Sucrase, Isomaltase and Starch: Why Some People With CSID Can Eat Starch (and Others Can't)
If you have Congenital Sucrase-Isomaltase Deficiency (CSID), or an acquired or secondary sucrase-isomaltase deficiency, you've probably been told to watch your sucrose intake.
But starch can be much more confusing.
Some people with sucrase-isomaltase deficiency seem to tolerate rice, bread or potatoes quite well. Others develop bloating, abdominal pain, gas or diarrhoea from starchy foods even when they contain very little sucrose.
Why?
This distinction between sucrose and starch was an important part of our recently published dietary framework for implementing a sucrose- and starch-reduced diet (SSRD). Rather than grouping these carbohydrates together, we used Australian food-composition data to classify foods according to their sucrose and starch content separately.
There is a physiological reason for doing this. Although sucrose and starch digestion are closely linked through the sucrase-isomaltase enzyme, they are not digested in exactly the same way — and a person with sucrase-isomaltase deficiency may have very different capacities for digesting each.
So why can sucrose and starch tolerance be so different?
The answer lies partly in the unusual structure of the sucrase-isomaltase (SI) enzyme itself — and in the fact that SI doesn't just digest sucrose. It also plays an important role in the final stages of starch digestion.
Step 1: Sucrose needs sucrase
Table sugar, or sucrose, is made from two smaller sugars: glucose and fructose, joined together by a glycosidic bond.
Before either sugar can be absorbed, that bond needs to be broken.
One of the catalytic parts of the sucrase-isomaltase enzyme — the sucrase domain — performs this job. It breaks sucrose apart, releasing glucose and fructose so they can be absorbed across the small intestine.
When sucrase activity is substantially reduced, some sucrose remains undigested as it moves through the small intestine.
This can create two problems. Unabsorbed carbohydrate can draw water into the intestine, and when it reaches the colon it can be fermented by gut bacteria. The result may be diarrhoea, bloating, gas, cramping and urgency.
But sucrose is only part of the sucrase-isomaltase story.
Step 2: Starch is made from glucose — but it isn't one simple chain
Starch is made from hundreds or thousands of glucose molecules joined together.
There are two main components: amylose and amylopectin.
Amylose consists predominantly of long, relatively linear chains of glucose. Most of its glucose molecules are joined by α-1,4 bonds.
Amylopectin is highly branched. Its long stretches of glucose are also joined by α-1,4 bonds, but the places where one chain branches away from another contain a different type of bond: an α-1,6 bond.
Most conventional food starches contain considerably more amylopectin than amylose — often around 70–80% amylopectin and 20–30% amylose, although these proportions vary considerably between foods and plant varieties.
Those branch points become particularly interesting when we look at sucrase-isomaltase deficiency.
Step 3: Amylase starts starch digestion — but it cannot finish it
Starch digestion begins with α-amylase, first in saliva and then much more extensively through pancreatic amylase in the small intestine.
Amylase cuts the α-1,4 bonds within starch, breaking enormous starch molecules into much smaller pieces.
But amylase cannot cut through the α-1,6 bonds at amylopectin branch points.
So after amylase has done its work, what remains includes shorter linear glucose chains and branched fragments known as α-limit dextrins.
These still aren't ready for absorption.
The final stages of starch digestion therefore take place at the brush border — the surface of the cells lining the small intestine.
Step 4: Brush-border enzymes finish the job
Step 4: MGAM and sucrase-isomaltase finish the job
Two important enzyme systems at the lining of the small intestine help finish starch digestion:
maltase-glucoamylase (MGAM) and sucrase-isomaltase (SI).
MGAM helps break the shorter starch fragments produced by amylase into individual glucose molecules that can be absorbed.
Despite its name, sucrase-isomaltase is not just involved in digesting sucrose — both of its domains also contribute to the final digestion of starch.
Both the sucrase domain and the isomaltase domain can break α-1,4 bonds in the starch fragments left behind by amylase. In other words, the sucrase part does not only act on sucrose; it also has maltase activity.
The isomaltase domain has an additional important role. It is particularly important for breaking the α-1,6 bonds found at the branch points of amylopectin.
So SI helps with starch digestion in two ways: both domains help digest α-1,4-linked starch fragments, while the isomaltase domain also helps clear the α-1,6 branch points.
That is why a deficiency of sucrase-isomaltase can affect starch digestion as well as sucrose digestion.
This means that SI contributes to starch digestion in two important ways:
it helps digest the ordinary α-1,4-linked starch fragments; and
its isomaltase domain does most of the work of dealing with α-1,6 branch points.
That is why sucrase-isomaltase deficiency isn't necessarily only a problem with table sugar.
Starch digestion is a team effort: amylase starts it; MGAM and sucrase-isomaltase finish it.
