WHY Foods Have a Low or High GI Classification

07 Nov 15:


I like to spend my time on the ElliptiGO indoor elliptical trainer productively and I find it is a good time to write short articles to address the more regular queries I get from various people to whom I am linked in my training community.

One such topic, which keeps ‘popping up’ under various different guises, is the issue of slow release & fast release foods, high & low glycemic / glycaemic index (high GI & low GI foods) – and, ultimately, what types of foods to eat for endurance training and performance.

Today I am writing specifically about WHY foods actually behave this way (because I’ve found that an understanding of this concept of WHY seems to be an even better way to explain the aspect of WHAT). I have already addressed the basics of high and low GI (slow and fast release) foods in an earlier article, so please initially refer to that short article for my explanation of the basics.

In this article I will break down the explanation of HOW and WHY foods get this description, in order to make it easier to remember (for those readers who are still not perfectly in sync with this concept and how to apply it to their training and lifestyle).

Slower release foods (lower GI foods) are generally better for our health – and are also better for our physical performance (and appearance), over the longer term. They help our bodies to function optimally and to utilise all their available ‘power sources’ – namely carbohydrate, protein and fat (which the body tends to selectively burn in that order of preference – based on how easily these nutrients are stored and accessed).

If in doubt as to what are slow or fast burn foods, an explanation of WHY they have these characteristics will often help you identify the broader food groups from the various selections of food you come across. A good way to explain this concept is to use the examples of everyday household food items.

  1. Consider a serving of powdered milk, used in your coffee. It might contain up to 50 calories.
  2. Then consider a small apple, which also contains the same amount of calories( 50 calories).
  3. And finally consider an even smaller piece of meat, which also contains 50 calories.

So we are assessing 3 different foods, with 3 different GI classifications – but a similar calorie content…

At the Start of PBP 2015 (1,230km in under 90 hrs)...

Let’s address each of the foods according to their ‘GI rating’.

1) The powdered milk generally has a high glyceamic / glycemic index. This means it’s called a “high GI food” and it will release the 50 calories it contains within minutes of being consumed. That’s because it does not require much digestion in order to break it down to its constituent parts (because it is a highly processed food, already very close in nature to pure sugar/glucose).

This fast release of the 50 calories of energy means that your body is less able to utilise the 50 calories completely, unless under very high exercise demands and struggling to keep up the energy supply. Your body does two things under high GI circumstances:

– Firstly it panics – and releases a lot of insulin (for a longer time than required to get rid of a few teaspoons of powdered milk). This high insulin level causes a rapid dropping of the blood glucose levels – which, if unchecked, can lead to the individual falling into a coma (from hypoglyceamia / hypoglycemia).

Much as a diabetic person would do under conditions of a hypolgycaemic / hypoglycemic ‘insulin overdose/imbalance’, such an individual must counter the rapid drop in blood sugar by eating more fast release sugars. These conditions cause the body to desperately crave those substances – and if those cravings are followed (and more sugary foods eaten) the cycle repeats. This cycle is the ‘trap’ of fast-release high GI foods.

– Secondly, when there is a sudden influx of sugar into the bloodstream due to eating a high GI food source, the body shuts off energy flow systems based on all its other energy sources (proteins and fats) in order to focus on burning off the sugar quickly. If fatty foods are being consumed at the same time as the high GI food source, or if the body needs energy for exercise, it will tend to rely heavily on the high GI food source (and completely shut out the fat stores and protein stores – meaning that people who exercise under these high GI conditions will not lose body fat and will not lose weight).

2) The apple, on the other hand, is a medium-to-low GI food, but it still releases the full 50 calories when it’s fully digested (a process which could take a few hours). This means your body can use the energy and nutritional content at a more natural, measured rate – as it slowly releases them throughout the process of being digested. These nutrients are used to fuel everyday activities and to gain minerals and vitamins, etc. This slow digestion allows precise production of insulin, only when it’s needed.

3) The 50 calorie piece of meat has a very low GI and takes much longer to fully digest and release its constituents than even the apple. It could take a whole day to fully digest the meat and release those 50 calories, meaning that the insulin requirement in the body to handle the energy released is, if anything, negligible.

The less insulin secreted in the processing of a food, the better. When people ask me how I am able to ride for more than 24 hours while fasting, I give the above explanation for how I derive my energy (which is kept in stored form in the muscles and fat). That type of energy is stored either in the muscle cells which need it (in the form of glycogen), or it can enter them without the involvement of insulin (in the form of blood-borne free fatty acids).

Low GI eating – like fasting – encourages the increased storage of energy in your muscles and a more efficient breakdown of fat for fuelling exercise. This reduces fat stores and reduces insulin production (which gets rid of sugar cravings – when you have adapted to the process). Remember: high insulin production fuels cravings for high calorie and high GI foods.

I hope this clearly explains why foods have high or low GI classifications and what the impact is of high or low GI foods on an athlete’s body. If you want to comment or query anything discussed in this article, just get in touch.