Fundamentals of Biochemistry - A. A. Anisimov 1986

Carbohydrates
Carbohydrate digestion and metabolism

Humans and animals are incapable of the de novo Biosynthesis of CARBOHYDRATES from inorganic substances; instead, they can only produce them via Gluconeogenesis from other organic precursors (such as organic acids, fats, and Amino Acids). However, diet remains the primary source of carbohydrates. Certain insects—such as wood-boring species and nectar feeders—derive almost their entire energy intake from carbohydrates and have adapted so thoroughly to this diet that they perish without them. Among various invertebrates, carbohydrate levels exhibit a pronounced seasonal rhythm. For instance, in mussels and shrimp, carbohydrate content surges sharply in autumn as they build up energy reserves for the winter. This process is further supported by the elevated carbohydrate concentrations in plankton, which serves as their food source. Carbohydrates constitute a vital component of the Human and Animal diet, accounting for 60–70% of total caloric intake in humans. They are particularly abundant in cereals, pasta, and noodles (65–75%), bread (roughly 50%), and potatoes (up to 25%), whereas meat and dairy products contain very minimal amounts (0.5–2%).

Carbohydrates are absorbed across the intestinal mucosa exclusively as Monosaccharides. Even highly soluble Disaccharides like sucrose and lactose cannot be absorbed in their intact form within the intestine¹. If introduced directly into the bloodstream by-passing the gastrointestinal tract, these disaccharides cannot be utilized by the Cells of various Tissues. This holds even more true for Water-insoluble Polysaccharides such as starch and Glycogen. Consequently, the Digestion of dietary carbohydrates requires their complete breakdown into monosaccharides—the sole form capable of being absorbed and utilized by the Organism.

Carbohydrate digestion begins in the Oral Cavity, where saliva supplies two Key Enzymes: α-amylase and maltase.

A distinctive feature of salivary α-amylase (formerly known as ptyalin) is its ability to hydrolyze starch exclusively from food products that have undergone thermal Processing during preparation. "Raw" starch is virtually unaffected in the Mouth, undergoing Hydrolysis only upon exceptionally prolonged chewing. Unlike the avian counterpart, mammalian salivary α-amylase exhibits high activity. Interestingly, this enzyme is absent in certain primates (such as baboons and rhesus macaques), whereas in humans it is exceptionally active.

Food remains in the oral cavity only briefly before entering The Stomach, where the bolus is gradually permeated by acidic gastric juice. Low pH levels inactivate salivary α-amylase; hence, its hydrolytic action is short-lived, meaning The breakdown of starch and glycogen merely initiates in the mouth. Amylolytic enzymes are entirely absent in the stomach.

The principal site for the digestion of starch and glycogen is the Small Intestine, where they are acted upon by pancreatic α-amylase. Unlike its salivary counterpart, this enzyme can digest raw, thermally unprocessed starch. Consequently, the appearance of undigested starch granules in the feces serves as a clinical indicator of impaired secretion or dysfunction of pancreatic α-amylase. Studies show that the intestinal α-amylase activity in various fish species correlates with their feeding habits: it is significantly higher in phytophagous species—which consume starch-rich phytoplankton—than in zooplanktivores.

¹ Only when disaccharides are present in the diet in massive excess can they be absorbed in very small quantities in the intestine, though they are rapidly eliminated unchanged in the urine.

Pancreatic juice also contains maltase, an enzyme that breaks down the disaccharide maltose. However, the bulk of dietary and α-amylase-derived disaccharides are hydrolyzed by enzymes located in the small intestine. This process occurs not within the intestinal lumen, but directly within the mucosal cells, driven by maltase, isomaltase, and sucrase (invertase). Invertase was first discovered in intestinal juice by V. V. Pashutin in 1870. Maltase and isomaltase typically form a stable complex with invertase. These enzymes operate within the brush border of the intestinal mucosal epithelium in quantities sufficient to ensure the digestion and assimilation of an adult human's diet. In addition to these enzymes, the intestinal epithelium contains β-galactosidase (with an optimal pH of 4.5), heterogalactosidase, and lactase.

Heterogalactosidase cleaves mixed-Structure Oligosaccharides via β-galactosidase linkages. Lactase exhibits relatively low activity. In several mammalian species, lactase activity disappears after weaning. Dietary lactose occurs exclusively as a component of milk, and its concentration in breast milk is nearly double that found in cow's milk. Lactose intolerance occurs in infants with a genetic lactase deficiency.

Through the sequential action of these enzymes, carbohydrates are converted into monosaccharides, which are readily absorbed by the intestinal wall. Their rates of absorption vary: galactose is absorbed most rapidly, followed closely by glucose, whereas fructose is absorbed at roughly half that rate; mannose and xylose are absorbed at about a quarter of The rate of glucose, and arabinose shows the lowest absorption rate. Pentoses and mannose traverse the epithelium via Facilitated Diffusion and cannot be transported against a concentration gradient. Consequently, the rate of pentose and mannose absorption depends entirely on how quickly these sugars diffuse outward toward the serosal side of the cells, where they are rapidly swept away by the bloodstream.

