Glycemic Index and Glycemic Load: What Do They Mean, and What Can They Tell Us

"Watermelon Sugar High" - High on GI, not GL

While Harry Styles captures the intoxicating allure of "Watermelon Sugar" in his song, his focus isn't on nutritional guidance. He explores the metaphorical sweetness of pleasure and desire. 

In contrast, let's take a closer look at the practicalities of how watermelon impacts blood sugar levels, considering both its glycemic index (GI) and glycemic load (GL). You might have heard the advice to avoid high-GI foods when trying to lose weight. This often includes warnings about grapes and watermelon due to their sugar content. But does this mean we need to completely banish these fruits, especially when they're in season, bursting with nutrients and fiber, and readily available fresh and local?

Well, to get to the bottom of that, you'll have to stick with me until the end of this (admittedly long) post, or you can simply scroll ahead if you're impatient! ☕

Glycemic Index and Glycemic Load

The glycemic index ranks foods based on how quickly they raise blood glucose levels after consumption. It is measured on a scale from 0 to 100, with pure glucose assigned a value of 100. GI indicates how rapidly a carbohydrate is digested and absorbed, affecting blood sugar levels. Foods with a high GI cause quick spikes in blood sugar, while low-GI foods result in more gradual increases.
High GI: 70-100, low GI: 55 and below.

But GI doesn't tell the whole story because it doesn't account for portion sizes. That's where glycemic load (GL) comes in. It's calculated by multiplying the GI by the amount of carbs in a typical serving.

This means that even low-GI food can have a high GL if you eat a large portion, and vice versa. So, for a complete picture of a food's impact on blood sugar, it's helpful to consider both its GI and GL.
High GL: 20+, low GL: 0 - 10 (for reference, 10 grams of pure glucose has a GL of 10).

So, while GI is useful for understanding the speed of glucose absorption, it does not account for the amount of carbohydrates consumed. GL provides a more accurate picture of how a food affects blood sugar levels by combining both the quality (GI) and quantity of carbohydrates.

Let's also introduce the insulin index, a newer concept compared to the glycemic index (GI). While GI tracks how fast a food raises blood sugar, the insulin index measures how much a food triggers insulin release, regardless of its carb content.
This distinction is important because some high-protein foods, for example, can significantly stimulate insulin without affecting blood sugar—something the GI doesn't capture. While the insulin index can help with meal planning, particularly for those with diabetes or metabolic concerns, its limitations emphasize the need for a holistic approach that considers individual responses and the overall nutritional value of foods.

For a clearer illustration, let's refer to the following graph

Schematic diagram of the influence of GI or GL on blood glucose (left axis) or insulin (right axis). 

Image source: Mens Sana in Corpore Sano: Does the Glycemic Index Have a Role to Play?

When you eat high-GI foods, your blood sugar shoots up quickly. This triggers a surge of insulin to get that glucose out of your blood and into your cells. However, this rapid influx of insulin can sometimes cause your blood sugar to drop too low, leading to feelings of fatigue, hunger, and cravings. Over time, repeated spikes and crashes can contribute to insulin resistance and an increased risk of type 2 diabetes.

Low-GI foods, in contrast, cause a slower, more gradual rise in blood sugar, requiring a less dramatic insulin response. This helps maintain stable energy levels and reduces the risk of blood sugar imbalances.

Remember: High GI carbs cause blood glucose levels to spike and crash. Low GI carbs are digested and released slowly for sustained energy.

Limitations of Glycemic Index (GI)

GI testing is conducted under controlled conditions where participants consume a portion of food containing exactly 50 grams of carbohydrates after an overnight fast. However, real-world eating habits rarely align with these conditions. We don't always eat isolated foods or consume precisely 50 grams of carbs in one sitting, and our meals often include a mix of macronutrients.

Moreover, GI doesn't account for individual variations in blood sugar response. Factors like genetics, gut microbiome, and overall health can significantly influence how our bodies process carbohydrates. Studies have shown considerable differences in blood sugar responses to the same food among individuals.

GI also focuses solely on carbohydrate content, neglecting the influence of other nutrients like protein, fat, fiber, vitamins, and minerals on digestion and blood sugar levels. These nutrients play a vital role in how our bodies metabolize food.

Let's Understand the Factors Influencing GI

While you can readily find online tables listing foods and their GI values, be aware that there might be significant variations for the same food. This discrepancy often arises from different testing methodologies or even a lack of clarity on how the data was obtained. For reliable information, I recommend seeking out food tables that explicitly state the source of their GI values, ideally with links to scientific references.

To truly grasp these numbers and even anticipate the glycemic impact of a meal, it's crucial to understand the key factors that affect GI.

The proportion of amylose and amylopectin in a food plays a key role in determining its glycemic index (GI).

Starch, the primary carbohydrate storage molecule in plants, consists of two main components: amylose and amylopectin. While both are composed of glucose units, their structural differences lead to distinct properties, affecting how they are digested and influencing the glycemic index (GI) of foods.

Amylose, a linear chain of glucose, forms a compact structure that's resistant to digestion, resulting in a slower release of glucose into the bloodstream. Foods rich in amylose generally have a lower GI.

Amylopectin, a highly branched chain of glucose, is easily broken down by enzymes, leading to a quicker release of glucose and a higher GI for foods containing more of it.

In essence, a higher amylose-to-amylopectin ratio translates to a lower GI and a more gradual increase in blood sugar. Conversely, a higher amylopectin proportion leads to a higher GI and a faster blood sugar spike.

