Glycaemic Index & Insulin Index 

New research is suggesting that just as all fats aren't necessarily bad, all carbohydrates are not necessarily good. And it seems the way we have thought about carbohydrates in the past, as simple and complex carbohydrates, is not very useful.
 View a Table of Low Glycaemic Index Foods.

Glycaemic Index

The glycaemic index or GI is a useful concept because it measures how rapidly the carbohydrates are absorbed and result in blood glucose and insulin elevations . The GI is not related to whether the carbohydrate is simple or complex. The GI measures the rise in blood sugar levels caused by a measured quantity of a particular food. High GI foods are rapidly absorbed and cause a large rise in blood sugar levels.People with diabetes are used to thinking about glycaemic index, they use it to help control their blood sugar levels, but until now we have not really been aware of  its significance for people without diabetes. 

Some of the so-called complex carbohydrates advocated, like potatoes and white bread and low amylose rice, are absorbed very quickly and give an undesirable metabolic response. The body tries to bring those blood glucose levels back down by secreting insulin. The high GI foods generate a demand for insulin. Since hyperinsulinaemia is linked with all facets of the ‘metabolic syndrome’ (insulin resistance, hyperlipidemia, hypertension and visceral obesity) the GI of foods may eventually be linked with all so-called diseases of affluence e.g. heart disease, obesity, diabetes.  Even small physiological increases in insulinaemia for as little as 3-5 days can induce severe insulin resistance in healthy young subjects with normal glucose tolerance and no family history of NIDDM  (Del Prato et al., Diabetologia 1994; 37: 1025-35). Higher day-long insulin levels are believed to promote carbohydrate oxidation at the expense of fatty acid oxidation, thereby promoting fat storage in adipose tissue and triglyceride synthesis in the liver (Friedman, Ma J Clin Nutr 1998; 67:513S-8S). Thus low GI diets may promote weight control by both enhancing satiety and promoting fat oxidation. A lower insulin level means the body stores less fat and can access existing fat stores more easily than when insulin levels are higher.

The Glycaemic Index and Health

Diabetes & Glycaemic Index
High GI foods are rapidly absorbed and cause a large rise in blood sugar levels and insulin. Thus, one may well expect that high GI foods could be linked long term to the risk of developing type 2 diabetes and of insulin resistance. Although not all studies have found this to be so, the weight of evidence suggests clearly that this is indeed the case (Am J Clin Nutr. 2004 Aug;80(2):243-4.), and two new studies (below) add to that weight.

Study 1: A US study published in the American Journal of Clinical Nutrition in 2004 (Vol 80, no.2, pp 348-56) on
91,249 adult women showed that a higher GI and lower amounts of cereal fibre in the diet (especially when combined with inactivity) are associated with greater risk of developing type 2 diabetes.

Study 2: Another US study published in Diabetes Care in 2004 (Feb;27(2):538-46) on 2,834 adults showed that the likelihood of developing insulin resistance (pre-diabetes) was reduced in those who consumed more
cereal fibre and whole grain and had a lower GI diet, but the association with glycaemic load was not significant.
The results are consistent with a recent Australian prospective study, involving 36,787 adults and 365 cases of diabetes (Diabetes Care. 2004 Nov;27(11):2701-6).

A study on  6500 nurses in the US showed that  those who consumed diets high in carbohydrate from white bread, potatoes and low amylose varieties of rice had 2-5 times the risk of developing diabetes than those who ate a diet rich in high fibre less processed cereals - even after controlling for known risk factors such as age and BMI (Salmeron et al., JAMA 1997; 277: 472-77). For the risk of diabetes, the type of carbohydrate (low versus high GI) was more important than the total amount of fat and carbohydrate in the diet. Furthermore, the total amount of refined sugar in the diet was not a risk factor for diabetes. 

Diabetes & Glycaemic load
Even though sugar per se has not been linked with any diseases of affluence, diets with a high glycaemic load have been associated with an increased risk of  diabetes, obesity (especially abdominal/visceral obesity) and heart disease  (Brand-Miller “The Glycaemic Index: Implications for Food-Based Dietary Guidelines” In: NH&MRC  Dietary Guidelines for Older Australians, 1999). The male health professional study has also demonstrated a link between GI load of the diet and risk of  developing diabetes in men (Salmeron et al. Diabetes Care 1997; 20: 545-550).

