What is the difference between Glycemic Index (GI) and Glycemic Load (GL)?

Your blood glucose rises and falls when you eat a meal containing carbohydrates. How high it rises and how long it remains high depends on the quality of the carbohydrates (the GI) and the quantity. Glycemic load or GL combines both the quality and quantity of the carbohydrate.

In addition to The GIF's great explanation above; a take home message I often tell my clients is that even though a food may be slowly absorbed, or low glycaemic index (GI), if you eat a large quantity it can still cause your blood sugar level to spike as you are eating a high glycaemic load (GL).

There are 2 principles to consider.
1. Choose low GI foods (e.g. sweet potato, quinoa, wholegrain bread, fruits)
2. Keep the glycaemic load in check by watching the portion size of your low GI carbohydrates.

Remember 1 slice of bread = 1/3 cup cooked quinoa, 1/2 cup sweet potato or 1/2 cup fruit salad. An APD can calculate more accurately how much carbohydrate you should have at each meal or snack. A general rule of thumb is to fill 1/4 of your plate with low GI carbohydrates.

The glycemic index of a food is a measure of the rate at which the carbohydrate from food is broken down into glucose in your bloodstream.  While choosing low GI foods has many benefits, the overall effect that a food has on blood glucose levels is dependent on both the nature (GI) of the carbohydrate it contains and the amount (ie grams of carbs).  Glycemic load takes both of these factors into account.. 
 
While foods with a high carbohydrate content and those with higher GI values will generally have the highest GL, this also means that small amounts of a high GI food may have only modest effects on blood glucose levels while, as Courtney points out above, large amounts of a low GI food can still raise blood glucose and insulin levels significantly.  What this means in practice, is that there is no need to avoid foods that have a high GI but are low in carbohydrate and nutrient-dense. On the other hand, just because a food is low GI, it doesn’t mean you can eat as much as you like, particularly if you are watching your weight or your blood glucose or insulin levels.

Glycaemic Load is a ranking system for carbohydrate-rich food that measures the amount of carbohydrates in a serving of food.

Foods with a glycaemic load (GL) under 10 are considered low-GL foods and have little impact on your blood sugar; between 10 and 20 moderate-GL foods with moderate impact on blood sugar, and above 20 high-GL foods that tend to cause blood sugar spikes.

The Glycaemic Index indicates how rapidly a carbohydrate is digested and released as glucose (sugar) into the blood stream. In other words, how quickly foods break down into sugar in your bloodstream. A food with a high GI raises blood sugar more than a food with a medium to low GI.

But the glycaemic index does not take into account the amount of carbohydrate in a food. So glycaemic load is a better indicator of how a carbohydrate food will affect blood sugar.

If a food has a high glycaemic index and a low glycaemic load -- like graham crackers have a GI of 74 and a GL of 8.1 -- how will that affect your blood sugar?

Food ranked high on the GI may represent a huge portion of a food because GI is not based on standard serving sizes. Basically, if a food is ranked high on the glycaemic index it has readily available carbohydrate for quick absorption. However, the same food can have a low glycaemic load because there may not actually be much total carbohydrate in a given serving of that food. A low GL is the better indicator that a food won't have much impact on blood glucose levels.

Here are two examples: Watermelon has a high GI of 72, yet a low GL of 7.21. The high GI is based on 5 cups of watermelon, not an actual serving size of 1 cup. The low GL means one serving of watermelon doesn't contain much carbohydrate, because it is actually mostly water. The low GL indicates that a serving of watermelon won't have much impact on your blood sugar.

Carrots are another example of a low GL food that many people think will raise their blood sugar a lot -- but it's not true. That's because carrots have a high GI of 71. However, what most people don't know, is that the GL for carrots is only 6. Therefore, unless you're going to eat ¾ kg of carrots in one sitting, an average serving of carrots will have very little impact on blood glucose levels. That said, juicing carrots -- which means consuming more carrots at once -- will have a greater impact on blood glucose.

How can knowing the glycaemic load and glycaemic index of foods be used to make healthier eating choices?

Everyone can benefit by eating a balanced diet of protein and fat, and foods that are lower on the GL and GI index. Foods with a lower GL and GI typically are high in fiber and nutrients and sustain your energy better throughout the day.

Also, knowing the GL of a food is a better indicator of whether that food will cause your blood sugar to spike. When your blood sugar spikes, the body releases extra insulin to bring down your blood sugar. If your body is asked to release extra insulin on a regular basis, it begins to lead to insulin resistance for many people and diabetes -- especially if diabetes is in your family.

Can knowing the glycaemic load of foods help people lose weight?

Yes. Consuming low GL and GI foods keeps us satiated longer because these foods are more slowly broken down for glucose utilization. The result is that you feel fuller for longer. When you consume high GL and GI foods, blood sugar levels spike which causes a short-term feeling of fullness, but then blood sugars plummet which causes you to crave food again and you ultimately end up consuming excess calories, which contributes to weight gain.

How seriously should we take all this?

Some evidence suggests GL and GI diets can benefit overall health -- especially for people with diabetes, although, more conclusive research is needed to know for sure.

People who favour these rankings feel that GL and GI diets can help in the treatment and prevention of chronic diseases. Detractors note that selecting foods only by using GL and GI could cause people to choose bacon rinds over watermelon. I think we each have to decide for ourselves.