The Glycaemic Index (GI) is a ranking system for carbohydrates based on their impact on blood glucose levels. It ranks foods on a scale from 0 to 100, with pure glucose assigned a value of 100. Foods with a high GI are quickly digested and absorbed, causing a rapid rise in blood sugar levels. In contrast, low-GI foods are digested and absorbed more slowly, leading to a gradual increase in blood sugar. This can be seen in the figure below.
The Use of GI as a Nutritional Tool
Dietitians and nutritionists use GI to guide individuals in choosing foods that provide a more gradual release of glucose into the blood. This can be particularly beneficial for people with diabetes, as it helps manage blood sugar levels more effectively. A diet focusing on low-GI foods can contribute to sustained energy levels and reduced hunger pangs for the general population.
Glycaemic Load (GL): A More Comprehensive Metric
While GI provides valuable information, it does not account for the amount of carbohydrates in a typical serving size. This is where Glycaemic Load (GL) comes into play. GL is calculated by multiplying the GI of a food by the carbohydrate content per serving and then dividing by 100. This gives a more accurate picture of a food’s actual impact on blood sugar levels.
For example, watermelon has a high GI (around 72), but because it contains a small amount of carbohydrate per serving, its GL is low (around 4). This demonstrates that GL can help make more informed dietary choices.
Moderation is Key: Low GI Doesn’t Mean Unlimited Consumption
It's important to remember that moderation is key when it comes to low-GI foods. A common misconception is that these foods can be consumed in unlimited quantities. However, low GI does not equate to low calories. Foods like nuts and certain dairy products have low GIs but are calorie dense. Overeating these foods may lead to weight gain and associated health issues despite their low GI. So, while low-GI foods are a healthy choice, consuming them in moderation is important.
A systematic review and meta-analysis of 54 studies revealed that low-GI diets offer significant benefits for individuals with impaired glucose tolerance or diabetes, including reductions in HbA1c, fasting blood glucose (FBG), BMI, and blood lipid levels. These effects, though modest, are comparable to those achieved with medications. The diets were particularly effective in lowering total cholesterol (TC) and LDL-cholesterol, especially in those with overweight or obesity. These findings are supported by another systematic review and meta-analysis of 29 studies, which also showed improvements in inflammation and blood pressure in adults with moderately controlled type 1 and type 2 diabetes.
The meta-analysis did not find evidence that low-GI diets contribute to weight loss in people with impaired glucose tolerance, type 1 diabetes, or type 2 diabetes, possibly due to reduced satiety in these individuals. It was concluded that while low-GI diets are effective in improving many aspects of diabetes, they may not be effective for weight loss in those with impaired glucose tolerance or type 2 diabetes. However, the second meta-analysis did find a beneficial effect on weight loss, highlighting how the effect on weight is not clear-cut. In both meta-analyses, low-GI diets had no effect on insulin or HDL-cholesterol.
This review highlights that most clinical trials show a significant improvement in postprandial hyperglycemia when either reducing carbohydrate intake or increasing soluble fibre in meals. The findings support previous research, indicating that low-GI and GL meals positively impact postprandial glucose levels in patients with type 2 diabetes (T2D). Since postprandial glycaemic response is primarily influenced by carbohydrates, both the quality and quantity of carbohydrates, as well as the amount of soluble fibre, are crucial in managing glucose levels after meals. Although low-GI diets generally improve overall glycaemic profiles and reduce HbA1c in T2D patients, they may not adequately control postprandial hyperglycemia unless the carbohydrate type and amount in each meal are considered. Additionally, low-GI diets often contain more fibre, which further helps in lowering glycaemic responses. Therefore, for effective management of postprandial glucose levels in T2D, dietary strategies should include careful consideration of both carbohydrate content and energy intake, alongside following a low-GI diet.
It has been put forward that other dietary factors, such as fibre intake and overall diet quality, may play a more critical role in influencing health outcomes than GI or GL alone. Despite some concerns that the effects of low GI diets might be attributed to dietary fibre, this meta-analysis found that fibre did not significantly influence glycaemic control outcomes, though it did appear to contribute to improvements in blood lipids and blood pressure. The study suggests that focusing on both carbohydrate quality and quantity through low GI/GL diets may offer broader health benefits than focusing on carbohydrate quantity alone. The critique that the benefits of a low GI diet cannot be separated from its components, such as fibre, applies to all dietary patterns, as foods high in fibre also contain various other nutrients and bioactive compounds that contribute to health.
In terms of the effect of low-GI and low-GL diets on cardiovascular health, a study analysed data from 137,851 participants across various countries and regions over a median of 9.5 years. It found that diets with a high GI and glycaemic load GL are linked to a higher risk of cardiovascular disease and death, particularly in individuals with preexisting cardiovascular conditions. A high GI diet tends to worsen insulin resistance, especially in those with higher BMI, leading to increased post-meal blood sugar levels and further health risks. Conversely, diets with a low glycaemic index are associated with lower blood sugar levels after meals, reduced cholesterol and inflammation markers, and lower blood pressure, which collectively contribute to a decreased risk of developing diabetes and cardiovascular disease. The findings underscore the importance of managing dietary glycaemic index to mitigate cardiovascular risks.
Conclusion
Understanding the Glycaemic Index and Glycaemic Load can be powerful tools in managing diet and health. While low-GI foods have many benefits, it is crucial to consume them in moderation and as part of a balanced diet. By combining low-GI foods with regular physical activity and mindful eating, you can maintain better blood sugar levels, possibly support weight management, and reduce the risk of chronic diseases.
Nicola Royce
Registered Dietitian
Postgraduate Diploma Diabetes,
Masters in Nutrition
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