Heart Disease & Glycaemic Index
A similar picture has emerged with acute coronary heart disease in the nurse's study (Liu et al.,  Fed Am Soc for Exp Bio J 1999; 12: 260). In another study on 1400 British adults (Frost, The Lancet 1999; 353: 1045-48)  blood lipids were more favourable with diets which consisted of foods with a low GI  i.e. "good carbohydrates and good cholesterol". 

Glycaemic Index and Food

Low GI foods include pasta, legumes, oats, whole grain bread/cereals, whole fruit, minimally processed/cooked foods. Since potatoes have a high GI and can constitute 15-20% of the total starch intake in a typical Western diet they could be a significant factor in the total glycaemic load. Current advice to increase the intake of starchy foods such as potatoes may lead to a greater risk of  diabetes (Eu J Clin Nutr 1999; 53: 249-54), abdominal obesity, elevated blood lipids, hypertension and heart disease.

An important finding of GI research is that foods containing refined sugars often have less glycaemic impact than starchy staples like bread. Moderate intake of refined sugars (10-12% energy) found in foods (e.g. cakes) or added to foods (e.g. coffee) is not associated with obesity, micronutrient deficiency or undesirable effects on blood lipids or insulin sensitivity  (Anderson  Nutr Res 1997; 17: 1485-8). This finding has helped to liberate the diabetic diet. However, many foods containing refined sugars also contain undesirable saturated fats and some undesirable high fat foods (e.g. potato crisps) can also have a low GI . Therefore, when selecting a low GI food one should also consider the total amount of carbohydrate, fat, fibre and salt and whether it is a nutrient or phytochemically dense food.

What factors can alter the Glycaemic Index of a food?

1.        The degree of processing/cooking/chewing: the more processed/refined a food is, the higher the GI.  Foods which are more textured, chewy, crunchy, fibrous tend to take longer to be digested and release their glucose into the blood stream more slowly than soft, refined or pre-cooked foods. Long grain white rice has a lower GI than quick cooking brown rice and multigrain bread has a lower GI than wholemeal bread. Food that has not been properly chewed also has a lower GI – it may also give you indigestion.  The method of baking bread appears to influence its GI; traditional slow rising bread dough (e.g. sour dough fermentation) can have a lower GI than breads made with rapid-rise dough.

2.        Fibre type:  soluble fibre slows down the digestion of starches and the absorption of glucose into the bloodstream  e.g. oat fibre (oats, oat bran, oat fibre flour), fruit pectin (especially cold climate fruits), legume fibre (baked beans, lentils etc) and psyllium (Metamucil, Fybogel).  Porridge has a lower GI than Weetbix (has only when bran) and plums have lower GI than watermelon.

3.        Starch type: Resistant starch is a type of starch which is slowly digested. Amylose (long chains of sugars joined together) takes longer to be broken down than starches made up of branches of sugars (amylopectins). Cold cooked potato has a lower GI than freshly cooked white potatoes,  new potatoes have lower GI than desiree potatoes and long grain rice lower GI than short grain rice.

4.        Degree of ripeness:  the riper the food the higher the GI e.g. yellow/black bananas vs. greenish bananas.

5.        Acidic VS Salty: acidic foods in a meal help slow digestion of starches generally, which reduces the GI of the meal e.g. lemon juice on vegetables, vinaigrette dressings on salad, pickled foods such as gherkins. Increasing the acidity of bread by using sour dough fermentation. Salt and salty foods/condiments tend to speed the rate of digestion of starches and increase the rate of absorption of glucose and increases the GI of the meal.

6.        Types of sugars:  pure glucose has a maximum effect on blood glucose e.g. Glucodin, glucose syrup (used in cake/confectionery manufacture), some sports drinks, Lucozade and as ‘dextrose’ in many foods. These foods will have a high GI.
Fructose occurs naturally in many fruits, some vegetables (corn, sweet potato), corn syrup, honey. Fructose is absorbed as fructose and contributes very little to blood glucose levels. High fructose foods have a lower GI. Lactose and sucrose have an intermediate effect on blood glucose levels. Honey has an intermediate GI (58).
The concept of GI has meant that people with diabetes do not need to be as strict about every teaspoon of sugar as they once were. A little sugar added to otherwise healthy and lower GI foods, such as sugar on porridge, marmalade on grainy toast is generally acceptable. For someone with diabetes, a moderate amount of table sugar, say the equivalent of 2 tablespoons over a day, is now generally acceptable.

