Resistant starch

Historically starch has been thought to be 100% digested to glucose in the small intestine. Research over the last few decades has found that a significant portion (about 10%) is not digested in the small intestine and passes into the large intestine where it is a substrate for bacterial fermentation. This starch is called resistant starch (RS) and many nutritionists think that it should be classified as a component of dietary fibre.
The bacteria in the large intestine produce short chain fatty acids from the RS which may help maintain the health of cells lining the colon (colonocytes) and prevent bowel cancer. These fatty acids are also absorbed into the bloodstream and may play a role in lowering blood cholesterol levels. A new study suggests that RS may also help with weight loss.

A study by Higgins et al, published in October 2004 issue of Nutrition and Metabolism showed that replacing 5.4% of the carbohydrate content of a meal with resistant starch increased fat oxidation by 23% in a sample of 12 study subjects. This increase is apparantly sustained throughout the day, even if only meal contains RS and the increased fat oxidation is sustained if one keeps eating RS on a daily basis. It appears that the RS changes the order in which the body burns food. Usually carbohydrates are used first, but when RS is present, dietary fat is oxidised first into energy before it has a chance to be stored as body fat. This study suggests that including foods high in RS in your daily diet may help with weight management.

Where is RS found?
intact wholegrain cereals/seeds/nuts (unprocessed) e.g oats, rye, wheat, barley, semolina, corn, linseed, sesame
processed starchy foods e.g some breakfast cereals (like cornflakes), white bread, rice, pasta
processed starchy foods with added RS called Hi-Maize derived from corn e.g some breads, cereals
legumes e.g lentils, baked beans (legumes have the highest content of RS)
unripe fruit, especially banana
Cooking and cooling the food can also increase the RS content
cooked cold rice (e.g sushi rice), cold pasta salad, cold boiled potato salad

So why is some of the starch resistant to digestion and what does cooking and cooling do to starch?

Starch is made up of glucose molecules linked together to form amylose and amylopectin. Amylose has a linear molecular structure and can stack to form tighly packed granules which is insoluble and hard to digest whereas amylopectin has a branched structure and thus cannot form tighly packed granules and and is thus easier to digest.

Most plants contain about 20-25% amylose. But some, like pea starch have 60% amylose and certain species of maize starch have 80% amylose (e.g. Hi-Maize(r)) - these plants are therefore very high in RS.

The physical and chemical composition of starch determines whether starch is digested in the small intestine or whether it ferments in the colon. There are several reasons why starch may not be digested:

Some starch may be physically trapped inside intact plant cells as in wholegrain foods like muesli and grainy bread. This starch is therefore inaccessible because digestive amylases are unable to penetrate or break down the cellulose cell walls.
The higher the amylose content of starch the greater its resistance to digestion because they form tighly packed granules in cells. Raw potato, green bananas, pulses and high amylose maize starch have a high amylose content.
When starch is heated, starch granules swell and are disrupted. This process, known as gelatinisation, makes the starch much more accessible to digestive enzymes. Starch with a high amylose content and starch which is inaccessible due to the physical structure in which it is located, are less susceptible to gelatinisation and hence are more resistant to digestion.
When starch that has been heated, is cooled, retrogradation occurs converting some of the gelatinised starch to a crystalline form which is resistant to digestion. Foods, such as bread, cornflakes, cold cooked potato, rice and pasta, contain retrograded starch which is resistant to digestion.

How much resistant starch is required for good health?
Some resistant starch is measured when total dietary fibre is measured. However, there is currently no official analytical method for measuring the resistant content of foods. It has been estimated that resistant starch intake in Australia is around 5-7 grams/person/day. Approximately 20 grams a day is recommended to obtain the beneficial health benefits of resistant starch.

Related articles - Carbohydrate and fibre
Cereals
The grains and cereals debate

Last Updated: July 2005