There is a lot of information available about the glycemic index (GI) and most of what you read will indicate that a low GI diet has various advantages to one’s health. But is it as black and white as it’s made out to be? Does it just come down to low GI being good and high GI being bad?
First, what is glycemic index? In simple terms, the glycemic index is a ranking system of 0-100 that is measured by any one particular foods ability to raise blood sugar levels after eating. It was designed to help in situations which glucose tolerance is impaired, like diabetes, to easier control their blood sugar levels. Large increases in blood sugar levels are also believed to be part of controlling hunger and satiety. Other purposes are to prevent chronic western diseases and controlling bodyweight better in a low-GI compared to a high-GI diet.
To measure it, the subject consumes a food that contains a fixed amount of carbohydrates; usually 50g. This means that if pure sugar is eaten, the volume is much smaller than if carrots, for example, are eaten. Foods that are considered to have a high GI are digested and absorbed rapidly and raise blood sugar quickly. Low-GI on the other hand, is supposed to have a slower digestion and absorption rate, thus maintain more stable blood sugar levels. White bread or plain sugar is most often used as a reference food with a score of 100.
Foods low in GI is thus supposed to be superior in terms of controlling hunger, ad libitum energy intake, body composition and weight, and overall health.
For a list of different carb sources and their glycemic index, see table 1 here. Note that this is food eaten in isolation – not something that happens often in real life.
Is the glycemic index so straightforward?
No, there are various problems with it. It’s not very reliable because responses are highly individual. Not only between people but also in the same person on different days . In some, however, it seems to be more reliable than in others. If we look into the data, specifically at white bread, the highest response was a GI of 140, while in another subject it was only 60. In potatoes, the results were also of high variability, with a GI score ranging from about 185 down to about 40. Other data has also found a low reliability within and between subjects .
Secondly, very few foods are eaten in isolation. Fat and protein added to the meal will influence the rate of digestion, absorption and hence blood glucose response. Some researchers have tried to create a formula to estimate the glycemic index of an entire meal . This, however, seemed to overestimate the GI of the meals by between 22 – 50%. Thus, there are still limitations.
Glycemic index and appetite
Contrary to popular belief, GI does not seem to effect appetite in most studies . However, some have found a higher satiety/fullness response with a lower GI diet , while others have found a higher satiety/fullness score with a higher GI diet . The impact of GI on appetite is therefore non-existent or at best very weak. What’s more, a meta-analysis which compared high vs low-GI breakfast on subsequent short-term energy intake found no difference .
What about long-term studies?
In studies >10 weeks, comparing low-GI diets with high-GI diets, much of the data shows there is not any difference in bodyweight . However, a meta-analysis from 2008 showed more weight loss in the low GI subjects  which is also supported by other studies . What’s more, a low-GI diet has shown to have an advantage in regards with LDL- and HDL cholesterol in overweight individuals  in addition to improved insulin sensitivity (only in obese) . Furthermore, cardiovascular disease risk reduction , lower triglyceride levels , lower inflammation  and decreases in gene expression of stress and inflammation . In contrast, Jebb et al (2010) found no difference in overweight individuals when it came to insulin sensitivity and blood pressure between the different GI diets .
This does not mean one should eat sugar instead of vegetables – don’t change your carb sources to candy just yet. Foods that are highly processed and low in fiber tend to correlate with a high GI diet. This means it may be the food quality that is the reason for some research showing a detrimental effect of a high GI diet, rather than GI itself.
The glycemic index should therefore not dictate your food choices, it’s not reliable enough, difficult to apply in practice, and the bigger picture is missing. In the case of sugar vs vegetables – vegetables have far more volume, fiber, water, micronutrients and are less calorically dense than sugar. A calorie from carb is a calorie from carb, but for your health the source is important! So choose your food sources wisely – whole foods before processed foods, but you can always make some room for sugar for increased flexibility in your diet.
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