7.        Combination of foods in mixed meals:  eating protein rich food in the same meal lowers the overall GI of the meal. Protein foods delay stomach emptying which delays digestion of the starches. Spaghetti with meat sauce has a lower GI than spaghetti with tomato sauce. Foods or meals with a higher fat content will show a lower GI than those with a lower fat content. Like protein, fats delay stomach emptying. However, in the interests of overall good health, restrict the fat to monounsaturated oils used in cooking and avoid butter and other hidden fats (meat fat, fats in fast food/commercial cakes/biscuits, vegetable ‘fats’, hydrogenated fats).

GI and Athletic performance

Research with sports people has shown that foods with a low GI eaten 2-4 hours before a sports event provides a slower release of energy and an improved ability to last longer in endurance activities. Pre-event low GI foods are now added to high carbohydrate loading practices which are used up to 4-5 days before an endurance event.  The GI index in foods is unlikely to influence performance during an event, as most athletes these days take quick acting glucose through glucose polymer drinks to replenish energy supplies while they are competing. ‘Re-loading’ however, after an athletic event to ensure maintenance of high energy levels for ongoing training, is still seen as important in sports and high GI foods are recommended for post event replenishment. Bread, potatoes and other high GI foods then, may have more value after the event than before, when fruit sugars and legumes are more likely to be useful.

Insulin Index

The Insulin Index (ID) is a relatively new concept which measures the amount of insulin the body produces in response to a set carbohydrate load in a particular food. This index is not necessarily proportional to the GI and consumption of large volumes of food with a high insulin index may play a role in the development of insulin resistance, although the link has yet to be conclusively established. High protein and high fat foods stimulate greater insulin responses than predicted by the level of glycaemia and in a study of insulin responses to 1000kJ portions of common foods, ordinary bread showed amongst the highest scores of any of the foods tested.  More exaggerated insulin responses are seen when people with underlying insulin resistance consume high GI foods. Hyperinsulinaemia has recently been shown to be an independent risk factor for coronary heart disease (Depres et al. NEJM 1996; 334:952-7). Thus an insulin index of foods may eventually be needed to supplement tables of GI (Holt et al. AJCN 1997; 66: 1264-76). Both indices compare the body’s response to a food on a scale where the response to the glucose control is set at 100. Choosing foods with a low GI and low ID can improve diabetic management and may possibly reduce the incidence of diabetes complications e.g. heart disease, renal disease.

Summary

What does this mean for the average person? If you have a family history of diabetes or if you are overweight/obese and/or inactive then consuming low GI foods may help prevent insulin resistance or diabetes developing. If you already have diabetes, low GI foods can help manage blood sugar levels.

Foods with a low GI are generally more filling, more sustaining, help control hunger and appetite as well as blood glucose levels and may help with weight loss. The lower the GI of a food, the better the carbohydrate food is for everyone, especially if it also low in fat and added sugar. The exception is if blood glucose falls below normal in people with diabetes. This is more likely if you are an insulin-dependent diabetic. A high GI food or drink will restore blood glucose levels more quickly. Follow up with a low GI food to maintain blood glucose levels.

It is not always possible to choose all lower GI foods. If you mix a low GI food with a high GI food, you will get an intermediate GI for that meal. Try to include a good proportion of ONE low GI food at each meal (MacDonald, Nutr Issues & Abstracts, 1995). It is better to have 4-5 small meals a day containing at least one GI food than to heave fewer larger meals. These strategies will allow a slow diffusion of energy through the body, thus eliminating peaks and troughs of blood sugars, tiredness and alertness.

Remember:
a) The GI list is a guide to relative effects of different carbohydrate foods on blood glucose levels, under research conditions. Individuals may react to individual foods and combinations differently.
b) The GI is not based on the typical serving size of a particular food, but rather on 50g of carbohydrate. For example, 50g of carbohydrate is found in about 2 slices of bread OR 500ml of soft drink OR about 1kg of carrots - all these foods have a similar GI when eaten in these amounts. So, if a food has a medium to high GI, but is not ordinarily consumed in large quantities then the value of the GI is not an issue. In contrast, if it is consumed in large quantities then the GI and total carbohydrate consumed is of concern.

The American Diabetes Association issued new nutrition guidelines for the treatment and prevention of diabetes and its complications in January 2002. The new guidelines outline strategies for controlling blood sugar levels, for example they state that "the total amount of carbohydrate is more important than the source or type; the glycemic index may reduce post-meal blood glucose, but studies do not show sufficient evidence of long-term benefit for it to be recommended". For example, even though pasta has a low glycaemic index it is not advisable for people with diabetes or impaired glucose tolerance to have a large serve because the total amount of carbohydrate will be too high.

See also: Glycemic index symbol on food products

Last Updated: March 2005.