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The pros and cons of the glycaemic index (GI) diet

Anna Wheeler
BSc RNutr

It was first discovered that meals containing the same quantity of various carbohydrate foods showed different glycaemic responses in the early 1970s. In 1981, Jenkins expanded on this and created the glycaemic index (GI) to classify carbohydrates according to the blood glucose response that they induced rather than their chemical structure.(1)
Thus the GI is a method of ranking carbohydrates according to how much they raise postprandial plasma glucose concentrations and overall blood glucose response. It is defined as the incremental area under the blood glucose curve after the consumption of 50g carbohydrate from a test food, divided by the area under the curve after eating a similar amount of a control food (usually glucose).(2) The area under the glucose curve is usually measured for two hours after the test food is eaten. Foods with a GI of 65 or above are classed as high; between 50 and 65 is considered as medium GI; and anything below 50 is low GI.

Characteristics of the GI
The GI of individual foods is difficult to predict. Values are affected by various factors, such as how the food has been cooked or processed, and other inherent factors, such as the ripeness of fruit. For example, mashed potato has a high GI, whereas boiled new potatoes have a much lower GI; and a ripe banana has a higher GI than one that is slightly green. The only reliable way to find out the GI of a food is to consult tables of foods that have been tested in order to evaluate their effect on blood glucose levels.

Tables of GI values
In 1995, the first list of GI values for different foods was published by Foster-Powell and Miller.(3) This was updated in 2002,4 and it now contains values for over 750 different types of foods tested with standard methods.
A criticism of GI has been that more than one value is often given for certain foods (usually including an average) and that it is not always clear which is the most appropriate value.
Table 1 shows the GI of various everyday foods.

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Mixed meals
There has been debate over whether the GI can be applied to mixed meals or only single foods. It is known that the glycaemic response to a carbohydrate food when consumed on its own differs from that when consumed as part of a meal. This is due to the effects of other nutrients. Protein and fat are known to decrease postprandial glucose absorption, and they also affect insulin response.
Foods containing little or no carbohydrate, such as meat, poultry, fish, cheese and eggs, are not given a GI value, as it would be difficult to consume a portion of these foods that would contain 50g carbohydrate. These foods are unlikely to cause a significant rise in blood glucose but will affect the glycaemic response in a mixed meal.

Glycaemic load
The glycaemic load addresses some of the problems with the use of GI in practice. It is calculated by multiplying the dietary GI of a food by the total amount of dietary carbohydrate it contains.6 It therefore takes into account the amount of carbohydrate in the food as well as the glucose response it produces. For example, it has been noted that carrots have a high GI (although values vary widely between 16 and 92), but the glycaemic load is low, as carrots do not contain large quantities of available carbohydrate.(4)

Health properties
Many research papers have referred to links between the GI and either health or disease. The mechanism by which it is thought that low-GI foods are beneficial to health is through decreasing serum glucose and insulin responses.(7,8) A number of large-cohort studies have reported the benefits of low-GI diets. These include improved glycaemic control in patients with diabetes and decreased risk of developing diabetes and cardiovascular disease. However, the evidence has been disputed and the debate continues. Also, evidence of long-term compliance with a low-GI diet and effectiveness over longer time periods has yet to be established.

Diabetes
Some epidemiological evidence suggests that regularly consuming high-GI foods may increase the risk of developing type 2 diabetes.(9)
A low-GI diet may be beneficial for people with established diabetes. Although traditional advice was to reduce consumption of refined carbohydrates and increase intake of starchy carbohydrates, it is now known that sucrose does not raise blood glucose concentrations any more than isocaloric amounts of starch.(10) It may be more useful for people with diabetes to consider the glycaemic response from carbohydrate foods in order to attempt to control blood glucose levels. Starchy foods and sugary foods exhibit varied glycaemic responses. Some sugars have a high GI (eg, glucose); others have a low GI (eg, fructose). Similarly, some starchy carbohydrates have a high GI (mashed potato), whereas others have a low GI (sweet potato, yam).
In The Implementation of Nutritional Advice for People with Diabetes,(10) Diabetes UK acknowledges the potential usefulness of GI for people with diabetes and recommends more active promotion of carbohydrate foods with a low GI. However, they also recognise the limitations discussed earlier. They say: "Patients should be advised against placing too much reliance on tables of GI values found in popular diet books, because the information can be misunderstood and misused. Moreover, excessive emphasis on the GI content of a meal may divert attention away from other more important aspects, such as fat or calorie content." Therefore GI may have a role in the diets of people with diabetes, but this should not be the only consideration.

