Glycemic Index (GI) and Glycemic Load (GL) are measures used to determine the effects of carbohydrates in food on blood sugar levels.  Of the two, GL is most useful as it refers to the effect of a portion of good on blood sugar levels. GL meanwhile is based upon GI.

Glycemic Index (GI) estimates how much each gram of available carbohydrate (total carbohydrate minus fiber) in a food raises a person’s blood glucose level following consumption of the food. This is then made relative to the effects of blood sugar from the consumption of glucose: by definition glucose has a glycemic index of 100, and other foods have a lower glycemic index.

Determining the GI of a food

• Foods with carbohydrates that break down quickly during digestion and release glucose rapidly into the bloodstream tend to have a high GI
• Foods with carbohydrates that break down more slowly, releasing glucose more gradually into the bloodstream, tend to have a low GI
• A lower glycemic index suggests slower rates of digestion and absorption of the foods’ carbohydrates and may also indicate greater extraction from the liver and periphery of the products of carbohydrate digestion
• A lower glycemic response usually equates to a lower insulin demand but not always, and may improve long-term blood glucose control and blood lipids
• The glycemic index of a food is defined as the incremental area under the two-hour blood glucose response curve (AUC) following a 12-hour fast and ingestion of a food with a certain quantity of available carbohydrate (usually 50 g)
• The AUC of the test food is divided by the AUC of the standard (either glucose or white bread, giving two different definitions) and multiplied by 100
• The average GI value is calculated from data collected in 10 human subjects

Glycemic index of foods

A low-GI food will release glucose more slowly and steadily, which leads to more suitable postprandial (after meal) blood glucose readings

A high-GI food causes a more rapid rise in blood glucose levels and is suitable for energy recovery after exercise or for a person experiencing hypoglycemia.

The glycemic effect of foods depends on a number of factors such as:

• The type of starch (amylose versus amylopectin)
• Physical entrapment of the starch molecules within the food
• Fat and protein content of the food
• Organic acids or their salts in the meal — adding vinegar, for example, will lower the GI
• The presence of fat or soluble dietary fiber can slow the gastric emptying rate, thus lowering the GI

Disease prevention

• Individuals who followed a low-GI diet over many years at a significantly lower risk for developing both type 2 diabetes, coronary heart disease, and age-related macular degeneration
• High blood glucose levels or repeated glycemic “spikes” following a meal may promote these diseases by increasing systemic glycative stress other oxidative stress to the vasculature and also by the direct increase in insulin levels
• The glycative stress sets up a vicious cycle of systemic protein glycation, compromised protein editing capacity involving the ubiquitin proteolytic pathway and autophagic pathways, leading to enhanced accumulation of glycated and other obsolete proteins
• High GI diets and high blood-sugar levels are related to kidney disease
• Mixing of high- and low-GI carbohydrates produces moderate GI values

Accuracy

• Glycemic index charts often give only one value per food
• Variations are possible due to ripeness, processing, the length of storage, cooking methods, and its variety (white potatoes are a notable example, ranging from moderate to very high GI even within the same variety)
• The glycemic response is different from one person to another, and even in the same person from day to day, depending on blood glucose levels, insulin resistance, and other factors
• Most of the values on the glycemic index do not show the impact on glucose levels after two hours. Some diabetics may still have elevated levels after four hours
• The GI of foods is determined under experimental conditions after an overnight fast, and might not apply to foods consumed later during the day because glycemic response is strongly influenced by the composition of the previous meal, particularly when meals are consumed within an interval of a few hours

Glycemic Load estimates how much the food will raise a person’s blood glucose level after eating it. One unit of glycemic load approximates the effect of consuming one gram of glucose.

Glycemic load accounts for how much carbohydrate is in the food, and how much each gram of carbohydrate in the food raises blood glucose levels and is based on the glycemic index (GI). Glycemic load is defined as the grams of available carbohydrate in the food x the food’s GI / 100.

For one serving of a food

• GL greater than 20 is considered high
• GL of 11-19 is considered medium
• GL of 10 or less is considered low
• Foods that have a low GL in a typical serving size almost always have a low GI. Foods with an intermediate or high GL in a typical serving size range from a very low to very high GI.

Glycemic load is beneficial in dietary programs targeting metabolic syndrome, insulin resistance, and weight loss

Glycemic load of a serving of food can be calculated as carbohydrate content measured in grams (g), multiplied by the food’s GI, and divided by 100.

• For example, watermelon has a GI of 72. A 100g serving of watermelon has 5 g of available carbohydrates (it contains a lot of water), making the calculation 5×72/100=3.6, so the GL is 3.6
• A food with a GI of 100 and 10 g of available carbohydrates has a GL of 10 (10×100/100=10), while a food with 100 g of carbohydrate and a GI of just 10 also has a GL of 10 (100×10/100=10).