar in the food (whether it is sucrose - table sugar-, lactose, fructose, glucose or some other sugar) and the nature and the form of the starch (some are more digestible than others). The cooking and processing methods used and the amount of other nutrients in the food, such as fat or protein, can also affect the response. In addition, individuals differ in their metabolism and the time of day the carbohydrate is ingested can affect the glycaemic response.

GI surprises
The finding of the glycaemic index resulted in a few surprises. Originally it was believed that "complex" carbohydrate foods, such as bread, rice and potatoes, were digested slowly, causing a gradual increase in blood sugar levels. However, it is now known that many starchy foods are digested very rapidly and absorbed quickly. They are high G.I. foods. On the other hand, moderate amounts of many “sugary” foods (those high in sucrose), such as confectionery, do not generally produce dramatic increases in blood sugar as has always been thought. Foods containing sucrose actually show quite low-to-moderate blood sugar (glucose) responses - lower than foods like rice.

Implications for health
The finding of the glycaemic response may have implications for several conditions including diabetes, overweight and obesity, and heart disease as results from some studies suggest that foods with a low GI factor could help to control hunger, appetite and blood sugar levels.

One of the most important implications of the GI factor is in the area of diabetes. A higher intake of low rather than high G.I. foods results in slower digestion of starches and sugars and slower absorption of sugar into the bloodstream. This in turn may help to regulate blood sugar levels although long-term studies on the overall health benefits are not yet available. In the past, people with diabetes were told to avoid sugar as it was thought to raise blood sugar levels rapidly. Most current recommendations allow a modest amount of sugar as the inclusion of sugar with a meal has little impact on either blood sugar (glucose) or insulin concentrations.

Weight reduction is often important for people with non-insulin dependent diabetes and low GI foods may help by promoting satiety. A diet that is comprised largely of carbohydrate-rich, low GI foods also tends to be low in fat, which may benefit weight control.

A diet with mostly low GI foods may also help reduce the risk of other diseases, such as heart disease and hypertension, although results are not conclusive. Insulin, a hormone needed for carbohydrate metabolism, has a profound effect on the incidence of many diseases. It is thought that high insulin levels are one of many factors in the development of heart disease and hypertension. The consumption of a diet rich in low GI foods will help to lower raised insulin levels.

The GI factor is also relevant for optimum sports performance. For athletes, high and moderately-high GI foods can be used to replenish energy stores faster than low GI foods when taken immediately after an event. One study has suggested that low GI foods can help to prolong endurance but this has not been replicated by other researches.

Look at the whole food
Findings from GI research suggest that a low fat diet comprising mostly carbohydrate foods with an emphasis on those foods with a medium to low GI factor, may offer health benefits. However, the G.I. factor should not be used in isolation. The total amount of carbohydrate, the amount and type of fat, the amount and quality of protein, dietary fibre and the vitamin, mineral and salt content of a food are also important considerations when choosing healthful diets.

The Glycaemic Index of Some Common Foods using glucose as standard.
Foods with a low G.I. Factor (G.I. less than 55)
Noodles and pasta
Lentils
Apples/apple juice
Pears
Oranges/orange juice
Grapes
Low fat yoghurt
Fruit bread
Baked beans
Chocolate

Foods with an Intermediate G.I. factor (G.I. 55-70)
Basmati rice
Banana
Rolled oats
Soft drinks
Sweet corn
Pineapple
White sugar
Foods with a High G.I. factor (G.I. > 70)
Bread (white or wholemeal)
Baked potato
Cornflakes
French Fries
Honey
Mashed potatoes
White rice (low amylose or “sticky rice”)
Source:

Foster-Powell, K., Holt, S.H.A., Brand-Miller, J.C. 2002. International tables of glycemic index and glycemic load values. Am J Clin Nutr, 76:5-56.
Foster-Powell, K and Brand-Miller, 1995. International tables of glycaemic index. Amer. J. Clinical Nutrition, 62 (supp) 871-93