Defining Fiber...Sex in the '70's!
...Crude fiber does not necessarily correlate exactly with dietary fiber. It consists of portions of cellulose, hemicelluboses, lignins, pectins, and a number of other materials. Furthermore, crude fiber is not entirely undigested but does undergo digestive decomposition, even in man, by the action of luminal bacteria.
Would you guess that this was written in 1976? These words seem to be written with a feeling of sacrilege. Could it be possible that MAN relies on hind-gut bacterial fermentation like a cow? What's next? Chewing our cud?
That quote is from the 1976 research paper, Natural Fiber and Bowel Dysfunction, and is possibly the very first introduction of the concept of prebiotics. The author goes on to describe our relationship with fiber over the years:
Diet has always been believed to play an important role in bowel function. The addition or omission of different dietary factors has been believed at different points in time, often rightly, to result in alterations in body states. Swings in opinion frequently occur and probably no where is this better exemplified than in relation to “roughage.”
Up until the '70's, fiber was considered "roughage." It is this vision that has clouded our judgment on the true importance of fiber. In this 1976 paper, fiber is referred to as "crude fiber." Did the word "crude" make fiber seem unimportant or even bad to early dieticians?
Further from 1976:
The negative attitudes exemplified by Meadle in the Lancet in 1886, “It is, I think, extremely doubtful if coarse food is useful in the long run. It causes atony [weakening] and weariness of the muscles eventually by overstimulation . . . “ were replaced in the 1920’s by the generally, but somewhat vaguely held view that roughage was advantageous. Bland diets had a vogue and popularity in the 30’s and 40’s only to be re- placed by the present cult for high fiber diets.
The "cult of high fiber diets" from the '70's described in the paper were undoubtedly low in fermentable fiber. At this time there was absolutely no thought given to gut microbes other than the strange notion that maybe man was more animal-like than we'd like to believe.
The word “fiber” has now become enshrined with the mystique that includes and involves a suggestion that the path of rectitude and health is trodden only by those who revert and convert to “natural” foods.
Think of what the '70's were like. Low fat, margarine, transfats, TV Dinners, and fast-food proliferation. We had put men on the moon! We don't need no roughage!
Doesn't this final thought from the paper just reek of the '70's?
It is important to differentiate clearly between the science and the sociology of dietary fiber. As I have indicated elsewhere “Human diet, almost as much as human sex, seems prone to become enmeshed in mystiques that draw on all sorts of attitudes that have nothing to do with the facts of the case and which can seriously hamper the scientist trying to get at the facts.” There is danger that much needed scientific inquiry into the role of natural fiber, both nutritional and metabolical, will be caught up in the mystique of “natural” foods that are deemed superior because they allegedly reflect some sort of harmony between man and nature that modern civilization has violated. Such considerations apart, however, the fiber hypothesis should be scientifically verifiable and could lead to important changes in food processing, nutrition, and preventive medicine.
In 1976, at least one researcher was seeing that there was more to "crude fiber" than "roughage." His "fiber hypothesis" was a call to his contemporaries to begin seeing fiber as therapeutic and to dig deep in understanding the magic that happens when certain fibers were consumed. Had Dr. Connell not seen that pectins, lignins, and gums acted differently than wheat chaff and bran, possibly we'd still be calling our fiber "roughage."
Suffice it to say, that, at this time, the argument is far from settled and considerable effort is needed to establish the effectiveness of wheat bran or, indeed, other fibers in the treatment of diverticular disease. It is quite possible that bran is not in itself the best product to use. Other fiber-containing materials, for example root crops such as carrot, rutabaga, or turnip might prove to be equally, if not more effective. Materials containing different proportions of celiuloses, hemicelluloses, and lignins such as psyllium seed or alfalfa have been tested only in a very limited way. Much study, therefore, remains to be done to provide a scientific basis for the current trends.
Dietary Fiber Defined
Shortly after this paper was written, a sort of "fiber explosion" took place. In 1978, JH Cummings authored, Colonic response to dietary fibre from carrot, cabbage, apple, bran. In this paper, he noted the differing effects of various fibers and noted:
Diet-induced changes in colonic function may explain international differences in the prevalence of colonic disease, whilst personal variation in the response to dietary fibre may determine individual susceptibility to large-bowel disease within a community.
Still, little talk of the microbiome. In 1979, AM Stephen (assisted by JH Cummings) wrote, Water-holding by dietary fibre in vitro and its relationship to faecal output in man where he overturned a long-held belief:
"Dietary fibre does not exert its effect on faecal weight simply by retaining water in the gut."
He was on to something! The very next year, AM Stephen, still assisted by JH Cummings, wrote, Mechanism of action of dietary fibre in the human colon. Here he described many inconsistencies he found concerning fiber, and even coined a new phrase, "Dietary Fiber" (Or rather, 'Fibre').
