Ruminant nutritionists – folks like me who work with sheep and cattle – have a fairly unambiguous definition of fiber. But human nutrition has a different definition of more or less the same thing, but with some additions and provisos. Confused? Let’s examine this topic a bit more carefully.
Here’s something to do: Go to your food pantry, pull out any commercial product and look at the outside of the box. The label “nutrition facts” tells an interesting story. For example, I’m in my kitchen looking at a package of whole wheat bread. Its nutrition facts specify that one serving is a single slice of bread weighing 43 grams (g). The USDA nutrient database lists the moisture content of whole wheat bread at 39%, which means one serving of this bread contains 26.2 g dry matter (which equals 61% of 43). The nutrition facts label also lists “dietary fiber” at 3 g. Thus, on a dry matter basis, the dietary fiber of this bread is 11.5% (equaling 3 divided by 26.2).
But here’s our question: What exactly is dietary fiber? It’s the term used on human foods, but this term does not exist in a vacuum. For background, let’s examine a standard term used in ruminant nutrition: neutral detergent fiber (NDF). This is a main fiber value used to balance rations and estimate the digestibility of feedstuffs, particularly forages. Originally developed in the 1960s by Peter Van Soest, NDF accurately identifies the fibrous cell walls of plants. All plant cells are enclosed in a cell wall (animal cells are not).
This cell wall is composed primarily of three main types of structural fiber: cellulose, hemicellulose and lignin. Although laboratories can routinely analyze NDF into its component parts, for this discussion, let’s step back and focus on the general concepts. Cell walls are extremely complex structures. In addition to those three main types of fiber, NDF includes minor components like chitin and silica and also Maillard products, which are indigestible fiber-like compounds that develop in heat-damaged hay or silage.
Under some conditions, the NDF assay will also include starch, which is a problem. Starch is a large, complex carbohydrate polymer that sometimes, depending on the type of starch and its molecular geometry, gets caught as a contaminant in the NDF assay. This artificially inflates the value of NDF. But starch is clearly not structural fiber, and rumen microbes ferment it quite differently than true fiber. Nutritionists have solved this problem by first treating the feed sample with the enzyme amylase which breaks down the starch so only the true structural fiber molecules are left in the sample. Laboratory reports now label this value as aNDF (amylase-treated NDF).
There are, however, three important plant compounds NDF does not include. Two are fiber-like molecules: pectins and galactans. Pectins are a class of gums common in fruits like apples, oranges and apricots. Galactans are complex molecules often involved with 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 is the fructans, which are common storage carbohydrates in plants. These are also quite soluble and are easily fermented by rumen microbes.
In the bigger picture, one critical nutritional characteristic of humans is that they don’t have a rumen. We are not herbivores who spend their time grazing high-fiber plants, and our digestive anatomy is quite different than a ruminant. Therefore, in human nutrition, the concept of fiber 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 CodexAlimentarius. I would guess this reference 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 originally developed by the United Nations to devise and standardize definitions to be recognized worldwide. I’ll summarize the definition 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? You might recall 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 plastic and nylon, but plants and animals also make lots of them. When the repeating unit is glucose, typical glucose-based polymers are molecules like cellulose and starch. 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 some 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 currently they have been included.
The primary issue here is how much polymer is broken down by human digestive enzymes. This depends greatly on the type of chemical bond between the subunits. If we have the enzyme that breaks the bond, the molecule is easily digested, like some forms of starch. If we don’t have the proper enzyme, the digestibility is much less or even zero, like cellulose. Then, it’s up to the microbes in the large intestine to ferment these molecules and provide some nutritive value or health benefits.
Starch, of course, is a mainstay of human nutrition, as it is the major storage molecule in all the grains. But it’s an oddball class of molecules that puts wrinkles in most fiber definitions because of the variations in its structure, the amount of cross-linkages, the degree of gelatinization, etc. In ruminants, most starch molecules are rapidly fermented by the rumen microbes. Some starch molecules, however, may be more crystallized than others, depending on processing and heat. These molecules may escape the rumen unscathed, but then they can be digested in the small intestine or fermented in the large intestine.
The amount that ends up in the manure is usually minor. In contrast, in humans, starch molecules move directly and quickly into the small intestine. The human amylase enzymes don’t have much time to digest these molecules, and if some of those molecules are gelatinized or resistant to amylase, they move intact into the large intestine. These are forms of resistant starch. The effect is that some starch molecules can act like fiber, and this amount can be quite 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 flow undigested through the human digestive tract until they reach the large intestine. Nutritionally, they act like fiber.
Fiber is important in human nutrition primarily because of its effects in the large intestine (also known as the colon). The human colon contains vast populations of microbes, and just like their counterparts in a rumen, they ferment the fiber molecules and release various compounds including volatile fatty acids, which are small molecules absorbed as nutrients. Additionally, the fermentation products cause an influx of water into the colon, which helps increase fecal bulk. And there is a growing body of research relating fiber to the microbiome in the colon – the ecology and species composition of those microbes. Our understanding of the colon environment may still be rudimentary, but it’s clear that fiber is critical for a healthy and balanced population of microbes.
Let’s come back to the original question about human food labels: What exactly is dietary fiber? More or less, 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. Of course, the definition 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 feeds – er, foods – that contain at least as much dietary fiber as alfalfa hay.
Woody Lane, Ph.D., is a livestock nutritionist and forage specialist in Roseburg, Oregon. He operates an independent consulting business and teaches workshops across the U.S. and Canada. His book, From The Feed Trough: Essays and Insights on Livestock Nutrition in a Complex World, is available through Woody Lane.