What Is Dietary Fiber and The Glycemic Index?

Last update November 11, 2021, article reviewed & updated multiple times since April 20, 2001.

What You Need to Know The human body secretes carbohydrate-digesting enzymes of several kinds both in saliva and in the gastrointestinal tract, but we don’t secrete an enzyme for the breakdown of every conceivable kind of carbohydrate. The glycemic index is a number that represents the speed of carbohydrate breakdown in an ‘average’ person’s body, and the ‘average’ effect on the blood sugar levels of that ‘average’ person of any particular carbohydrate food. The glycemic index shows that complex carbohydrates and starches are not necessarily better for our blood sugar levels than simple carbohydrates and sugars are.

A Review Of Carbohydrates

We have previously discussed what is actually meant by the words ‘carbohydrate‘, ‘sugar‘, and ‘starch,’ and how these dietary elements relate to one another. As a quick review, you will recall that carbohydrates are either simple sugars, complex sugars, or starches.

Remember that both starches and complex sugars very easily and quickly break apart into simple sugars by coming apart at their water bond connections. The simple sugars are broken down (digested) by the body, into the simple sugar called glucose, because the body runs on glucose. Since all carbohydrates that can be digested become simple sugars, there is no such thing as a ‘bad’ or ‘good’ carbohydrate. There are only carbohydrates that can be digested and carbohydrates that can’t be digested. This is where we encounter the concept of dietary fiber.

What Is Dietary Fiber?

Although words ending is the letters ‘-ose’ are sugars, some of them, like cellulose, are not available to our bodies as sugar.

You will recall that the actual definition of the word ‘carbohydrate’ is ‘one of many combinations of carbon, hydrogen, and oxygen that come apart at water bonds.’ But there is nothing in this definition that implies that all carbohydrate combinations come apart during the digestive processes of our bodies.

Consider this. The bonds holding carbohydrate substances together can’t merely come apart by themselves. If that were so, they would just fall apart all over the place, and there couldn’t be such a thing as a ‘complex’ carbohydrate. The bonds must have structural strength, and that means something is required to cause or permit the breakdown of the carbohydrates to occur. This something is called a ‘catalyst.’ For digestion to occur, the necessary catalysts for digestive breakdown are substances in our bodies called ‘enzymes.’

The human body secretes carbohydrate-digesting enzymes of several kinds both in saliva and in the gastrointestinal tract, but we don’t secrete an enzyme for the breakdown of every conceivable kind of carbohydrate. Since we don’t secrete an enzyme that digests cellulose, we can’t eat trees. Wood is made of several types of carbohydrates, but we don’t have the right enzymes to break the connecting bonds of those kinds of carbohydrates. So when we say that a certain carbohydrate (like cellulose) is not digestible, we mean that we do not produce the necessary enzymes to break the water bonds, and therefore the simple sugars that would be in those non-digestible complex carbohydrates are not available to us as food.

Non-digestible carbohydrates are variously called dietary fiber, crude fiber, indigestible residue, gums, and roughage. Although the fiber doesn’t contribute to our nutritional needs directly, it is essential to our bodies because it causes the action necessary to clear the intestinal tract. In particular, fiber helps with diets designed for weight loss because it takes up room in the digestive tract without adding useful sugars. This is the reason some low-carb diets tell us we don’t have to count fiber. A person could mix a pound of sawdust into a cup of ice cream, and it wouldn’t make the slightest difference in how much simple sugar reached his or her digestive system. The only food available to his or her body would be in ice cream.

Some dietary fiber dissolves in water or absorbs water. We call this type of fiber ‘soluble fiber.’ Fiber that does not interact much with water, we call ‘insoluble fiber.’ Soluble fiber is very useful as a prevention for constipation since the water causes the fiber to swell up, thereby producing more bulk in the intestinal tract. But do not confuse solubility with digestibility. They aren’t the same thing.

We now understand that how (or if) carbohydrates are (or can be) digested in the body depends upon how they are put together by nature, and whether or not we have the enzymes to break the bonds to get at the simple sugars. This leads to the next subject: the glycemic index.

What Is The Glycemic Index?

The word ‘glycemic’ means ‘having to do with glucose in the blood.’ The glycemic index is a number that represents the speed of carbohydrate breakdown in an ‘average’ person’s body, and the ‘average’ effect on the blood sugar levels of that ‘average’ person of any particular carbohydrate food. It is not a very scientific measurement, and we should all take it with many grains of salt!

Different carbohydrate foods have different glycemic index numbers for four basic reasons: the differences in the way the carbohydrates are put together by nature (as we discussed above), the ease in which the complex carbohydrates come apart under the action of our bodies’ enzymes, the degree to which the carbohydrate foods are already broken down by other processes outside our bodies (such as the difference between cooked carrots and raw carrots), and how much fiber, protein, and/or fat is in the food, in addition to the digestible carbohydrates.

How Is The Glycemic Index Of A Particular Food Determined?

A group of people is assembled, and the fasting blood sugar levels for each individual are measured. The test subjects than all eat a specified amount of a particular food. Their blood sugar levels are measured frequently for the next several hours, and an average of those various blood sugar levels is determined. From this information, a glycemic index value is given on a scale of from one to one hundred, comparing it to a so-called reference food, which is given the arbitrary value of 100.

Now it gets even more confusing. There is no standard for what constitutes reference food. Some glycemic index systems use glucose as the standard, but others use white bread. On the scale where glucose is 100, white bread is 70. But on the scale where white bread is 100, glucose is over 125.

While it is true that the lower the number, the less the impact that food will have on the ‘average’ person’s blood sugar, the actual numerical values depend on which scale has been used, and there is no particular reason why someone else might not come up with another scale in the future, based on a completely different reference food. Furthermore, there is really no such person as ‘the average person.’ We are all unique.

Why Is Knowing The Glycemic Index Of Foods Valuable To Low Carbers?

Even with all its faults, the glycemic index has value to low carbers. It shows that complex carbohydrates and starches are not necessarily better for our blood sugar levels than simple carbohydrates and sugars are. The glycemic index of some complex carbohydrates is actually much higher than the index number of some simple sugars. For example, pure sugar has a glucose scale index of about 65, while rice cakes have a glucose scale index of more than 75!

A low fat/high carb diet increases the problems brought on by high glycemic index foods because high index foods are recommended, and because foods that would reduce the effect of the rising blood sugar, such as meat and fat, are shunned. This results in swings in blood sugar levels, increased insulin release into the blood, hypoglycemia episodes, higher blood sugar levels for diabetics, and all the problems we recognize as being associated with insulin production.

In Conclusion & What’s Next

As we go along in this series of articles, we will be discussing diabetes and the effects of insulin in greater detail. But in the next article, we’ll continue our discussion of carbohydrate breakdown under the influence of enzymes, and what happens to the carbohydrates once they are in the body.

I’ll be here. I hope you’ll join me.

The Science of Low-Carb & Keto Diets Presented by CarbSmart.com This article is number 2 of 24 articles describing The Science of Low-Carb & Keto Diets. Visit this link or click the above image for all of the articles in the series.	Article History Last Update November 9, 2021 Updated January 8, 2018 Updated May 24, 2010 Updated August 1, 2003 Original Article April 20, 2001
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About Dr. Beth Gruber Dr. Gruber is a graduate of the Southern California University of Health Sciences and has been in private chiropractic practice in Long Beach, California since 1964. She also received both a Bachelor’s Degree and a Master’s Degree from California State University at Long Beach. She has written on health-related subjects for over 30 years, for several different publications. She lives in Southern California with her husband of 33 years. Both she and her husband follow and live the low-carb lifestyle full time.

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