Glycemic Index Explained

Many of you may be familiar with this term, its used to market certain carbohydrate and grain products as being “better for you”
But what does it mean?
​
The glycemic index (GI) is the measure of the rate of which an ingested food causes the level of glucose in the blood to rise. This tells us how much blood sugar goes up when we consume 50 g of usable carbohydrates from a particular food.
It is a relative measure determined against a specific reference food.

In general the less processed and higher fibre a food is, the more complex its carbohydrate molecules usually are. Due to this , these foods typically take longer to digest and have a lower GI.

High glycemic foods include sugar, candy, breakfast cereal and bagels. Lower GI foods include legumes, whole grains and vegetables.

When blood sugar goes up quickly, insulin is usually quick to respond. The amount of insulin released usually matches the amount of glucose present.

This had led to some people to suggest that a low GI diet is a healthy one.

The problem with gi as a measure

While GI is an interesting measure of the physiological response to carbs in the diet, it does not tell the entire story.

1. We don’t eat most foods by them selves (for example you would not just eat bread on its own). Protein, fat and fibre all change GI  level , eating carbs as part of a full meal will change GI levels.
2. Many factors can affect how quickly a food is converted to glucose, including:
   when we eat that meal during the day
   Wether we have been active
   This can vary person to person, day to day and meal to meal.
3. The GI score can be easily manipulated by the TYPE of sugar in the food. Foods with large amounts of fructose tend to have low GI scores, because fructose does not immediately raise blood glucose levels. But that does not mean foods any healthier.
4. The GI index is based on a standardised amount of carbohydrates not a standardised amount of food. In order to calculate GI of carbohydrates we’d only need a small amount of sugar, but quite a lot of carrots for example. Making this an unrealistic comparison.

Glycemic Load

Researchers often use glycemic load as a more realistic measure. The glycemic load of a food is based on the GI X by the serving size of the food.
While this gives a better picture of how fast blood sugar may go up after a meal, GL still has some of the same problems as GI. And it does not take into account other elements like fibre, micronutrients, phytonutrients etc.

Choosing carbs wisely 

The GI and GL does not give us the whole story. Most people should not be using these (or any numbers) to dictate what and how they should eat and enjoy.
Nutrition is not black and white, carbohydrates and most foods are more complicated than this.

References 

Barardi et all 2018 The Essentials of Sport and Exercise nutrition Third Edition.
​