What is the Glycemic Index & Examples

Glycemic index (GI) refers to the rate at which carbohydrates convert into blood glucose.

While virtually all carbohydrates convert into blood glucose eventually, some convert much faster than others. Pure glucose is given a GI score of 100; everything else is compared to the digestion/absorption rate of glucose. 

What the numbers represent is the percentage of carbohydrate that turns into blood glucose in the first two hours. Foods with a high GI (greater than 70) tend to digest and convert to blood glucose the fastest, with a significant blood glucose “peak” occurring in 30–45 minutes. Foods with a moderate GI (56-69) digest a bit slower, resulting in a less pronounced blood glucose peak approximately one to one and a half hours after they are consumed. Foods with a low GI (below 55) tend to make a gradual appearance in the bloodstream. The blood glucose peak is usually quite modest, and may take several hours to occur. 

Most starchy foods have a relatively high GI; they digest easily and convert into blood glucose quickly. Exceptions include starches found in pasta and legumes. Foods that have dextrose in them tend to have a very high GI. Fructose (fruit sugar) and lactose (milk sugar) are slower to convert into blood glucose. Table sugar (sucrose) has a moderate GI because it contains a combination of glucose (which is very fast) and fructose (which is slower). Foods that contain fiber or large amounts of fat tend to have lower GIs than foods that do not.

Glycemic Index Facts

• High-fiber foods raise BG slower than low-fiber foods
• High-fat foods raise BG slower than low-fat foods
• Solids raise BG slower than liquids
• Cold foods raise BG slower than hot foods
• Unripe foods raise BG slower than ripe foods
• Raw foods raise BG slower than cooked foods 

GI Index and Timing of Meal Bolus 

For those who take insulin, knowing the glycemic index of a food helps in determining the optimal time to take the mealtime insulin. Foods with a high GI (greater than 70) tend to raise blood sugar the fastest. For these types of foods, it is best to take mealtime rapid-acting insulin 20-30 minutes prior to eating. This will allow the insulin peak to coincide as closely as possible with the blood sugar peak. Taking insulin for high-GI foods just before or while eating would produce a significant after-meal blood sugar “spike”, as the insulin would lag behind the blood sugar rise by about half an hour. 

Foods with a moderate GI (approximately 56–69) as well as most “mixed” meals digest a bit slower, with a slightly less pronounced blood sugar peak approximately 60–90 minutes after eating. Taking rapid-acting insulin 10–15 minutes before eating these types of foods is recommended.

With low-GI (below 55) foods, the blood sugar “peak” is usually quite modest, and may take several hours to appear. Taking insulin prior to eating foods such as pasta or yogurt is likely to lead to hypoglycemia about one hour later, followed by a delayed blood sugar rise. Instead, try taking the insulin 10–15 minutes after eating. A second option is to split the rapid-acting insulin into two parts: half given with the meal, the other half about an hour later. A third option is to take regular insulin with the meal, rather than a rapid-acting analog. One other option, available to insulin pump users, is to extend the bolus delivery over 60–90 minutes. 

A Matter of Accuracy

How accurate is the glycemic index? Given the degree of inter-individual variability in digestion and the impact of mixing foods with different glycemic index values in the same meal, there is no way to tell exactly when a specific food or meal will cause the blood glucose level to peak. What glycemic index is good for is categorizing foods according to their relative impact: fast, moderate, and slow. 

When looking at complete meals, the main source of carbohydrate should dictate the rate of blood glucose rise. For example, a meal where pasta is the primary carbohydrate should be treated as low-GI. A meal where potato is the major source of carbs should be treated as high-GI. And don’t forget to take the fat content and size of the meal into account. Higher-fat meals will produce a slower blood glucose rise regardless of the nature of the carbohydrate, and larger meals usually take longer to digest than smaller meals. 

So while it is true that subtle differences between GI values mean very little, major differences can certainly be used to your advantage. And we can use all the advantages we can get! 

Examples

Reference

About Glycemic Index

©Gary Scheiner MS, CDCES – Integrated Diabetes Services. May be reproduced and used for patient education, but not sold.