This article explore the meaning of Glycemic Index and Glycemic Load and their impact on weight loss.

What is Glycemic Index?

Glycemic Index (GI) is a measure of the carbohydrate in foods to raise blood sugar or glucose levels upon consumption. A low GI value represents that the food (or carbohydrate) is absorbed slowly and has a slow impact on blood sugar and insulin levels. Similarly, a high GI value represents that the food is causing considerable impact and rise on blood sugar and insulin levels.

Typically GI value of carbs are classified into three categories:

  • Low = <55
  • Medium = 55 to 69
  • High = >70

Calculate the GI value of food here.

Why is monitoring GI levels important?

Consuming high GI foods over a period of time can lead to serious health issues including type 2 diabetes, obesity, cancer, and high blood pressure.

When the body’s blood sugar levels rise, insulin activity in the body increases as well. Insulin, known as the “storage” hormone, does not like to have the body’s stores depleted. It attempts to hoard as much nutrients as it can. This deters the breakdown of fat. Subsequently, the body’s attempt to lose weight is halted and levels of bad cholesterol rise leading to chronic health diseases.

The greater problem with high GI food is not limited to the rise in blood sugar level but also the nature of the rise. Typically, it causes a spike and then drops rapidly. This unnatural rise in blood sugar level is what the body fails to adjust to.

A low GI food, on the other hand, has a more gradual and limited rise in the level of blood glucose.

glycemic index

Image courtesy: Glycemic Index.com

 

Find a list of low GI healthy carbohydrate food items here.

How is the GI measured?

Calculating the GI values of foods requires specific and valid scientific methods, currently calculated only by a limited number of research institutions.

Usually, the GI score of a food is measured by tracking the blood glucose levels of about a dozen people and measuring the impact of a particular food on their blood glucose levels.

What affects the actual GI value?

The GI value for a particular food depends on various factors including:

  1. Chemical Structure of the Carb: Simple structured carbs like glucose is easily broken down the body leading to a quick rise in blood sugar level. (GI of glucose is 100). However, complex carbs like fructose take much longer to be broken down, and thus have a much lower GI.
  2. Physical Structure of the Carb: The size and surface area of a particular carbohydrate also impacts the GI value of a food. If the carbohydrate molecules are small/fine (such as in white flour) the body’s digestive enzymes have a much larger surface area to metabolize the molecules. At the same time, larger or more spread-out carbs like bread (made from white flour) makes it much easier for the body to digest it. Hence, the GI index for such foods is higher.
  3. Level of Refinement of Carb: When a carb is refined, the amount of natural fiber and other constituents that might help in slowing down its digestion is also reduced. Thus, greater the food is refined or processed, higher the GI value.
  4. How Carbs are Cooked: The greater a food is cooked, the structure of the carbohydrate is broken down further. Thus, allowing the body to absorb the food easier, leading to increased GI index.
  5. Amount of Fat and/or Acid accompanying the Carb: Additional fat and/or greater acidic value in a food makes it more difficult for the body to absorb it and hence lower its GI value. Food such as vinegar, lemon juice, pickles, etc. all help in lowering the GI of a meal.

Glycemic Load: An equally important consideration

While, the GI of a food does tell you the impact on blood sugar levels, it does not provide the entire picture. It does not indicate the amount of glucose the food will deliver to the bloodstream, which is the primary concern.

Besides knowing how quickly the food makes glucose enter the bloodstream, one also needs to know the amount of glucose the food will add to the bloodstream.

This is determined by the Glycemic Load calculated as:

Grams of a carbohydrates in a meal x GI / 100.

For example, the GI of watermelon is extremely high at 80. But, a serving of watermelon only delivers half a dozen carbohydrates. So, effectively its Glycemic Load is only 5, and thus watermelons are healthy to consume even though their GI value is on the higher side.

As an indicator, Glycemic Load for food is classified as:

  • Low = <10
  • Moderate = 10 to 20
  • High = >20

Conclusion

Consider both the GI value and Glycemic Load of a food to determine whether it is healthy or not. In case, you are planning to join a low GI diet like Ketogenic diet do consult your doctor or dietitian before starting.

Find a detailed analysis on all popular carbohydrate foods in a spreadsheet here.

More sources available at:

The complete list of the glycemic index and glycemic load for more than 1,000 foods can be found in the article “International tables of glycemic index and glycemic load values: 2008” by Fiona S. Atkinson, Kaye Foster-Powell, and Jennie C. Brand-Miller in the December 2008 issue of Diabetes Care, Vol. 31, number 12, pages 2281-2283.