Introduction

The human digestive tract produces α-amylases and brush-border enzymes capable of hydrolysing simple starches all the way to glucose (Figure 1).  Not all starches can be completely utilised by humans, however. Starches may have different structural properties that endogenous enzymes can't bind to, or physical properties that limit enzyme access during the time it takes for starch to transit the GI tract (Birkett2007).  Any type of starch that obstructs or hinders its digestion is classified as 'resistant starch'.

Figure 1. If amylase can access the straight-chain starch (amylose) it can be degraded efficiently in the small intestine (top).  Hydrolysis of amylopectin rapidly occurs (middle) until the enzyme encounters a branch point (bottom) at which point the enzyme cannot catalyse hydrolysis and the resulting 'limit dextrin' is further process by isomaltase (brush border cells) or remains undigested.

Amylose and amylopectin

Starches are biopolymers of glucose created by plants much the same way glycogen is used by animals to store glucose monomers. Starch is made up of two main types: branched polymers with α-1,4 and α-1,6-glycosidic linkages called amylopectin (typically 65-85% of starch content), and straight-chained polymers with α-1,4-linkages called amylose (15-35%, Figure 1 and Table 1).  Both forms generally exist as granules.

Table 1. Comparison of the properties of amylose and amylopectin (from Birkett2007).

Upon cooking (heat + moisture; <70°C, Doublier1989), amylopectin loses its granularity to become a waxy, gelatinous substance.  The structure unravels and allows many of its branched sidechains to be exposed to amylase enzymes (Figure 2).  As such, amylopectin is typically rapidly broken down, but when the enzyme encounters an α-1,6 branch point it stops because it is unable to cleave this type of glycosidic linkage.  As a result, about 30% ends up as variously-sized fragments (Hurtado2019b) called 'limit dextrins' and these can either be further hydrolysed by brush-border isomaltase enzyme or proceed unchanged to the large intestine where bacteria can restart the process of hydrolysis and fermentation.

Figure 2. Molecular model of amylose forming a double helix structure through hydrogen bonding (left).  Amylopectin (middle, Perez2010) can be rapidly digested because its organised structure can be relatively easily unravelled allowing access to many amylase molecules (right).

In contrast to amylopectin, amylose, the straight-chained polymer of various lengths, is able to resist digestion to varying degrees. The structure of amylose lends itself to being packed together as linear, double helical structures stabilised by hydrogen bonding and these are further able to organise themselves to form two sixfold crystal structures (Figure 3, Perez2010).

The degree of amylose indigestibility is often source-dependent: high-amylose corn starch, for instance, has a high tolerance to cooking and baking (154–171°C, Doublier1989) whereas most other amylose sources tend to lose their granularity and gelatinise under these conditions, rendering them digestible in the small intestine.  As a consequence of the cooking process, about 95% of starches obtained from our diet are actually digested (Cassidy1994).

It has been shown in rats, however, that high levels of amylose in meals (600-850g amylose/kg of total starch, Brown2003) - whether cooked or uncooked - reduces the post-meal ('postprandial') insulin response in these animals (Brown2003).  There is a reasonable assumption the same would occur in humans as well and suggests not all the amylose is gelatinized and digested.

Figure 3. Molecular model of double-helix amylose (left)

Resistant Starches

Starches are often classified by rate of digestion, and in effect, the ratio of digested glucose vs the amount passed to the colon. As mentioned above, resistant starches (RSs) are those glucose biopolymers that tend to be indigestible.  The amount of RS occurring in foods varies (Table 2).

Table 2. Resistant starch content of foods (from Birkett2007).

Four main types are generally recognised (see Table 3).

Physically inaccessible starches (RS1)

Starches of this type are inaccessible to enzymes because the polymers are compartmentalised within resistant cell walls of cereal grains and legumes (Livesey2014). Starches are considered to be 'physically trapped within the food matrix' and require milling and grinding to become more digestible (Birkett2007). Steel-cut oats that have been left to soak overnight (uncooked) are a good source of this type of starch.

Table 3. Types of starch and their digestibility (from Livesey2014)

Resistant granules (RS2)

Some plants efficiently store starch as dense, well organised packages called granules ranging in size from <1 µm to 100 µm.  Perez and Bertoft (Perez2010) have published a very comprehensive review of the architecture of these structures.  Granules are made up of layers of amylopectin in the form of alternating semi-crystalline shells ('growth rings') and amorphous layers. Raw starchy foods, such as green bananas, potatoes, corn and ginger, produce grains that limit access by digestive enzymes.

Retrograded starch (RS3)

When amylose is repeatedly heated and cooled the single strands of the polymer tend to aggregate, forming double-helix structures (Figure 3, above). Such starches are said to have been retrograded. Chemists use a similar principle to purify compounds using 'recrystallisation' techniques. The key to successful recrystallisation is slow cooling to allow the molecules time to optimally organise themselves.

Retrograded starches are most often seen in high-amylose containing foods, such as breakfast cereals, potatoes and bread (Livesey2014). Again, enzyme accessibility is limited because of the dense nature of these starches.

Chemically modified starch (RS4)

Native starches that have been chemically, physically or enzymatically modified prior to being added to foods are called 'chemically modified' starches.  The food industry produces these compounds for various reasons (thickeners, improved solubility, emulsifiers, etc.), but some modifications also limit digestibility.  Cross-linked strands produced by treating starch with sodium trimetaphosphate to create distarch phosphate are such an example of RS4 food additives (Chen2018).  Modifications of hydroxy sidechains of the glucose monomers (etherification and esterification) inhibit enzyme substrate recognition (Birkett2007).  Even the process of baking (non-burning) can produce RS4 maltodextrins (Livesey2014).

Undigested carbohydrates

Resistant starch and other undigestible carbohydrates (e.g. cellulose) are not broken down in the small intestine and so don't contribute to plasma glucose levels. Instead the undigested chyme passes into the large bowel where bacteria can fully or partially ferment the carbohydrates to produce short-chain fatty acids (SCFAs), such as acetate, propionate and butyrate (for more details, see Fate of Undigestible Carbohydrates).

It is believed that about 95% of SCFAs are absorbed and provide energy for our body (Cummings1991).  Butyrate is a primary energy source for colonocytes, while acetate can be burnt by muscles and propionate converted to glucose by the liver.  The amount of useable energy is not insignificant: SCFAs may provide 5-10% of our energy intake (Cummings1997, Cummings1987).

Not all carbohydrates can be metabolised by bacteria and around 20% of it is excreted (Phillips1995), depending on an individual's diet and health status.