Examples:    

Low GI: Lentils, beans, whole-grain bread, and long-grain rice are examples of foods with a higher amylose content and, therefore, a lower GI. 🍏🍆🌾🫘

High GI: White bread, white rice, potatoes, and many processed foods tend to have a higher amylopectin content, contributing to their higher GI. 🍰🍞🍟🍩

While the amylose/amylopectin ratio is a major determinant of GI, several other factors also come into play:

Level of processing

Highly processed carbohydrates typically have a higher GI, meaning they raise blood sugar levels more quickly.
Ripeness

The sugar in fruit breaks down as it ripens, resulting in a higher GI. For example, ripe bananas (GI 51) will cause a greater blood sugar spike compared to unripe bananas (GI 42).
Preparation

Cooking can break down carbohydrates, increasing the GI of a meal. The cooking process swells starch molecules and softens foods, making them easier to digest. Al dente spaghetti has a lower GI (44) than overcooked spaghetti (64).
Retrogradation

This process occurs in starchy foods as they cool. During cooking, starch forms a gel that crystallizes upon cooling. This recrystallized starch is less digestible, resulting in a lower GI and a slower rise in blood sugar. Some of this starch even transforms into resistant starch, a type of fiber that benefits gut health and blood sugar control. Foods like pasta, potatoes, legumes, rice, or grains, when cooked and then cooled or reheated, have a reduced impact on blood sugar, much like stale bread, due to retrogradation.
Acidic seasoning

Adding an acidic seasoning, such as lemon juice, can lower the GI of a meal. Acid slows down stomach emptying, which in turn slows down starch digestion.

Emerging Frontiers in GI and GL Research

Personalized Glycemic Response

Recent research underscores that individuals can react differently to the same foods, even with identical GI values. Factors such as gut microbiome composition, genetics, and lifestyle seem to play a role. Scientists are exploring ways to tailor dietary recommendations based on these individual glycemic responses, potentially revolutionizing blood sugar management.
Gut Microbiome Connection

New studies highlight a complex relationship between dietary GI/GL and the gut microbiome. Diets rich in low-GI foods may foster a healthier and more diverse gut microbiota, which has positive implications for metabolic health and overall wellness. Understanding this connection could unlock innovative dietary strategies for gut health and disease prevention.
Technological Advancements

Continuous, non-invasive blood sugar monitoring technologies are on the horizon. These advancements could provide real-time data on individual glycemic responses to various foods, further personalizing dietary recommendations and improving blood sugar control.
Human Studies

Large-scale studies continue to emphasize the importance of minimizing blood sugar spikes after meals for overall health. This underlines the significance of both glycemic index and glycemic load, along with fiber and whole grains, as vital factors in dietary guidelines (Association of glycaemic index and glycaemic load with type 2 diabetes, cardiovascular disease, cancer, and all-cause mortality: a meta-analysis of mega cohorts of more than 100 000 participants).

And finally, the key question: "So, what about watermelon?"

Let's do some calculations!
Here's the formula for calculating GL for watermelon or any other food:
GL = (GI x Grams of available carbohydrates in a serving) / 100
Available carbohydrates are the carbohydrates that your body can actually digest and absorb, impacting your blood sugar levels. Fiber, on the other hand, is a type of carbohydrate that your body doesn't digest, so it doesn't contribute to the GL.
Available Carbohydrates = Total Carbohydrates – Fiber.

Let's calculate the GL for watermelon:
Assume a serving size of 120 grams (about 1 cup of diced watermelon)
Watermelon has a GI of approximately 72 (there might be some variation in this number depending on the source)
Total Carbohydrates in 120g watermelon: ~7.55 grams
Fiber in 120g watermelon: ~0.4 grams

Available Carbohydrates = 7.55 grams (Total Carbs) - 0.4 grams (Fiber) = 7.15 grams

GL = (72 x 7.15) / 100 = 5.15

Therefore, the glycemic load of 120 grams of watermelon is approximately 5.15, which is considered low.
This means that even though it can cause a quick rise in blood sugar, the overall impact on blood sugar levels is relatively small.

Here are some tips on how to further reduce glucose and insulin fluctuations

Reduced Blood Sugar Spikes 

Eating a single portion at once allows your body to process the carbohydrates and release insulin in a more controlled manner, minimizing fluctuations in blood sugar levels.

Balanced Meal 

Consuming watermelon with a meal containing protein, fat, and fiber further helps slow down the absorption of sugars, leading to a more gradual rise in blood sugar.

Reduced Snacking 

Eating a full portion with a meal can also help reduce the temptation to snack on other high-GI foods throughout the day, contributing to overall better blood sugar control.

It's generally better to consume a single portion of watermelon within a meal rather than splitting it up and eating it throughout the day.

🍉 🍉 🍉

But to return to the question about watermelons, I personally have no intention of giving them up during the summer season. I'll enjoy them in moderation, along with grapes and other seasonal fruits. Despite their high GI, their GL is low if consumed in normal portions. I'll simply be mindful not to snack on grapes throughout the day, but rather enjoy a smaller amount at once, perhaps as part of a snack or light meal, paired with nuts and feta cheese. The protein and fat from these additions can help slow down carbohydrate digestion and absorption, potentially mitigating the impact on blood sugar.

It's crucial to note that I don't have diabetes or pre-diabetes, so these practices shouldn't be seen as universal recommendations.

And to be clear, this post isn't advocating for carbohydrate avoidance. It's about achieving better glycemic control. Carbohydrates are essential, breaking down into glucose to fuel our organs, especially the brain, and muscles!

Literature:

• Low glycaemic index, or low glycaemic load, diets for diabetes mellitus
• Associations of the glycaemic index and the glycaemic load with risk of type 2 diabetes in 127 594 people from 20 countries (PURE): a prospective cohort study
• Glycemic Load Explained: Definition, Formula, Benefits, and Examples
• A Beginner’s Guide to the Low Glycemic Diet
• glycaemic index and diabetes
• Are glycemic index charts a good way to understand your responses to food?