Demystifying the Glycemic Index (GI): Carbohydrate Impact on Blood Sugar

Author: Yean Toh | Published date: September 25, 2023 | Category: Nutrition

Understanding the Glycemic Index (GI)

In the past, doctors and scientists tended to treat all carbohydrates equally. Now, they realise that there is a big difference between carbohydrates that are quickly digested and absorbed as glucose versus those that are slowly digested and absorbed. To help consumers make better food choices, scientists have developed the Glycemic Index (GI), which indicates how quickly carbohydrates get digested into glucose and absorbed into the bloodstream. Glucose, which requires no digestion and is immediately absorbed, is assigned a Gl of 100. Most other foods have lower indices, which means they take a longer times to be digested and absorbed.
On the Glycemic Index chart, foods are generally classified as follows:

High Glycemic Index foods (Gl above 70) are rapidly digested and they raise the blood glucose level quickly.

  • GI 100: Glucose
  • GI 80-90: Cornflakes, carrots, maltose, honey, white potatoes
  • GI 70-80: Whole grain bread, millet, white rice, new potatoes, watermelon, soda drinks

Medium Glycemic Index foods (GI 50-70) have a moderate impact on blood glucose levels.

  • GI 60-70: White bread, bananas, raisins, Mars Bar chocolates
  • GI 50-59: Spaghetti, corn, whole cereals, peas, yam, potato chips

Low Glycemic Index foods (Gl below 50) foods have minimal impact on blood glucose levels.

  • GI 40-49: Oatmeal, sweet potatoes, navy beans, oranges, orange juice
  • GI 30-39: Peaches, cherries, blue berries, apples, ice-cream, milk
  • GI 20-29: Kidney beans, lentils, fructose
  • GI 10-19: Soybeans, peanuts
  • GI below 10: Most green vegetables

Limitations of the Glycemic Index

The Glycemic Index, however, is not perfect. Do take note of the following:

  • There is some dispute over what is "high", "moderate" or "low" Gl. The general consensus is spelled out as above. If you are obese, however, you need to be more stringent in selecting your foods and perhaps avoid some of the moderate Gl foods. 
  • Foods with a lower GI have a lesser impact on blood glucose levels and are generally considered healthier. But there are some puzzling surprises. For example, brown rice (GI 87) has a higher GI than white rice (GI 83). This goes against common sense since the fibre in brown rice would logically slow down digestion and glucose absorption.
    Moreover, brown rice, white rice and carrot (GI 85) are rated higher than millet, cabbage, soda drinks and white sugar (all GI 70). Does this mean we are worse off eating rice and carrots? And that millet, cabbage, soda drinks and sugar all have a similar impact on our blood glucose levels?
    Another problem is inconsistency. In the case of French baguette, most Gl charts give a high figure of 95. Yet at least one chart gives a very much lower figure of 57 plus or minus 9, meaning it could be as low as 48! Meanwhile, some foods in the medium-Gl range are equally questionable. They include ice cream (GI 60), Ketchup (GI 55) and Nutella® (GI 55). Are these foods supposed to be healthier than foods like brown rice and carrots?

This content is adapted, with permission, from Book 1 of 2 : The Wonders of Nutrition by Dr Ang Poon Liat. MBBS, M.MED (PAED), MRCP (UK PAED), FAMS, MD.



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