Coronary heart disease
There is limited evidence to suggest that low-GI diets improve insulin sensitivity in various populations, including patients with coronary heart disease (CHD).(11)
Prospective short-term studies have shown that low-GI diets lead to lower levels of blood triglycerides, while not adversely affecting HDL-cholesterol concentrations.(12)
Conversely, some researchers have refuted the long-term reliability of these claims. The Cochrane Review of Low GI Diets for Coronary Heart Disease states: "There is no evidence that low glycaemic index diets have an effect on LDL cholesterol or HDL cholesterol, triglycerides, fasting glucose or fasting insulin levels."(13) Further longer-term randomised controlled studies are needed in order to assess the effects of low GI diets for CHD.

Obesity
Low-GI diets may be useful in weight control, perhaps because of increased satiety and consequent reduced energy intake. It has been reported that, after consumption of high-GI foods, lower satiety, increased hunger and increased voluntary food consumption were observed.(14) However, other studies have shown no difference in body weight between low- and high-GI diets.(15) At the present time, definitive studies have not been completed, with most studies either short in duration or of poor study design. The evidence for this hypothesis remains to be verified in longer-term
intervention trials.

Cancer
Some studies suggest there may be a link between high-GI diets and cancer; higher risks of colorectal cancer and breast cancer have been reported,(16,17) but these studies require confirmation. There is insufficient evidence to draw conclusions on any links between GI and other types of cancer.

Sports nutrition
One area of research that has shown a positive role for moderate- or high-GI foods is sports nutrition. Research has shown that a diet high in carbohydrate, obtained from either simple sugars or complex carbohydrates, is effective in improving exercise performance. However, the glycaemic index may be utilised to confer extra advantage to the athlete. For example, carbohydrate-rich foods with a moderate-to-high GI provide a fast and readily available source of carbohydrate for glycogen storage and therefore could be valuable in recovery meals to accelerate postexercise refuelling.(5)

Implications within clinical practice
The GI is a difficult concept to explain to patients. There is wide variability in the GI of foods and meals, due to factors such as food processing, cooking methods and presence of other nutrients, as well as variability between individuals in the way foods are digested. If people were to restrict themselves to only low-GI foods, such as chocolate and ice-cream, their diet is likely to be unbalanced and high in fat. It is therefore important to use the GI in context as part of a varied healthy eating plan and not to rely on it totally for food choice.
Tables of GI values can be misunderstood or misinterpreted. For example, it is easy to assume that a low- GI food will have less effect on glycaemia than a high- GI food. However, this is only the case if equal amounts of carbohydrate are consumed. In the case of a bowl of pasta (low GI) compared with a slice of bread (high GI), the pasta would give a higher glycaemic response due to the portion sizes involved. Thus there is an argument that glycaemic load, which accounts for the amount of available carbohydrate, is a more useful measure.
However, small changes to the diet, such as including some lower-GI foods, may have a positive effect in reducing glycaemia. A practical way to apply the principles of the GI in the diet is to replace high-GI carbohydrates with lower-GI options. This can be done by choosing a low- or moderate-GI breakfast cereal, (eg, porridge or muesli), to replace a higher-GI cereal (eg, cornflakes or branflakes). For lunch, patients could choose granary bread or pitta bread, rather than white/wholemeal bread or baguettes, which have higher GI. And for an evening meal, mashed potato can be replaced by sweet potato or yam, and white rice by basmati rice. These simple changes can lower the overall glycaemic response from a meal and may help to attenuate increases in glucose in the bloodstream.

Conclusion
At present there is insufficient evidence to draw firm conclusions on the effectiveness of the glycaemic index as a form of prevention or treatment of disease, or as a means of body weight control. Therefore the glycaemic index should not be recommended to patients as a complete dietary approach. However, it may be beneficial to suggest low-GI foods within conventional dietary and lifestyle advice. Low-GI foods are often associated with other desirable factors, such as high fibre, and therefore may have a role to play in a healthy diet. Use of the glycaemic index may have some merit for certain groups of the population, for example people with diabetes. However, it is important that patients do not focus exclusively on GI and that they concentrate on the overall balance of their diet.

References

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  2. Wolever TMS, Jenkins DJ, Jenkins AL, Josse RG. The glycemic index: methodology and clinical implications. Am J Clin Nutr 1991;54:846-54.
  3. Foster-Powell K, Miller JB. International tables of glycaemic index. Am J Clin Nutr 1995;62:871-93S.
  4. Foster-Powell K, Holt SHA, Brand-Miller JB. International table of glycemic index and glycemic load values: 2002. Am J Clin Nutr 2002;76(1):5-56.
  5. Stear S. Fuelling fitness for sports performance. London: The Sugar Bureau; 2004.
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  7. Slabber M, Barnard HC, Kuyl JM, Dannhauser A, Schall R. Effects of a low-insulin response, energy-restricted diet on weight loss and plasma insulin concentrations in hyperinsulinemic obese females. Am J Clin Nutr 1994;60:48-53.
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  13. Kelly S, Frost G, Whittaker V, Summerbell C. Low glycaemic index diets for coronary heart disease. Cochrane Database Syst Rev 2004;4:CD004467.
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