Fibre, more than any other dietary component, affects human large bowel function, causing an increase in stool output, dilution of colonic contents, a faster rate of passage through the gut and changes in the colonic metabolism of minerals, nitrogen and bile acids. (Fibre here refers to 'dietary fibre', which comprises plant cell wall polysaccharides and lignin, and not to 'crude fibre'.) It is thought that these changes are brought about by fibre passing through the gut undigested and holding water within its cellular structure...
This man was definitely on to something BIG!
This is because fibre is extensively degraded in the gut, probably by the colonic microflora. Using a newly developed method modified from ruminant nutrition for isolating bacteria, we have shown that the main component of human faeces is bacteria.
While AM Stephen went on to publish many more papers on "Dietary Fibre," his protege really picked up the ball! In 1980, JH Cummings (assisted by AM Stephen!) wrote: The role of dietary fibre in the human colon. And here was an opening for some real science to begin:
Several effects of dietary fibre on colonic function have been documented by experiment or deduced from epidemiologic observation. The magnitude of these changes depends on the source and the physical and chemical composition of the fibre used, and on the individual response of the subjects. Three theories of the mode of action of fibre are discussed; they relate to the water-holding capacity of fibre, the production of short-chain fatty acids from fibre in the colon and the alteration by fibre of the colonic microflora.
In his paper on dietary fiber, JH Cummings notes:
Fiber digestion in the human gut has been reported in the literature on several occasions in the past 80 or 90 years. Despite this, the popular view persists that it is not broken down but it is an inert and unimportant component of the diet.
The identification of the microbial contribution to human fecal mass adds a new dimension to theories as to how fibre exerts its effects on the colon...The differing response of individuals to the same fibre source may relate to the characteristics of their colonic bacteria.
Reading this 1980 paper is like reading an ancient history text. It's fun to read along and see as an important discovery was made. This paper is guaranteed to raise goosebumps on everyone who is just now discovering fiber and the gut flora. Here is a full text pdf if you'd like to read.
Bacteria finally recognized!
Between 1980 and 1993, several papers had been published, but they all seemed stuck on fecal weight and osmotic pressure. However, in 1993, Dr. Roberfroid published, Dietary fiber, inulin, and oligofructose: A review comparing their physiological effects. In this paper he worked off of research over the last 13 years since Cummings proclamation that colonic bacteria is behind the effects of dietary fiber:
The main physiological effects of dietary fiber are primarily on gastric emptying and small intestinal transit time, resulting in an improved glucose tolerance and a decreased digestion of starch; second, on colonic transit time and large bowel functions due to fermentation by ceco-colonic microbial flora or bulking action.
Roberfroid was really digging deep! He was wading through the fibers and seeing what they did, why they did it, and cataloging the characteristics of the most important fibers:
The so-called soluble dietary fibers are fermented to a large extent by a wide variety of anaerobic bacteria that result in an increase in bacterial biomass, an increase in fecal mass, a change in intracolonic pH, and production of short chain fatty acids and various gases as metabolic end products. The insoluble fibers are only marginally fermented; they serve almost exclusively as bulking agents that result in shorter transit time and increased fecal mass.
Here you see new terms defined, "soluble and insoluble" fibers. Roberfroid also noted that the bacteria responsible for this fermentation are "anerobic." I wonder if he ever imagined in his wildest dreams that in 20 years people would be seeking individual fibers to favor individual bacteria and engage in all-out Akkermansia wars?
His conclusion was that the dietary fibers with the most benefit were inulin and oligofructose due to their chemical structure and "degrees of polymerization." He recommended in his paper that the term "dietary fiber" was too broad and called for reform. He said the term "dietary fiber" should only be used when the fiber showed, "resistance to digestion, access to the colon, and effects on some gastrointestinal function." He also called for two sub-classes to dietary fiber: One that regulates gastric functions AND systemic functions such as guar gum and pectins, and another that regulates only gastric functions such as wheat bran.
Roberfroid certainly was astute in his closing remarks:
Indeed, trying to find a universal method for assaying all products to be classified as Dietary Fiber is hopeless!
Prebiotics enter the scene...
Yet, in 1994, Roberfroid again stunned us with a watershed paper, Dietary Modulation of the Human Colonie Microbiota: Introducing the Concept of Prebiotics. A new term is now about to enter our vernacular, "Prebiotics!"
Because the human gut microbiota can play a major role in host health, there is currently some interest in the manipulation of the composition of the gut flora towards a potentially more remedial community.
