Is fiber important in human nutrition? Ask your doctor. Ask the internet. Ask anyone this side of Neptune. But then follow this up with a simple question: “What exactly is fiber?” Hmmm.
Ruminant nutritionists – folks like me who work with cattle, sheep and other herbivores – have an unambiguous definition of fiber. But the human nutrition world is a bit different, and so is its definition of fiber. Human nutritionists call it dietary fiber, and it contains some additions and provisos. Confused? Let’s examine this topic a bit more carefully.
Here’s a homework assignment: Go to your food pantry, pull out any commercial product, and look at the outside of the box. All commercial food packages have a “Nutrition Facts” label that contains quite a few numbers. These numbers tell an interesting story. For example, I’m in my kitchen looking at a package of whole wheat bread. Its Nutrition Facts label specifies that one serving equals a single slice of bread weighing 43 grams. The USDA Nutrient Database lists the moisture content of whole wheat bread at 39%, meaning that one serving of this bread contains 26.2 grams of dry matter (= 61% of 43). The Nutrition Facts label also lists dietary fiber at 3 grams. Thus, on a dry matter basis, the dietary fiber of this bread is 11.5% (= 3 ÷ 26.2 as a percentage).
But back to our question: What is dietary fiber? It’s the standard terminology used on human food labels, but what precisely does it mean?
First let’s review some background. The standard term used in ruminant nutrition is neutral detergent fiber (NDF). This is a main fiber value used to balance livestock rations and estimate the digestibility of feedstuffs, particularly forages. Originally developed in the 1960s by Peter Van Soest, NDF accurately identifies the amount of fibrous cell walls of plants. All plant cells are enclosed in a cell wall that is composed primarily of three main types of structural fiber molecules: cellulose, hemicellulose and lignin. Analytical laboratories routinely analyze NDF into its component parts. But cell walls are extremely complex structures. In addition to those three main types of fiber, the NDF number also includes minor components like chitin, silica and Maillard products (indigestible fiberlike compounds that develop in heat-damaged hay or silage).
Under some conditions, the NDF assay may also include starch, but this is a serious problem that must be corrected. Starch is a large, complex carbohydrate polymer that plants use for energy storage. But sometimes, depending on the type of starch and its molecular geometry, it gets caught up as a contaminant in the NDF assay, artificially inflating the value of NDF. But starch is clearly not part of the cell wall. Starch is not a structural fiber, and rumen microbes ferment it quite differently than true fiber. The good news is that nutritionists have solved this analytical problem by first treating the feed sample with the enzyme amylase. This breaks down the starch and makes it soluble. The amylase treatment thus removes starch from the fiber, leaving only the true structural fiber molecules in the sample. Laboratory reports now label this value as aNDF (amylase-treated NDF).
There are, however, three important plant compounds that NDF does not include. Two – pectins and galactans – are fiberlike molecules. Pectins are a class of gums common in fruits like apples, oranges and apricots. Galactans are complex molecules often involved in the three-dimensional structure of cellulose. Although both are associated with the cell wall, they are both relatively soluble and are highly digestible by ruminants because they ferment easily in the rumen. The third substance not included in NDF is the class of molecules called fructans. These common storage carbohydrates in plants are also quite soluble and easily fermented by rumen microbes.
In the big picture, a critical nutritional characteristic of humans is that they don’t have a rumen. We are not herbivores. We do not spend our time grazing high-fiber plants, and we don’t chew our cud. Our digestive anatomy is quite different than that of a ruminant. Therefore, the concept of fiber in human nutrition must encompass a wider spectrum of molecules than those analyzed by NDF.
Human nutritionists have been grappling with this problem for a long time. A few years ago, they settled on the term dietary fiber, which has an official definition in the CODEX Alimentarius. I would guess that this reference book is not exactly a household word to most folks who buy food in supermarkets, but it’s quite important in the human nutrition world. It’s a formal set of codes published by the CODEX Alimentarius Commission — an organization developed by the United Nations to devise and standardize definitions that could be recognized worldwide. I’ll summarize the definition of dietary fiber to avoid the convoluted legalese of the actual code. Basically, the CODEX Alimentarius defines dietary fiber as the nondigestible molecules in food that are carbohydrate polymers with a degree of polymerization of 3 or more, with the added caveat that these molecules show health benefits.
Degree of polymerization? Whoa! Yes, this is a bit of jargon, but let’s translate. Recall that polymers are molecules composed of many repeating units, kind of like a string of pearls in a long necklace. Classically, we think of polymers as synthetic molecules like nylon and polyethylene, but plants and animals also make lots of polymers. When the repeating unit is glucose, common glucose-based polymers are molecules like cellulose and starch. For those molecules, the repeating unit is also called the base unit. The degree of polymerization (DP) is simply the number of repeats of the base unit. Large molecules like cellulose can have a DP in the thousands; small polymers like the breakdown products of starch can have a DP of 20 or less. There is some controversy among human nutritionists about including fibers with DP values between 3 and 9, as these are molecules with very short chain lengths, but as of this writing they have been included in the list.
The primary issue here is how much of the carbohydrate polymers are broken down by human digestive enzymes. This depends greatly on the type of chemical bond between the repeating units. If we have the digestive enzyme that breaks the bond, the molecule is easily digested in the small intestine. For example, many forms of starch are easily digested. But if we don’t have the appropriate enzyme, the digestibility is much less or even zero, like cellulose, and the molecule then flows farther down the tract into the large intestine. Then it’s up to the microbes in the large intestine to ferment these molecules and provide some nutritive value and/or health benefits.
Starch, of course, is a mainstay of human nutrition because it is the major storage molecule in all our grains. But starch is also an oddball class of molecules because it has so many variations, and each variation can affect its digestibility in the small intestine. What variations? Well, things like variations in its structure, the number of cross-linkages, the degree of gelatinization, etc.
In ruminants, most of these variations don’t matter very much, and most starch molecules are rapidly fermented by the rumen microbes. Some starch molecules, however, may be more crystallized than others, depending on their processing and heat. These molecules may escape the rumen unscathed, but then they can be digested by enzymes in the small intestine or fermented by microbes in the large intestine. So, with ruminants, the amount of starch that ends up in the manure is usually insignificant.
But humans have no rumen to ferment the starch. In humans, starch molecules move quickly into the stomach and then into the small intestine. The human amylase enzymes don’t have much time to digest these large molecules, especially if some of these molecules are gelatinized or are resistant to amylase. These are forms of resistant starch which then flow intact into the large intestine. The ultimate effect of this process is that these resistant starch molecules can act like fiber in the human GI tract, and this amount can be significant.
Those other non-NDF compounds – pectins, galactans and fructans – also have characteristics like resistant starch. Humans don’t have the enzymes to digest them. These molecules all flow undigested through the human digestive tract until they reach the large intestine. Nutritionally, they act like fiber.
Let’s come back to the original question about human food labels: “What exactly is dietary fiber?” Very simply, dietary fiber is composed of the components of NDF, plus a few additional substances like pectins, galactans, fructans and some forms of resistant starch.
There you have it. Dietary fiber is more than ruminant fiber. The definition clearly depends on whether or not you chew your cud. But for homework and extra credit, you can visit your favorite supermarket and read some package labels. Bring your calculator. See if you can find any common foods that contain at least as much dietary fiber as alfalfa hay.
