Turning Waste Into Food: Cellulose Digestion

Introduction

Fiber constitutes an essential element in the human diet. It has been shown to prevent cholesterol absorption and heart disease and help control diabetes (1). The National Academy of Sciences Institute of Medicine recommends the adult male consume at least 38 grams of soluble fiber per day – the only kind of fiber humans can digest (1). The other more abundant type of fiber, insoluble fiber, passes through the human digestive system virtually intact and provides no nutritional value.

What if humans could digest fiber? Cellulose, the main type of insoluble fiber in the human diet, also represents the most abundant organic compound on Earth (2). Almost every plant has cell walls made from cellulose, which consists of thousands of structurally alternating glucose units (Fig. 1). This configuration gives cellulose its strength but prevents it from interacting with human enzymes. Cellulose contains just as much energy as starch because both molecules consist of glucose subunits. It is only possible to use that energy by burning wood and other cellulose materials. However, if that energy were physiologically available, humans could lower their food consumption and produce much less digestive waste than they currently do.

Figure 1: Structure of cellulose

The Human Digestive System

Disregarding cellulose digestion, human digestion is still a very efficient process (Fig. 2). Even before food enters the mouth, saliva glands automatically start secreting enzymes and lubricants to begin the digestive process. Amylase breaks down starches in the mouth into simple sugars and teeth grind up the food into smaller chunks for further digestion. After swallowing the food, hydrochloric acid and various enzymes work on the food in the stomach for two to four hours. During this time, the stomach absorbs  glucose, other simple sugars, amino acids, and some fat-soluble substances (3).

Figure 2: The organs of the human digestive system.

The mixture of food and enzymes, called chyme, then moves on to the small intestines where it stays for the next three to six hours. In the small intestines, pancreatic juices and liver secretions digest proteins, fats and complex carbohydrates. Most of the nutrition from food is absorbed during its journey through over seven feet of small intestines. Next, the large intestines absorb the residual water and electrolytes and store the leftover fecal matter.

Although the human digestive system is quite efficient, discrepancies among the human population exist concerning what individuals can or cannot digest. For example, an estimated seventy percent of people cannot digest the lactose in milk and other dairy products because their bodies gradually lost the ability to produce lactase (4). Humans can also suffer from various other enzyme or hormone deficiencies that affect digestion and absorption, such as diabetes.

Comparative studies show that the human digestive system is much closer to that of herbivores rather than carnivores. Humans have the short and blunted teeth of herbivores and relatively long intestines-about ten times the length of their bodies. The human colon also demonstrates the pouched structure peculiar to herbivores (5). Yet, the human mouth, stomach, and liver can secrete enzymes to digest almost every type of sugar except cellulose, which is essential to a herbivore’s survival.

In the case of lactose intolerance, lactase supplements can easily rectify the deficiency, so what rectifies the inability to digest cellulose?

Ruminants and Termites

Ruminants-animals such as cattle, goats, sheep, bison, buffalo, deer, and antelope – regurgitate what they eat as cud and chew it again for further digestion (6). Ruminant intestines are very similar to human intestines in their form and function (Fig. 3). The key to specialized ruminant digestion lies in the rumen. Ruminants, like humans, also secrete saliva as the primary step in digestion, but unlike humans, they swallow the food first only to regurgitate it later for chewing. Ruminants have multi-chambered stomachs, and food particles must be made small enough to pass through the reticulum chamber into the rumen chamber. Inside the rumen, special bacteria and protozoa secrete the necessary enzymes to break down the various forms of cellulose for digestion and absorption.

