One of the most common pieces of advice in the health-nutrition world is that we should focus our carbohydrate intake on slowly-digesting carbohydrates, because they make us feel more full than rapidly-digesting carbohydrates. Rapidly-digesting carbohydrates, such as potatoes, stand accused of causing us to overeat, resulting in obesity, diabetes, and many other chronic ailments. Is this true?
This concept is usually discussed in terms of the glycemic index. Foods with a high glycemic index cause blood glucose to rise more than foods with a low glycemic index, and the former are usually digested more rapidly. This measure was originally developed to help diabetics control their blood sugar levels, and it may indeed be useful for that purpose. However, more recently it has been suggested that non-diabetics should focus on low-glycemic foods as well.
A number of single-meal studies do suggest that low-glycemic carbohydrates provide more prolonged satiety than high-glycemic carbohydrates (1, 2, 3, 4, 5, 6, 7). Yet these studies are often difficult to interpret, because the test foods frequently differ in many ways besides glycemic index (e.g., fiber content, calorie density). Also, a few studies have either failed to replicate the finding, or found that high-glycemic carbohydrates are actually more filling (8, 9, 10, 11, 12).
Many observational studies have shown that people who eat lower-glycemic carbohydrates tend to fare better over time. However, again these studies are difficult to interpret, because the most common high-glycemic foods are generally highly processed, high-reward, and low in fiber. Adding to my skepticism, nearly all longer-term (10+ weeks) randomized, controlled trials have found that the glycemic index of the diet makes little or no difference in calorie intake, body weight, or metabolic health (13, 14, 15, 16, 17, 18, 19, 20). This is consistent with the fact that many traditional cultures have remained lean and healthy eating diets that are extraordinarily rich in high-to-medium-glycemic carbohydrate such as cassava, yams, taro, sweet potatoes, millet, rice, palm starch, and potatoes (Western Diseases. Trowell and Burkitt. 1981)
The single-meal studies that have been conducted to date have usually only focused on one food, or a few foods, which makes it difficult to generalize the findings. What I really want to know is this: when people eat a variety of typical foods, is there any correlation between the food's glycemic index and how full that person feels? In other words, can we increase satiety by favoring lower-glycemic foods over higher-glycemic foods in the context of a typical diet?
I finally found a study that answers this question. It was published in 1996.
Susanna Holt and colleagues recruited healthy college students, mostly lean but including a few that were overweight (21). The research team fed the volunteers 240-Calorie portions of 38 common foods individually, and measured satiety, glucose, and insulin levels every 15 minutes for two hours after each test meal. Test foods included many of the most common foods people eat in their everyday lives, for example, cereal, potatoes, fish, meat, fruit, nuts, pastries, rice, popcorn, and bread.
I'll cut right to the chase: there was no association whatsoever between the glycemic impact of a food and the satiety it provided.
Potatoes (plain) were a remarkable outlier: they were the most satiating food of all, despite a high glycemic index.
This result shows that among a wide variety of common foods, the glycemic index doesn't predict which ones will be more filling than others, per unit calorie. Other papers based on the exact same data set show that a food's protein content, calorie density, fiber content, and palatability are all important predictors of satiety (22), so we know this experiment was adequately designed to detect effects on satiety.
One important limitation is that the study only lasted two hours. It's possible that differences would have emerged if the volunteers had been studied for three or four hours.
In the end, I'm not convinced that non-diabetic people benefit much from the glycemic index concept. As always, if you find that low-glycemic foods help you control your appetite, then by all means keep eating them. Yet for most of us, the glycemic index seems like an unnecessary complication.