Roberfroid was looking mostly at Bifidobacteria and Lactobacillus as these two types of bacteria have had a long-standing association with good health. His prebiotioc of choice was inulin, but he noted there were probably more. Roberfroid's 1994 definition of prebiotics was stated:
Prebiotics are nondigestible food ingredients that beneficially affect the host by selectively stimulating the growth and/or activity of one or a limited number of bacterial species already resident in the colon, and thus attempt to improve host health.
And to further clarify:
Among the natural non-digestible oligosaccharides that fulfill the criteria of a colonic food, fructooligosaccharides are the only products presently recognized and used as food ingredients that meet all the criteria allowing classification as prebiotics...Among the colonic foods, fructooligosaccharides such as oligofructose and inulin are naturally occurring ingredients for which convincing experimental evidence in favor of a health-promoting effect is available.
Roberfroid was a chemist, not a dietician. He really didn't have any good recommendations as to dietary sources of prebiotics, that would come later. To clarify further, his choice of "colonic food":
Inulin is prepared by hot water extraction of chicory roots, and oligofructose is obtained by partial enzymatic hydrolysis of inulin under strictly controlled conditions...the term fructooligosaccharides will be used here to encompass both oligofructose and inulin, which are commercially available as RAFTILOSE® and RAFTILINE®, respectively.
Already we were having problems defining exactly what we should EAT to get the benefits of prebiotics. And this continues right up to the present. Between 1994 and 2015, there are hundreds of papers describing the potential prebiotic potential of various plant materials.
Resistant Starch was first mentioned in a scientific journal in 1985 and a few follow up papers concerning RS in potatoes and bananas followed. It wasn't until 1992 when RS was classified into the types we recognize today and the prebiotic potential explored. Still, today, much confusion abounds about what is or is not a prebiotic.
Now a whole slew of prebiotics!
The prebiotic potential of seaweeds and algae has been examined, giving us classes of alginates, fucans, laminarins, agar, carrageenans, and more. Man-made prebiotics have really stolen the show, though, recently and have become a boon to food manufacturers looking for ways to label their processed goods as "healthy." Some of the man-made prebiotics are actually just pre-digested versions of fibers that are found in nature such as Partially Hydrolyzed Guar Gum and Resistant Dextrins. Others such as Fructooligosaccharides, though found in nature, can only be accessed in isolation through chemical processes.
Here is a list of prebiotics you may be familiar with:
- Human Milk Oligosaccharides - the carbohydrates found in human breast milk, a human’s first (and most important) taste of prebiotics.
- Resistant Starch - the most common storage carbohydrate of plants. Found in tubers, roots, green bananas, green plantains, legumes, peas, oats, nuts, carrots, maize, sedge nutlets, and grains.
- Oligosaccharides - (inulin, fructo-oligosaccharides, galacto-oligosaccharides, xylo-oligosaccharides) the second most common storage carbohydrate of plants (chicory, onion, leek, dandelion, endive, asparagus, green bananas, legumes, lentils, oats, rice bran, maize, grains).
- Non-Starch Polysaccharides (NSP):
- Arabinogalactan - a storage carbohydrate of trees and many plants (carrots, radish, black gram beans, pear, maize, red wine, tomatoes, sorghum, coconut meat)
- Glucomannan - found in the cell walls of certain plant roots and wood, also a component of bacterial and yeast membrane. Konjac roots contain 40% by dry weight and are a great source of glucomannan
- β-Glucans - found in oats, barley, whole grains, shiitake, oyster, maitake, mushrooms, dates, yeast
- Pectin - found in avocados, berries, citrus, fruits, vegetables
- Gums and mucilages - found in seed extracts (guar, locust bean), tree exudates (gum acacia, algal polysaccharides (alginates, agar, carrageenan), psyllium
There are also a few man-made prebiotics derived from plants and animals, often called "functional foods" by food manufacturers:
- Galacto Oligosaccharides - derived from cow’s milk to simulate human breast milk for infant formula.
- Fructo Oligosaccharides - separated from natural inulin, used in sweeteners.
- Mannan Oligosaccharides - made from yeast cells, approved only for animals.
- Resistant Dextrins - Formed through thermolysis, transglucolysis, regrouping and repolymerisation of starch granules.
- Partially Hydrolyzed Guar Gum - Produced by subjecting guar gum to digestive enzymes.
- Polyphenols and Flavonoids - found in many places in trace amounts; colorful plants, dark chocolate, seaweed, and mushrooms.
- Glycoprotein and glycolipids - Found in raw meat, raw blood, cartilage, gelatin, collagen, chondroitin, and animal cells.
- Chitin and chitosan - found in fungi, yeasts, insects, worms.
In Part 5 we'll discuss the future of fiber and some hurdles we still must overcome to define 'the perfect fiber.'