The glycemic index (GI) is often considered an inherent characteristic of a food, meaning it should provide consistent blood sugar responses regardless of who eats it or the circumstances. However, research reveals that GI values can vary significantly between individuals and even within the same person, depending on factors like the time of day the food is consumed or what other foods are eaten alongside it.
People’s blood sugar responses to the same food can differ widely. Studies show that even under controlled laboratory conditions, individual variations in glycemic response are noticeable. For instance, Wolever and colleagues (1990) studied 12 people with diabetes and measured their blood sugar responses to foods like white bread, rice, and spaghetti. The average variation in blood sugar response (AUC) between individuals was about 45%. However, when responses were expressed relative to a reference food (like glucose or white bread), the variation dropped to just 10%.
This method of calculating GI standardizes the data and makes it more consistent, but it doesn’t reflect the real-world reality that individuals often respond differently to the same food. As a result, knowing a food’s GI might not accurately predict how someone’s blood sugar will react to it. .
Blood sugar responses can also vary significantly in the same individual at different times. In fact, within-person variation can sometimes be greater than differences between people. For example, Wolever et al. (1985) found that when subjects were given 50 grams of glucose or bread multiple times under controlled conditions, the variation in their blood sugar response ranged from:
The stage of ripeness or maturity of a food can also significantly influence its GI. As fruits ripen, their starch content is converted into sugars, which lowers their GI. For example, a green, unripe banana has a higher GI than a fully ripened banana (Englyst & Cummings, 1986). Similarly, research suggests that “new potatoes” (younger, freshly harvested potatoes) tend to have a lower GI compared to older, matured potatoes (Soh and Brand-Miller, 1999).
The time of day also affects blood sugar responses and GI values. Blood sugar responses are generally higher after an overnight fast compared to later in the day. For instance, Wolever and Bolognesi (1996) found that the blood sugar response (AUC) to two different breakfast cereals was significantly higher when eaten after a 12-hour fast compared to midday after a standard breakfast. Specifically:
Similarly, a study by Brand-Miller (1994) showed that blood sugar responses to the second meal of the day were similar, regardless of whether the first meal was high-GI or low-GI. This suggests that GI values are most reliable when measured for the first meal after an overnight fast. If measured later in the day, the differences in GI between foods would likely be much smaller.
While GI is a useful tool for comparing how foods affect blood sugar levels, it is not an absolute measure and should be used alongside other considerations for managing blood sugar and diet.
At Food Research Lab, we specialize in providing comprehensive Glycemic Index (GI) testing services that empower food and beverage manufacturers to validate their product’s impact on blood sugar. Our testing follows internationally recognized protocols and is conducted at our ISO-certified clinical research facility, ensuring accuracy, reliability, and global compliance.
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