Part 8

Beyond Macronutrients: The Glycemic Index

If net calories stored as fat, metabolism and hunger each vary by macronutrient, might they also vary by other differences in food?

They might and they do.

Glycemic Index:

Glycemic Index is a very popular concept today (2011) and it deserves a great deal of explanation.

Glycemic index is a measure of how foods affect human blood glucose (sugar) concentration over a short time (several hours). Specifically, GI reflects how rapidly and how long human blood glucose levels rise in response to eating a measured amount of a food on an empty stomach. Foods that cause a rapid rise in blood sugar are "HIGH glycemic index" food whereas foods that cause a slow rise in blood sugar are "LOW glycemic index" foods.

Why Glycemic Index Matters:

Recent medical research shows that rapid and significant rises in blood glucose levels caused by "high GI" foods can harm human health and contribute to weight gain.

Let's examine this by using a graph to illustrate the effects of low and high GI foods on human blood sugar levels:

Glycemic Index and blood sugar over time

Figure 10: Effect of high and low glycemic index foods upon normal (non-diabetic) fasting human blood glucose concentration over time.

Notice several things.

1.     High GI foods cause glucose levels to rise very fast and very high.

2.     High GI foods cause glucose levels to FALL very fast and very low. In fact, high GI foods consumed after a fast actually cause blood sugar levels to fall BELOW normal. This causes people to experience great hunger.

3.     The "area under the curve" is greater for high GI foods than low GI foods.

Now, to understand how high GI foods can harm human health we need to add one more parameter to our graph and that is insulin level.

Insulin is the hormone, made in an organ called the pancreas that tells the cells in our muscle and other places to absorb blood glucose. Insulin is secreted into the blood by the pancreas (in non-diabetic people) in response to increases in blood sugar. Insulin levels fall when blood sugar returns to normal.

Glycemic Index, blood sugar and insulin levels over time

In the graph above, I have added curves representing blood insulin levels associated with the blood sugar curves in the earlier graph. Let's focus upon the purple insulin curve which belongs to the red "high GI" curve. Notice that insulin levels rise in response to the rapid rise in blood sugar but also notice that the response takes time and is not instantaneous. This is because insulin synthesis and secretion takes time. The effect of this delay is that a bit too much insulin in produced in response to a very rapid increase in blood sugar so that blood sugar not only falls rapidly, but it also falls too low before finally recovering to a normal level. During the time that blood sugar has fallen below normal, a person generally experiences intense hunger; more than the normal hunger associated with fasting. This leads us to the first problem with high GI foods:

High GI foods trigger rebound low blood sugar that increases hunger.

The second problem with high GI foods has to do with insulin. High levels of insulin, repeated over and over again, day after day and year after year will eventually lead to insulin resistance. This means that the muscle and other tissues in the body that normally respond to insulin by absorbing blood glucose (and thereby lowering blood sugar)--that these tissues begin responding less and less well to insulin and in turn are less and less effective at lowering blood sugar levels. This sort of insulin resistance has another more familiar name: type 2 (adult-onset) diabetes.

High GI foods cause high insulin levels that MAY contribute to Insulin Resistance, Metabolic Syndrome and Type 2 Diabetes.

To be clear, I am not suggesting that a high-GI diet will CAUSE diabetes. In fact, the biggest risk-factor for type 2 diabetes is obesity, particularly abdominal obesity, but… high GI foods can contribute to obesity and, because of their independent effects upon insulin resistance, can contribute to type 2 diabetes.

THIS is why glycemic index matters so much and why it is far more than just a curiosity.

Understanding Glycemic Index

Glycemic Index is not easy to understand and several points need to be emphasized:

1.     GI is non-intuitive: Nobody can predict the exact GI of a food without measuring it.

2.     GI can change dramatically by adding other foods

3.     Generally, adding low-glycemic or non-glycemic foods to high GI foods will LOWER and not raise the overall GI of a meal.

4.     GI is only useful for foods that contain absorbable carbohydrate. This means the GI is not measured for "non-glycemic" foods like pure meat, eggs, fat or pure fiber.

5.     GI can only be determined through measurement of blood sugar level in ten human volunteers over many weeks.

6.     Because of this, it is very labor-intensive and expensive to measure the GI of foods.

7.     Because of the work and high cost involved in measuring the GI of foods, only about 4000 foods have actually had their glycemic indices measured. In other words, we don't know the GIs of many foods.

8.     It is easier to predict a low-GI food than a high-GI food. To put it another way, nearly all foods with dilute carbohydrate that is mixed with lots of fiber, protein or fat will have a relatively low GI, BUT, not ALL foods that are nearly pure carbohydrate will necessarily have a high GI. This means that generally, you're safe eating whole grains, whole vegetables and fruits (except watermelon which is high-GI). Also remember that foods with very little carbohydrate like meats have almost no effect on blood sugar.

Eating a Low-GI Diet:

Avoiding foods that rapidly raise blood sugar is not difficult. You will need to memorize perhaps twenty high-GI foods to avoid, but other than that you simply need to know that if a food contains carbohydrate (starch or sugar), it's GI will be low if it also contains abundant fiber, protein or fat. For example, boiled white rice eaten plain is a high-GI food, but boiled brown rice is not and especially not if you eat it with meat, vegetables and a little fat. I will discuss these sorts of GI-lowering strategies later in the book's cooking chapter.


Glycemic Load:

The idea behind glycemic load is that the amount of carbohydrate containing food consumed can alter its effect upon blood sugar. For example, a teaspoon of a very high-glycemic food like pure glucose will hardly change blood sugar at all while a large amount of a medium-GI food might. In other words, amount matters. NOT a big surprise.

For the record, the formal definition of glycemic load is:

Glycemic Load = Glycemic Index X Grams of Absorbable Carbohydrate

Do NOT bother to remember this. The point is simply, once again and as with all foods, AMOUNT MATTERS.