Glucose and galactose can be absorbed in the intestine even against a tenfold concentration gradient, indicating the operation of an Active Transport mechanism. This process is presumed to involve specialized carriers with specific affinities for individual monosaccharides, operating within a Na⁺-dependent transport system. The active transport mechanism Functions when sugar concentrations in the intestinal lumen are low. When luminal glucose or galactose levels are high, a secondary transport system based on facilitated diffusion takes over. This system functions only as long as the sugar concentration at the cellular brush border significantly exceeds that within the cells. Data regarding the exact mechanism by which sugars exit toward the serosal side remain scarce, though simple diffusion down a concentration gradient is currently hypothesized.

Humans and mammalian species lack cellulase, the enzyme responsible for hydrolyzing crude fiber (Cellulose). Similarly, plant pentosans resist the digestive Enzymes of the mammalian gastrointestinal tract. Only a fraction of these compounds are partially broken down by Bacteria in the Large Intestine, yielding organic acids, CO₂, and other metabolites. Because many mammals are herbivorous, cellulose digestion is vital to their survival; consequently, they possess specialized digestive tracts highly adapted for the symbiotic Fermentation of cellulose. For instance, ruminant stomachs feature multiple chambers, the first and largest of which is the rumen. The rumen teems with bacteria and Protozoa (such as Ciliates) that produce cellulase. It functions essentially like a fermentation vat, where food mixed with saliva undergoes intense microbial breakdown. The fermentation products (predominantly butyric, acetic, and propionic acids) are absorbed and utilized by the body, while CO₂ and CH₄ are expelled via eructation. Crucially, the organic acids derived from cellulose can be directly channeled into Lipid Biosynthesis, serving as the primary energy source for ruminants (supplying up to 70% of the total Energy Requirements in cattle).

Rumen microorganisms benefit ruminants in yet another way: they synthesize Proteins from inorganic nitrogen salts, such as ammonium compounds. Consequently, urea can be added to ruminant feed to boost Protein Synthesis—a significantly more cost-effective approach than supplementing diets with expensive protein-rich feedstuffs. Microbial Protein synthesis in the rumen is especially critical when animals subsist on low-quality forage. For instance, camels fed a near protein-free diet (poor-quality hay, dates) excrete virtually no urea in their urine; instead, urea is recycled back into the rumen—partly across its walls and partly via saliva—to resynthesize proteins. A similar mechanism of urea recycling has been observed in sheep. Adding inorganic sulfates to a ruminant's diet enhances microbial PROTEIN SYNTHESIS AND, most importantly, incorporates sulfur into Essential Amino Acids such as Methionine and Cysteine.

Thus, ruminal microorganisms also improve protein quality by synthesizing all essential Amino Acids and numerous vital Vitamins. Upon microbial death, these nutrients remain within the host's intestinal tract and are assimilated. A prime example is the synthesis of vitamin B₁₂, which ruminants obtain exclusively from microorganisms. Symbiotic cellulose digestion also occurs in certain non-ruminant herbivores that lack a well-developed rumen. However, because fibrous plant matter is typically bulky, its microbial fermentation proceeds slowly and requires a voluminous digestive tract. In some non-ruminant herbivores, the stomach is enlarged and multi-chambered, whereas in others, cellulose is broken down in the cecum. Multi-chambered stomachs are also found in certain primates, sloths, and one species of kangaroo.

Enhanced digestion of plant material via symbiotic microbial fermentation also occurs in certain birds, such as the willow ptarmigan, which subsists primarily on buds and young twigs throughout the winter months. Most gallinaceous birds possess dual ceca dedicated to cellulose fermentation. However, localizing this process to the hindgut is less efficient than in the rumen, as it precludes any subsequent digestion of the nutrients as they pass through the remainder of the gut. In many rodents, rabbits, and hares, this limitation is compensated for by coprophagy (the ingestion of feces).

The mechanisms of Cellulose digestion in invertebrates remain a subject of considerable debate. Convincing proof of this capability (independent of microbial Symbiosis!) has been established for the silverfish (*Ctenolepisma lineata*) using ¹⁴C-labeled cellulose and germ-free specimens. While autonomous cellulose digestion is also considered plausible in shipworms (*Teredo*) and garden snails, conflicting evidence has been raised. Meanwhile, the crucial role of symbiotic bacteria and flagellates in cellulose assimilation by termites is undisputed.

In humans, the bulk of dietary cellulose passes through the gut unchanged and appears in the feces. Only a minor fraction of dietary fiber is degraded in the human large intestine by the resident microflora and subsequently absorbed. The proportion of fiber utilized in this manner depends on the quantity and composition of other dietary components. Nonetheless, the presence of dietary cellulose is far from useless, as fiber stimulates the secretion of digestive juices and enhances peristalsis, thereby facilitating overall digestion.