Glycemic Index: What You Need to Know About Carbs

Carbs have become a bit of a hot topic in nutrition lately. Beliefs vary significantly from person to person, with carbs variously interpreted as good or detrimental to health. Truthfully, the answer isn’t quite as black and white as it may seem. Carbs come in a wide variety of shapes and sources, with various ways to evaluate their impact. One such method is the glycemic index (GI). So, let’s examine the GI, and talk about both its usefulness as well as limitations.

Glycemic Index

What is the Glycemic Index?

The glycemic index (GI) is a concept introduced in a 1981 paper. The GI’s purpose is in comparing different carbohydrate sources based on their effects on the body’s blood glucose, i.e. blood sugar levels. In the experiment itself, 5-10 subjects were tested on 62 commonly eaten foods, with the portions for each carb source standardized to contain equal amounts of carbohydrate. The researchers then drew blood samples from the subjects two hours later, and compared the  corresponding blood glucose level changes between carb sources.

The range of values on the GI span from 0 to 100, with pure glucose (pure simple sugar) used as the reference point for a 100 score. The foods are broadly divided into three groups based on their GI scores: 

Low GI = 55 or lower

Midrange GI = 56-69

High GI = 70 or higher

On this scale, the lower GI sources tend to cause slower, steadier rises in blood glucose levels. The higher the GI value climbs, the more rapidly the food elevates blood glucose levels.  Figure 1 shows select examples of generic carb sources, their GI values, and their grouping:

The last thing to note about the GI is that there are several factors that can potentially influence the GI value of a food. For example, the presence of fat, protein, and/or fiber in a carb source generally decreases its GI value. Conversely, things such as fruit ripeness, cooking, and processing all tend to drive the GI value up. Finally, there is significant variability in blood sugar response between individuals eating the same food. Much of this has to do with starting blood glucose levels, insulin resistance, and other sources of genetic variation between individuals.

What Do We Use It For?

The glycemic index is primarily used to predict the effects of different carb sources on a person’s blood sugar levels. This is particularly important with conditions such as diabetes, where maintaining blood sugar levels is crucial to overall health and well-being. In this context, low GI foods may make it significantly easier to keep blood sugar levels constant. High GI foods are also useful in their ability to rapidly elevate blood sugar, as they can quickly drive a person out of hypoglycemia. This is a reason why diabetics commonly keep convenient sources of simple sugars on hand, e.g. hard candy.

However, the GI is much less effective at predicting the exact response an individual will have to a given carb source. As explained above, this is because there are other factors at play besides the raw carb content of the food itself. Variables such as insulin sensitivity/resistance can cause two individuals to have significantly different responses to the same food and quantity. Therefore, the GI is more useful for predicting general trends as opposed to an exact physiological response.

Does the Glycemic Index Matter?

This really depends on the context at hand. Diabetics can benefit from knowledge of the GI when making food choices, as they tend to be much more concerned with maintaining a specific range of blood sugar levels. They can thus adjust their carb choices according to their needs. If they’re hypoglycemic, a high GI food will be most effective at bringing their blood sugar levels up quickly. For those who tend toward hyperglycemia, the opposite is true – low GI foods will cause more gradual increases in blood sugar levels, without the potential trouble of a spike in blood sugar levels.

Outside of that particular circumstance, though, the GI is significantly less useful for most other people. The body is generally quite good at regulating its own blood sugar levels, and rapid elevations in blood glucose levels are usually titrated back down to normal levels with little trouble. In other words, the GI value of a given food tends to have negligible effects on net blood glucose levels over time. 

It is thus likely a better use of one’s time to focus on the quality and quantity of carbohydrate consumed, not necessarily the source. Many common “junk” foods actually have a low GI, while some “healthy” foods actually have a high GI. For example, chocolate cake has a GI score of about 38, while brown rice has been measured at values as high as 95.

As for weight control, there are differing opinions about the GI’s effectiveness. Some sources find that lower GI foods are more effective at producing weight loss. Other reviews indicate that low GI foods have no significant impact on weight loss. It is worth noting that the sources reporting a difference use epidemiological and population data, which tends to be skewed by other factors than the variable in question. In this case, lower GI foods tended to be associated with increased protein and fiber intake, both of which are believed to have significant effects on weight loss endeavors. 

Final Thoughts

The glycemic index is an interesting and useful tool in specific scenarios. It can predict the effects of certain carb sources on blood glucose levels, making it a valuable resource for populations with specific concerns over regulating blood sugar levels, e.g. diabetics. Outside of this scenario, though, there is limited evidence that GI values correlate with overall health or assist with weight loss.   

So, get those carbs in and enjoy. Carb-e diem!