So why can some people with CSID tolerate starch?
This is where individual variation becomes important.
People diagnosed with CSID don't all have exactly the same amount of remaining enzyme activity.
Sucrase activity is reduced, sometimes profoundly. But residual isomaltase activity can vary considerably, from very little activity to relatively well-preserved activity.
Overall starch-digesting capacity can also vary.
This means that two people who both have CSID may have very different capacities for completing starch digestion.
Someone with substantial residual SI and isomaltase activity — together with functioning MGAM — may tolerate relatively large amounts of starch.
Someone with much less residual SI activity may have difficulty digesting both sucrose and starch.
And there are many possibilities between those two extremes.
One enzyme system, but different things can go wrong
It is tempting to imagine sucrase-isomaltase as having a simple "sucrose half" and a "starch half".
The biology is more complicated.
Sucrase-isomaltase is initially produced as a single protein with two functional parts — the sucrase and isomaltase domains — which work together at the surface of the small intestine.
A genetic variant can affect the enzyme’s ability to break down carbohydrates, but it can also interfere with how the protein folds, matures, moves through the cell or reaches the intestinal brush border.
Some genetic variants may leave more isomaltase activity intact than others.
Others interfere more broadly with processing or trafficking of the SI protein, potentially reducing both sucrose- and starch-digesting capacity.
This helps explain why sucrase-isomaltase deficiency exists as a spectrum, rather than every person having identical sucrose and starch tolerance.
Importantly, the position of an SI genetic variant alone cannot reliably tell us exactly which foods a person will tolerate.
Residual enzyme activity, the specific combination of SI variants, complementary activity from other digestive enzymes, the amount of carbohydrate eaten and other gastrointestinal factors can all influence symptoms.
Acquired sucrase-isomaltase deficiency is different again
Not all reductions in sucrase-isomaltase activity are caused by inherited SI variants.
Because SI sits at the intestinal brush border, conditions that damage or disrupt the small-intestinal lining can cause secondary or acquired reductions in SI activity.
How much sucrase, isomaltase and other brush-border enzyme activity is affected will depend on the underlying condition and the health of the intestinal mucosa.
People with acquired or secondary SI deficiency may therefore also have very different patterns of sucrose and starch tolerance.
And not all starch behaves the same way
There is one final complication.
Knowing that amylopectin contains α-1,6 branch points does not mean that foods containing more amylopectin will necessarily cause more symptoms.
Starch digestibility is influenced by much more than the number of branch points.
The botanical source of the starch, its physical structure, cooking and processing, the amount eaten and the composition of the rest of the meal can all change how quickly and completely starch becomes available to digestive enzymes.
Although amylose has fewer α-1,6 branch points than amylopectin, higher-amylose starches are generally digested more slowly and may contain more resistant starch. However, whether this translates into better tolerance in people with sucrase-isomaltase deficiency remains uncertain.
Resistant starch adds another layer. Resistant starch escapes digestion in the small intestine, so it does not need to be fully broken down by sucrase-isomaltase. Instead, it passes into the large bowel, where it behaves more like fermentable fibre. This may reduce the small-intestinal digestive burden, but it can also lead to gas or bloating in people who are sensitive to colonic fermentation.
So two foods containing the same number of grams of starch may not necessarily behave identically in the gut — and even the same food may behave differently depending on how it is cooked, cooled and eaten.
This is another reason why individual tolerance matters.
The takeaway
Sucrase-isomaltase isn't only a "sugar enzyme." It also makes a major contribution to starch digestion.
The sucrase domain is responsible for digesting sucrose, but SI also contributes substantially to digestion of α-1,4-linked starch fragments.
The isomaltase domain is particularly important for dealing with the α-1,6 bonds at starch branch points.
MGAM provides additional starch-digesting capacity and works alongside SI.
People with CSID can retain very different amounts of sucrase, isomaltase and overall starch-digesting activity.
Genetic variants can affect catalytic activity, protein folding, processing or trafficking, so genotype does not translate neatly into a "sucrose-only" or "starch-only" phenotype.
The type, structure, processing and amount of starch eaten can also influence digestion.
This is why some people with CSID can eat substantial amounts of starch, while others find that starch is just as important a trigger as sucrose.
And it is why neither "people with CSID only need to avoid sugar" nor "everyone with CSID needs a low-starch diet" is particularly helpful advice.
The more useful question is:
How much sucrose- and starch-digesting capacity does this particular person have?
That needs to be worked out individually rather than assumed from the diagnosis alone.
This article provides general education about carbohydrate digestion and sucrase-isomaltase deficiency and is not a substitute for individual medical or dietary advice. If you have sucrase-isomaltase deficiency and are unsure which carbohydrates you tolerate, individual assessment can help identify an appropriate level of sucrose and starch without unnecessarily restricting both.
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