Watermelon has a high glycaemic index (GI)

If you're making a conscious effort to eat low GI foods, does this mean you shouldn't eat watermelon?

Watermelon has a high GI, but compared to other foods you might eat, it doesn’t have as much carbohydrate because it is mainly water.

The small amount of carbohydrate in watermelon is absorbed very quickly making it a high-GI food, but because there is so little it won’t have much effect on your blood glucose levels, so in terms of impact, it is very low. This is termed ‘glycaemic Load’.

Glycaemic load

Although a food may have a low GI it’s also important to consider how much carbohydrate it has. Glycaemic load considers the amount of carbohydrate in a food as well as its GI. Both of these together determine the blood glucose response.

To work out glycaemic load (GL), multiply the GI x the grams of carbohydrate, then divide by 100.

For example:

• 1 cup of watermelon: (GI of 72 x 9 grams carb) / 100 = 6.5. 
  
• 1 cup of sweetcorn (GI of 37 x 32 grams carb) / 100 = 11.8. 
  

The differing amount of carbohydrate affects the glycaemic load and therefore the blood glucose response in your body.

Portion size also affects glycaemic load. For example, one cup of sweetcorn has a GL of 11.8 but half a cup only has a GL of 5.9.

What this demonstrates is that using GI in isolation does not give you the full story. You need to consider the amount of carbohydrate in the food you are eating.  For this reason, GI is most useful when choosing between foods with a high percentage of carbohydrate and is fairly irrelevant when foods contain a low percentage of carbohydrate. 

It also highlights that when a food has a low GI it does not mean you can eat huge volumes of it.

In short

When considering a low- or high-GI food we need to consider how much carbohydrate a food contains.

Bread, rice, pasta and cereals are mainly carbohydrate so choosing low-GI varieties makes a difference. But with foods that have little or no carbohydrate such as pumpkin, nuts, meat, fish, chicken, avocado and eggs, the glycaemic index doesn’t really matter.

In fact, some of these foods do not even have a GI value because there’s not enough carbohydrate to be bothered with.

All fruit and vegetables are fantastic for health because they are packed full of antioxidants and nutrients.