Cellulose has many forms, some of which are more complex and harder to break down than others. Some of the microbes in the rumen, such as Fibrobacter succinogenes, produce cellulase that breaks down the more complex forms of cellulose in straw while others such as Ruminococci produce extracellular cellulase that hydrolyzes the simpler amorphous type of cellulose (7). Conveniently, cellulose hydrolysis produces several byproducts, such as cellobiose and pentose disaccharides, which are useful to rumen microbes. The reactions produce other byproducts such as methane, which is eventually passed out of the ruminant (7). Thus, the microbes and ruminants live symbiotically so that the microbes produce cellulase to break down cellulose for the ruminants while gaining a food source for their own sustenance.

Figure 3: The ruminant digestive system

The various microbes within ruminants may hydrolyze certain types of cellulose, but ruminants still cannot eat wood or cotton. Termites, on the other hand, can feed on various types of wood. It was believed for a long time that termites also depended on microorganisms that lived inside their bodies to digest cellulose for them, but research in the late 1990s showed that certain types of termites had the ability to produce enough cellulases and xylanases in the midgut to support their own survival (8). However, other species of termites do not have the capacity to produce enough cellulase independently and must depend on microbes from the domains Archaea, Eubacteria and Eucarya to break down cellulose. Regardless of the various levels of termite independence, there exists a symbiotic relationship between termites and over 400 species of microorganisms, analogous to that of ruminants and their microbes (8). The termite gut is even designed to provide energy-yielding substrates for the microbes (8).Both protists and fungi are attributed to the production of supplementary enzymes, but their specific roles and mechanisms are still being debated because isolating pure cultures has proven technically difficult. Despite the ubiquity of these microbes and the benefits they bring to ruminants and termites, research has yet to fully elucidate their mechanisms.

Current Technologies

People have long been interested in tapping into the energy in cellulose. However, most companies and research groups are only focused on ways to harness that energy as biofuel and not as food. Major research is aimed at converting cellulosic material into ethanol, although that process is still inefficient and requires refinement.

Cellulose must first be hydrolyzed into smaller sugar components such as glucose, pentose or hexose before it can be fermented into bioethanol (9). One method uses acids to hydrolyze cellulose but this can destroy much of the sugar in the process. Another way to hydrolyze cellulose is by mimicking the microorganisms inside ruminants and termites. Bioenergy engineers can use the enzymes produced by microbes to break down cellulose. However, enzymes have biological limitations and implement natural feedback inhibition that poses a problem for industrial manufacturing (9). Other technical barriers to efficient enzymatic hydrolysis include the low specific activity of current commercial enzymes, the high cost of enzyme production, and a lack of understanding of the mechanisms and biochemistry of the enzymes (9).

Companies and governments all over the are eager to invest heavily in research to turn biomass into biofuel, which could bring enormous benefits to the world economy and environment. Biomass is readily available, biodegradable, and sustainable, making it an ideal choice as a source of energy for both developed and developing countries. This could also help reduce waste problems plaguing society today. The United States produces 180 million tons of municipal waste per year, and about fifty percent of this is cellulosic and could potentially be converted into energy with the right technology (10).

Cellulose Digestion in Humans

The benefits of turning cellulose into biofuel are just as relevant when considering engineering humans to digest cellulose as a food source. Right now, technology focuses on controlling cellulose hydrolysis and processing in factories, but perhaps in the future humans could serve as the machine for extracting energy from cellulose, especially since the enzymes used to hydrolyze cellulose are hard to isolate in large quantities for industrial use. Termites themselves are tiny creatures, but as a colony, they can break down houses and entire structures. A healthy human digestive system already carries an estimated 1 kg of bacteria, so adding a couple of extra harmless types should not pose a problem (11).

