Blood sugar levels can be measured using samples from different types of blood and methods. Blood can be collected from veins, arteries, or capillaries, and glucose levels can be measured in whole blood or in the liquid part of blood (serum or plasma)</b. For clinical tests like glucose tolerance tests, venous plasma or serum samples are typically used. However, early glycemic index (GI) studies often used capillary blood (from finger pricks) because it’s simpler and requires minimal processing. Participants could even collect their own samples using finger pricks, which made studies more convenient.
There is a debate, however, about the accuracy of capillary blood samples. Finger pricking sometimes mixes blood with interstitial fluid (fluid between cells), especially when people squeeze or “milk” their fingers. Despite this, research by Wolever and Bolognesi (1996) found that capillary blood measurements provided more consistent results and showed clearer differences between foods’ glycemic responses.
Today, many GI studies still use capillary blood sampling to calculate the area under the glucose response curve (AUC) and determine GI values. However, the method used for blood sampling can affect the GI results, so it’s important to carefully review a study’s methodology before comparing or interpreting its findings.
When blood samples are taken also plays a key role in determining GI values. Typically, blood sugar levels are measured every 15-30 minutes for two hours after eating a food with carbohydrates. This timing mirrors the glucose tolerance test, which is used to diagnose type 2 diabetes or prediabetes. However, this standard may not be ideal for measuring the full post-meal blood sugar response, as glucose metabolism often takes longer than two hours, especially in people with diabetes or slower digestion.
For example, research by Gannon and Nuttall (1987) found that if you measure blood sugar over a longer time, differences in GI between foods become less noticeable. This means the two-hour window commonly used in GI testing may not capture the complete picture of how different foods affect blood sugar over time. As a result, the timing of blood sampling should be carefully considered when interpreting GI studies.
The way a food is prepared or processed can significantly influence its glycemic index (GI), particularly in foods high in starch. Grinding, mashing, rolling, or even thoroughly chewing carbohydrate-rich foods breaks down their starch molecules, making them easier to digest and increasing their GI. For example, research shows that simply mashing a cube of potato can raise its GI by about 25% (Wolever et al., 2001).
Altering the chemistry of starchy foods can also impact their GI. Adding acid, such as vinegar, reduces the GI by lowering the pH. For instance, potato salad made with vinegar has a lower GI than plain cooked potatoes. Similarly, certain chemical treatments like acetylation or adding beta-cyclodextrin to potato starch have been shown to lower its GI (Raben et al., 1997).
Cooking affects starch in food, changing its digestibility and thus its GI. Raw starch, like that in uncooked potatoes, is difficult for digestive enzymes to break down, resulting in a low GI. Cooking, however, causes starch molecules to gelatinize, making them easier to digest and increasing the GI. Interestingly, if a cooked potato is cooled after cooking, some of the starch molecules revert to a form known as “resistant starch,” which is harder to digest and lowers the GI (Fernandes, 2005).
A study by Fernandes et al. (2005) demonstrated how cooking methods impact the GI of potatoes. The GI ranged significantly depending on the preparation method:
Pre-cooking and reheating also affected GI. Precooked and reheated Russet potatoes had a lower GI compared to freshly cooked ones, while pre-cooking had no effect on boiled white potatoes.
The type of cooking—baking, boiling, frying, or microwaving—can influence GI, but results are mixed. Some studies suggest that baked potatoes may have a lower GI than boiled potatoes, while other studies found no significant differences (Wolever et al., 1994; Soh and Brand-Miller, 1999; Fernandes et al., 2005).
In summary, the GI of starchy foods like potatoes can vary widely depending on how they are processed and prepared. Factors such as mashing, adding acids, cooling after cooking, and the specific cooking method can all play a role in determining how quickly a food raises blood sugar levels. Understanding these variations can help people make better dietary choices to manage blood sugar responses.
At Food Research Lab (FRL), 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.
Food Research Lab strives for excellence in new Food, Beverage and Nutraceutical Product Research and Development by offering cutting edge scientific analysis and expertise.
