The Fate of Digestible Carbohydrates

Carbohydrates are an important class of food that provides energy for our bodies.  This class is made up of a huge variety of compounds, but the most important for energy needs are polymers of glucose (starches), simple sugars, such as glucose, galactose and fructose, and disaccharides, like sucrose, maltose and lactose (Laurentin2013).

Fuel for the cells

For most peoples of the world, carbohydrates provide about half of our energy requirements.  While no single sugar is essential, glucose is the most prominent and can be utilised by all cells in our body, and most of the bacteria in our gut. Indeed, the brain operates optimally when glucose is used as the energy source, consuming ~130 g/day, while red blood cells and the kidney have a requirement for it (Livesey2014).

Aerobic respiration of a single glucose molecule yields ~17 kilojoules of energy (Dashty2013), or 36-38 adenosine triphosphate molecules (ATP; see also Bacterial digestion: Energy).  Other readily available non-glucose sugars, such as galactose, mannose and fructose are mostly converted to glucose in the liver before they can be utilised for energy. Glucose is the most accessible energy source for tissues in the body, so-much-so the liver and muscles store glucose as a special polymer, called glycogen.  This provides a readily deployable reservoir of glucose for energy generation and to maintain a healthy blood glucose concentration (3.9 mM - 5.6 mM; WHO).

Digestive process

Traditionally, free glucose is rarely found in foods.  Instead, glucose and other simple sugars, are typically derived from plant-derived polymers such as amylose, amylopectin and more complex carbohydrates (e.g. fibre). As a result, the process of digestion leads these non-absorbable polymers through a series of diverse chemical and physical transformations, ultimately converting them into glucose, a form easily absorbed by our bodies.

Chewing food stimulates the release of saliva and the presence of carbohydrates promotes the release of a saliva-soluble amylase. This enzyme starts the process of starch hydrolysis by releasing small amounts of glucose, maltose and smaller polymer fragments.  Since the time in the mouth is limited, so too is the degree of digestion: only around 5% of hydrolysable carbs are broken down at this stage (see Figure 1).  The purpose of the limited amount of digestion might have more to do with aspects of taste than anything else.  Indeed, too much glucose release promotes bacterial growth that could lead to tooth decay.

After chewing and swallowing, the saliva-moistened food bolus travels down to the stomach. During this journey, the amylase present in the bolus continues its hydrolytic action (Livesey2014), breaking down complex starches further. However, once the stomach's acidic environment reaches the enzyme, it loses its activity.  Some resident stomach commensals, such as Lactobacillus kalixensis or Actinomyces bouchesdurhonensis, can also make minor contributions to carbohydrate digestion (Livesey2014),

Figure 1. Digestible carbohydrate progression from ingestion to absorption.  Most digestion occurs in the small intestine and involves pancreatic amylase and brush border enzymes, which are bound to microvilli membranes.

Small intestine

Next stop is the small intestine, where digestion and absorption occurs to the greatest extent. After neutralisation of the acidic digesta with bicarbonate, pancreatic amylase is released into the duodenum.  The role of this enzyme is to catalyse the hydrolysis of the α-glucoside bonds that link the glucose monomer units together (Figure 2).  

Figure 2.  Crystal structure of human pancreatic α-amylase (Qin2011; left). The enzyme is released into the duodenal lumen, where it attacks sugars that contain α-glycosidic bonds.  The result is cleavage of simple starches into smaller glucose-containing pieces that can be further broken down or absorbed (i.e. glucose).

In addition to this amylase, there are 3 other important enzymes: sucrase-isomaltase, which hydrolyse sucrose; lactose phlorizin-hydrolase, responsible for attacking lactose; and maltase-glucoamylase, which can break down maltose and smaller amylose fragments (Figure 3).  They are produced by an epithelial layer of specialised cells known as enterocytes. Also called brush border cells, because of the brush-like appearance of many microvilli (very fine finger-like protrusions extending from the apical face of the cell), these cells embed the enzymes into membranes of vesicles produced at the tip of the microvilli (Figures 3 & 4).  Immobilisation is important because the products of enzyme activity are right where they will be absorbed by the cell.

Figure 3. Pictorial representation of brush border enzymes attached to the membrane of the vesicles produced at the end of microvilli.

The enzyme-catalysed degradation of disaccharides (sucrose, lactose, maltose) and glucose polymers always produces at least 50% glucose, with various amounts of fructose and galactose, depending on the diet.  

Figure 4. Enterocytes - or brush border cells - are responsible for completing the digestion and absorption of intestinal carbohydrates.  Fine extensions, called microvilli, protrude from the apical membrane.  This creates a greater surface area and makes digestion and absorption much more efficient.  Mucus (yellow in figure, secreted by goblet cells) is more viscous and retains digestion products longer.  Two main transporters, GLUT5 and SGLT1, are responsible for moving monomers across the membrane.

Carbohydrate absorption

Two main membrane-bound transport proteins expressed by enterocytes are responsible for recognising and facilitating movement of carbohydrate monomers across the membrane. The most important is SGLT1 - or Sodium-Glucose Transport Protein-1 (Figure 5). As the name implies, it transports a glucose molecule, but at the same time brings across 2 sodium ions (symport). Sodium uptake is important because it allows the cell to take up glucose against a higher glucose gradient. Galactose, which is derived from the hydrolysis of lactose, is also transported by this protein.

Transport of glucose, fructose and galactose out of the enterocyte is facilitated by another transporter, GLUT2 (Glucose Transporter-2). Transport is driven by an ATP-mediated sodium-potassium pump, which swaps 3 sodium for 2 potassium ions. Presumably, this process stops glucose moving back into the enterocyte.

The other usual transporter present is GLUT5, or Glucose Transporter-5 (Figure 5). Here, the name is misleading because it only transports fructose. The transported sugar is quickly dealt with, and so fructose goes from a higher concentration in the lumen to a lower one inside the cell. This is a form of facilitated (or passive) transport. 

Figure 5. Pictorial impression of an enterocyte transporting normal levels of sugar monomers via transporters.

Small amounts of ingested fructose (alone or derived from sucrose) can be transformed to useful energy metabolites by, and for use in, the brush border cells; however, a larger intake of either fructose or sucrose results in fructose mainly being transferred to the liver where it is converted to glucose or fat (Figure 6)(Chiu2018).

A heavy carb meal can result in very high local concentrations of glucose in the lumen.  When this occurs GLUT2 can be recruited to the apical membrane to assist GLUT5 in dealing with glucose backlog (Figure 6). After exiting the basolateral membrane of the cell, the vascular system carries the sugars to the liver via the portal vein where they are metabolised or stored.

Figure 6. Pictorial impression of an enterocyte subjected to large local lumen concentrations of fructose. Rather than fructose being used primarily by the cell to generate energy, much of the sugar monomer accompanies glucose and is exported, via the GLUT2 transporter, into the bloodstream. Circulating fructose is processed almost exclusively by the liver and is often turned into triglycerides though de novo lipogenesis (Chiu2018).