Termites and ruminants serve as a great example of how organisms can use microbes effectively. However, the human body would need some adjustments to introduce the microbes into the body. Our stomach is much too acidic for most microbes to survive.  The acid, among other secretions and enzymes, follows the food into the small intestine, where the microbes might end up competing with us for food. By the time the food has reached the large intestines, only the cellulosic material is left for dehydration and possibly hydrolysis. However, our large intestines lack the ability to absorb the sugars that the microbes would produce from hydrolysis. Perhaps another organ could be added to the end of the human gastrointestinal tract to especially accommodate cellulose-digesting microbes. Modern medicine allows safe inter-species transplantation, but the ideal solution would be to genetically engineer humans to develop the organs themselves to avoid he complications of surgery and organ transplantation.  Genetic engineering for the purpose of treating disease and illness is still undergoing intense debate, so nonessential pursuits such as cellulose digestion will not be possible until the scientific and medical communities accept genetic engineering as a safe and practical procedure.

A simpler solution would be to take supplements similar to the ones used to treat lactose intolerance. Cellulose broken down in the stomach can be absorbed as glucose. Extracting the right enzymes to work in the human stomach can bypass the problems of supporting microbes inside the human body. Additionally, since the process would occur inside the human body, the limitations that posed a problem for commercial hydrolysis of cellulose would become necessary biological controls. In the case of lactose intolerance, lactase is easily extracted from yeast fungi such as Kluyveromyces fragilis, so perhaps the easiest solution for cellulose indigestion is to extract the appropriate enzyme from the right microbes (12). As mentioned previously, the commercial extraction of enzymes is not yet practical. As previously stated, this field of human enhancement does not receive much research because companies and funding institutions are much more interested in the lucrative biofuel industry. Consequently, many questions remain unasked and unanswered. For example, what would the removal of cellulose weight from stool do to the process of defecation? What other effects might the microbes have on the human body? How do we deal with the other byproducts of cellulose hydrolysis such as methane production?

These questions could be analyzed through observation. Other mammals have survived many millennia by digesting cellulose with microbes, and since humans are mammals, there are no underlying reasons why human bodies cannot be compatible with these organisms. The microbes that currently reside in the human body already produce gases inside the digestive system, ten percent of which is methane (3). Methane production used to be viewed as a problem at cattle ranches and dairy farms, but methane itself is a highly energetic biogas that can be used as fuel. Harnessing it might prove difficult considering that current social graves do not favor open flatulence even for the sake of renewable energy.  However, certain diets richer in alfalfa and flaxseed have been proven to reduce methane production in cows, which could potentially solve that problem (13).

Conclusion

Vegetation, which is severely lacking in the modern diet, is the major source of insoluble fiber. Vegetables contain many vitamins, nutrients, and soluble fiber, which has numerous health benefits as mentioned in the introduction. Adding these foods to our diet after adding cellulose-digesting capabilities could help assuage the obesity epidemic and significantly improve human health.

Ultimately, improving human digestion could vastly reduce waste generated by humans and increase the efficiency of human consumption. We only need to better observe and understand those particular microbes to integrate them into our bodies, which are already structurally favorable for such a change. With the successful integration of microbes, we could cut down on food intake by making use of the energy in previously indigestible cellulose, reduce cellulosic waste by turning it into food, solve problems of food shortages by making algae, grass, straw, and even wood edible, and eventually turn human bodies into a source of renewable energy.

References
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6. D. C. Church,  Digestive Physiology and Nutrition of Ruminants (O & B Books, Corvallis, Oregon, 1979).

7. R. L. Baldwin, R.L., Modeling Ruminant Digestion and Metabolism (Chapman & Hall, London, UK, 1995).

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9. A. Demirbas, Biofuels (Springer-Verlag London Limited, London, UK, 2009).

10. S. Lee, Alternative Fuels (Taylor & Frances, Washington D.C., 1995).

11. Friendly Bacteria in the Digestive System (2000). Available at  http://www.typesofbacteria.co.uk/friendly-bacteria-digestive-system.html (19 April 2010).

12. Lactase (2006). Available at http://www.vitamins-supplements.org/digestive-enzymes/lactase.php (20 April 2010).

13. L. Kaufman, Greening the Herds: A New Diet to Cap Gas (2009). Available at http://www.nytimes.com/2009/06/05/us/05cows.html (20 April 2010).