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What Is the Glycaemic Index?

Posted by Manuela Boyle on 11 October 2022
What Is the Glycaemic Index?

The glycaemic index (GI) is a measure of how much certain foods increase blood sugar levels.

It scores foods on a 0–100 scale and ranks them as low, medium, or high 

Here are the score ranges for the three GI values:

Low: 55 or less
Medium: 56–69
High: 70 or above
Foods high in simple carbs or added sugar are broken down more quickly in the bloodstream and tend to have a higher GI.

Meanwhile, foods high in protein, fat, or fibre have less of an effect on blood sugar levels and typically a lower GI.

Several other factors may also influence GI value, including food particle size, processing techniques, and cooking methods.

For example:

Glycaemic index of sweet potatoes

The way that foods are cooked can have a major impact on the glycaemic index of the final product. This is particularly true of sweet potatoes.

Boiling is thought to alter the chemical structure of the sweet potato, preventing spikes in blood sugar levels by allowing the starch to be more easily digested by enzymes in your body 

When boiled, they’re also thought to retain more resistant starch, a type of fiber that resists digestion and has a low impact on blood sugar levels 

Boiled sweet potatoes have a low to medium GI value, with a greater boiling time lowering the GI.

For example, when boiled for 30 minutes, sweet potatoes have a low GI value of about 46, but when boiled for just 8 minutes, they have a medium GI of 61 

The roasting and baking processes destroy resistant starch, giving roasted or baked sweet potatoes a much higher glycaemic index 

Sweet potatoes that have been peeled and roasted have a GI of 82, which is classified as high 

Other foods with a similar GI value include rice cakes and instant oat porridge 

Baked sweet potatoes have a significantly higher glycaemic index than any other form.

In fact, sweet potatoes that have been peeled and baked for 45 minutes have a GI of 94, making them a high-GI food 

This puts them on par with other high-GI foods, including white rice, baguettes, and instant mashed potatoes 

Compared with roasted or baked versions, fried sweet potatoes have a slightly lower glycaemic index due to the presence of fat. This is because fat can delay the emptying of the stomach and slow the absorption of sugar in the bloodstream 

Still, when they’re fried they have a relatively high GI.

Although the GI value can vary, sweet potatoes that have been peeled and fried in vegetable oil typically have a GI of around 76 

This puts them on par with cake, doughnuts, jelly beans, and waffles 

The GI of sweet potatoes varies based on the cooking method. While boiling gives a low to medium GI value, roasting, baking, and frying all give high GI values.

The bottom line
Sweet potatoes can have a low, medium, or high glycaemic index depending on how they’re cooked and prepared.

Boiled sweet potatoes affect blood sugar levels far less than other varieties, such as fried, roasted, or baked versions. Longer boiling times reduce the GI further.

To support better blood sugar control, it’s best to select healthy cooking methods and enjoy sweet potatoes in moderation.

Still confused? Ask us!!

Author:Manuela Boyle


  • The Institute for Functional Medicine
  • Society for Integrative Oncology
  • Naturopaths and Herbalists Association of Australia
  • Australian Traditional-Medicine Society
  • British Naturopathic Association