How Foods Affect Blood Sugar Levels: The Glycemic Index and Glycemic Load

The glycemic index and glycemic load are two important concepts for understanding how different foods affect blood sugar levels. This article provides an overview of how these measures are determined, how they differ, and what research says about the health implications of following a low glycemic diet.

What is the Glycemic Index?

The glycemic index (GI) is a measure of how much a food raises blood glucose levels compared to pure glucose. More specifically:

It measures how much a food with 50g of available carbohydrates raises blood sugar compared to 50g of pure glucose.
Glucose is used as the reference food and set at 100.
The glycemic response to other foods is measured as a percentage compared to glucose.

For example, a food with a GI of 70 raises blood sugar 70% as much as pure glucose. A food with a GI of 40 raises blood sugar only 40% as much as glucose.

How Glycemic Index is Determined

To determine the GI of a food, participants are given a test meal containing 50g of available carbohydrates from that food. Their blood glucose levels are measured at set intervals over 2 hours. This is repeated several times.

The incremental area under the blood glucose response curve is calculated. The same process is done with 50g glucose for comparison. The GI is calculated as:

GI of Test Food = (Blood Glucose Response to Test Food ÷ Blood Glucose Response to Glucose) x 100

The final GI value for a food represents the average response in a group of participants. There is variation between individuals, but the relative differences between foods remain fairly consistent.

Glycemic Index of Various Food Groups

Here are some examples of the GI values for different types of foods:

Grains: White rice (89), instant oatmeal (82), whole wheat bread (73), bran cereal (51), whole grain barley (22).
Fruits: Bananas (51), pineapple (66), grapes (43), apples (38), strawberries (32).
Vegetables: Baked white potato (111), sweet potato (70), corn (60), carrots (35), spinach (15).
Dairy: Skim milk (32), full-fat yogurt (34), ice cream (38).
Legumes: Chickpeas (33), red kidney beans (24), lentils (26).

As you can see, refined grains, potatoes, and sugary foods tend to have higher GIs, while fruits, non-starchy vegetables, beans, and unsweetened dairy have lower GIs.

What is Glycemic Load?

The glycemic index has some limitations. Namely, it measures the response to a fixed 50g dose of carbohydrates. But foods contain widely varying amounts of carbs.

Glycemic load (GL) accounts for this by factoring in the carbohydrate content of a typical serving.

GL = (GI x Grams of Carbs per Serving) / 100

For example:

Table sugar has a GI of 66 and 5g carbs per tsp. Its GL = (66 x 5) / 100 = 3.
An apple has a GI of 38 and 15g carbs. Its GL = (38 x 15) / 100 = 6.

GL provides a more realistic measure of blood sugar impact from standard servings. High GL foods (?20) elicit larger spikes. Low GL foods (<10) cause more gradual rises.

Determining Glycemic Index/Load in Practice

To demonstrate how GI and GL are determined, let’s walk through an example from the transcript:

A participant consumed a meal of plain yogurt with apple and nuts for breakfast.
Their blood glucose was measured repeatedly after eating. It rose from 105 to 115 mg/dL over 2 hours.
The yogurt had 200g carbs per serving. With a GI of 27, its GL was (27 x 5)/100 = 1.
The apple had 15g carbs. With a GI of 44, its GL was (44 x 15)/100 = 7.
The total meal GL was around 8, indicating a low blood sugar impact.

Monitoring blood sugar responses in this way allows GI and GL values to be calculated for different foods.

Health Implications of Glycemic Index/Load

What does research say about the health effects of choosing low versus high GI/GL foods? Here are some key points:

Blood sugar control: Low GI/GL diets reduce post-meal blood sugar spikes in people with diabetes. This improves HbA1c levels.
Insulin resistance: A low GI diet may improve insulin sensitivity and reduce insulin resistance. This helps lower diabetes risk.
Weight loss: Low GI foods may enhance weight loss by controlling appetite and prolonging feelings of fullness after meals.
Heart disease: Some studies link high GI/GL diets to increased heart disease risk. Low GI foods may reduce inflammation and LDL cholesterol.
Cancer: High GI foods may promote higher blood insulin levels, which can stimulate tumor growth. The impact on cancer rates is less clear.

Overall, research suggests sticking to a low GI diet with plenty of whole grains, fruits, vegetables, legumes, nuts, yogurt and low-fat dairy can provide health advantages. But GI/GL values should not be viewed in isolation. The overall nutrition profile of foods matters too.

Tips for Following a Low Glycemic Diet

Here are some practical tips for shifting your diet towards lower GI options:

Focus on fiber: Choose whole grains and fruits/veggies with skins for extra fiber. Fiber helps slow digestion and flatten blood sugar curves.
Pair carbs with protein/fat: Eating carbs alongside protein, fat or both helps moderate the glucose response.
Avoid refined carbs: Limit foods made with white flour and added sugars, which spike blood sugar rapidly.
Don’t overcook starches: Cooking starchy foods like potatoes and pasta al dente keeps the GL lower.
Watch portions: Larger servings = more carbs and a greater blood sugar impact, even of low GI foods.
Spread carbs evenly: Don’t overload a single meal. Spread intake evenly across meals & snacks.
Check labels: Compare GL values on nutrition labels and choose lower GL options.

The Glycemic Index and Glycemic Load

The glycemic index and glycemic load provide useful metrics for comparing how different foods influence blood glucose levels. Following a low GI/GL diet can offer health benefits by promoting better blood sugar control and insulin sensitivity. When combined with an overall nutrient-rich diet, low GI eating patterns can be a smart approach for many people.