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
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.
is RS found?
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,
processed starchy foods with added RS called
Hi-Maize derived from corn e.g some breads,
legumes e.g lentils, baked beans (legumes
have the highest content of RS)
unripe fruit, especially banana
and cooling the food can also increase the
cold rice (e.g sushi rice), cold pasta salad,
cold boiled potato salad
why is some of the starch resistant to digestion
and what does cooking and cooling do to
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.
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:
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.
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
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
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.
much resistant starch is required for good
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
grains and cereals debate
Updated: July 2005