Until recently nutritionists believed that the starch from cereals, breads, pasta and potatoes was completely broken down and absorbed from the small intestine - that's the uppermost part of the bowel just after the stomach and is where most of our digestion occurs. Dietary fibre was the only food component that was believed to enter the large intestine.

However, over the last 10 years, scientists realised that a significant portion of starch escapes or "resists" digestion in the small intestine and actually reaches the large intestine or bowel where it behaves in a similar manner to fibre.

In fact, many of the health benefits attributed to fibre may well be due to this resistant starch. For example, this fibre-like starch could explain why countries like Japan and India, where intake of fibre is low, have one of the lowest incidences of bowel cancer in the Western world. 

What are the health benefits of resistant starch?

Resistant starch works like soluble fibre. It:

• produces a mild laxative effect - the bacteria that live in the bowel use it as a food source and this increases the mass of bacteria and so adds to the faecal bulk.
• stimulates the bacteria to create short-chain fatty acids, particularly butyrate, a substance which maintains the bowel lining and is believed to protect against bowel cancer.
• could assist in weight management by increasing satisfaction, creating a feeling of fullness, and by lessening the number of kilojoules absorbed from carbohydrate.
• reduces the rise in glucose after eating, so demands less insulin, which is important for people with diabetes.
• minimises tooth decay, being resistant to fermentation by bacteria in the mouth and less likely to stick to the teeth.

Where is resistant starch found?

Most starchy foods have 5% or less resistant starch (see table below), but certain foods carry much higher natural levels. The levels of resistant starch can be increased by cooking and processing. Foods naturally high in it are:

• Unripe bananas (54%)
• Legumes
• High-amylose corn starch (50-85%)

Source: The Resistant Starch Report, S Landon, CGB Colyer and H Salman, Goodman Fielder and National Starch, no date supplied

Current levels of intake

Australians currently consume about 6 grams of resistant starch per day. With the main sources being cereals and bread (36%), vegetables (26%) and fruit (22%).

Nutritionists suggest increasing this to around 20 grams a day to promote good bowel health.

Types of starch

Starch is a complex molecule made up of many glucose units joined together. It is found exclusively in plant foods such as:

• grains like maize, wheat and barley,
• vegetables like peas, beans and potatoes and
• fruits such as green bananas.

Starch occurs in two forms - amylose and amylopectin. Amylopectin is readily digested, but amylose is not. 

Hi-maize™

Hi-maize™ is a special high-amylose variety of corn. It contains more of the harder-to-digest amylose than the easier-to-digest amylopectin and so is high in resistant starch (50 to 85%).

It can increase the fibre content of many foods without interfering with their taste, appearance or texture. It does not make bread look brown or make bran cereal chewy and coarse. It's also gluten-free.

It is the main ingredient in high fibre white breads and hamburger buns (those with ‘invisible fibre') and certain breakfast cereals such as Goodness Superfood with BarleyMAX - it's an easy way to add fibre to kid's diets.

Varieties of wheat with high amylose levels are also now being bred and will soon produce flour and bread with plenty of resistant starch.

Classification of resistant starch

Resistant starches are usually classified into four different groups.

1. RS1
Found in whole or kibbled grains, legumes and seeds where the granules of starch are physically inaccessible, preventing access by digestive enzymes to the starch.

2. RS2
Found in unripe banana, raw potato, some legumes and Hi-maize. This starch is naturally resistant, containing more of the hard-to-digest amylose.

3. RS3
Found in cooked-and-cooled potato, corn flakes and bread - the cooking or processing has altered the crystalline form of starch and rendered it unable to be digested by human enzymes (retrograded).

4. RS4
These are chemically modified starches and are used to thicken and set foods. Unlike natural starch, modified starch is less likely to breakdown during processing and storage and has a lower temperature at which it thickens. The food can therefore be cooked at a lower temperature so retaining important nutrients.

Regulations

Along with lignin, resistant starch is one of the substances detected by the official AOAC method for measurement of fibre. Hence it can be claimed as "fibre" on food labels. 

The bottom line

Resistant starch is just ONE of the elements that comprise dietary fibre. Dietary fibre is all the components of plant food that pass through the stomach and small intestine undigested. As such it is not one single element but a number of substances - largely carbohydrate in nature - all reaching the large bowel undigested where they are subject to varying degrees of breakdown by bacteria. These components non-starch polysaccharides (NSP), lignin and resistant starch.