Family Doctor Books
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Published in association with the British Medical
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Family
Doctor Books |
Preview of Understanding Food & Nutrition
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Everything that goes into
your stomach is mixed with enzymes, chemicals that break food into its
basic components (nutrients). This mixture is then passed from your stomach
into your intestines, where the nutrients are absorbed into your bloodstream.
The nutrients are transported around your body to the cells where they
are used or stored. Food components that are not absorbed are excreted.
Digestion
starts in the mouth and continues in the stomach and small intestine.
Nutrients are absorbed in the intestines and waste materials
excreted through the anus. |
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Your gastrointestinal tract
Your gastrointestinal tract is a tube of around seven
metres in length.
Your mouth and stomach
Digestion begins as soon as you start chewing food.
Saliva, which is secreted by glands in your mouth, is mixed with the
food as you chew it to make it easier to swallow. Saliva contains the
enzyme amylase, which breaks down starchy carbohydrate foods into simpler
sugars that can be absorbed into your body. Amylase can work only in
an alkaline environment.
Once food is swallowed, it travels down your oesophagus
(gullet) to your stomach. At its entrance and exit, your stomach has
rings of muscle called sphincters that act as valves. When food arrives
at your stomach, the top sphincter opens, so that food can enter. The
top sphincter then closes, keeping the food and digestive juices inside
your stomach. If this sphincter leaks, digestive juices, including acid,
are regurgitated into your gullet. When this happens, you experience
heartburn, as your stomach’s acid contents irritate the lining of
your oesophagus.
Digestive juices are added to food from glands in your
stomach wall. These juices contain chemicals that break down food into
a usable form. Two of these chemicals are the enzyme protease and hydrochloric
acid. Protease breaks down proteins. Hydrochloric acid destroys most
of the bacteria present in food and provides the acid conditions in which
protease works. The only substance that is not subject to these digestive
processes is alcohol, which is absorbed into your bloodstream directly
from your stomach.
Your stomach acts as a reservoir. Semi-liquid food
remains there for two to four hours before being released in small amounts
through the bottom sphincter into your small intestine.
Saliva
is mixed with food as you chew, making it easier to swallow and
starting digestion. |
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The stomach
has rings of muscle at its entrance and exit, which act as valves,
sealing the stomach. |
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The small intestine
This is the longest part of your gastrointestinal tract,
being five to six metres long. It is called the ‘small’ intestine
as it is narrow, only two to four centimetres in diameter, compared with
the large intestine, which is six centimetres in diameter. Your small
intestine consists of three distinct parts. Your duodenum lies just after
your stomach, and is the shortest part of your small intestine. It is
followed by your jejunum and your ileum, which connects to your large
intestine.
When food enters your duo-denum, it is still acid from
the stomach juices. Alkaline digestive juices are now added to neutralise
it. These digestive juices are produced in an organ lying below your
stomach called the pancreas and contain enzymes that continue to digest
food. Bile is also added to the mixture. This green, watery fluid, which
is produced in your liver and stored in your gallbladder, helps to keep
fatty material in solution.
Once the digestive juices have done their job, the
major food components have been broken down into their constituents:
- Proteins into amino acids
- Carbohydrates into glucose and other simple sugars
- Fat into fatty acids and glycerol.
Further down your small intestine, in your jejunum
and ileum, the end-products of digestion are absorbed through the intestinal
wall into your bloodstream. Food is passed along your intestine by wave-like
contractions of muscles in your intestinal wall; this is called peristalsis.
Your intestinal wall is not smooth, but consists of millions of tiny
finger-like protrusions called villi. The villi give your intestines
a large surface area through which food is absorbed. Water-soluble vitamins
and minerals are also absorbed at this stage of digestion.
Food passes
from the stomach into the small intestine, where absorption of
nutrients begins. The large intestine comes after the small intestine,
and absorbs water and eliminates undigested waste. |
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Once the nutrients have been absorbed, the remaining
undigested food passes through another sphincter muscle into your large
intestine. Your body can store some nutrients, such as those providing
energy and certain vitamins and minerals. Excesses of nutrients that
cannot be stored are lost in the faeces.
The large intestine
Your large intestine consists of your colon, rectum
and anus, and is up to a metre in length. It reabsorbs the water that
is used in digestion and eliminates undigested food and fibre. It has
recently been shown that very little food is really passed through the
body undigested, because bacteria in the colon break down fibre residues,
and release fatty acids, which are important for the nutrition of the
colon itself. Once water has been reabsorbed in your colon, the faeces,
which are now drier and more solid, are passed along your rectum by peristalsis
and are finally expelled through your anus.
When faeces reach your rectum, they trigger the desire
to defaecate, due to reflex contractions of your rectum and the relaxation
of your anal sphincter muscles. Your anal sphincters are circular muscles
that control the opening and closing of your anus.
It usually takes between one and three days for food
to pass from your mouth to your anus. Some people defaecate two or three
times a day, others daily and some only every two to three days. All
of these patterns are normal.
The intestinal
wall is not smooth but consists of millions of tiny finger-like
protrusions called villi. |
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The importance of fibre
Fibre or non-starch polysaccharides (NSPs) are derived
from plant material. Fibre cannot be broken down by digestive enzymes,
so it passes through your gastrointestinal tract without being absorbed.
As it adds bulk to your diet, fibre makes you feel full and also regulates
your bowel movements. It does this by retaining water in your gut, increasing
the bulk of the gut contents.
If you eat a high-fibre diet with plenty of fluids,
your faeces will be bulky. As a result, they stimulate your gut wall,
increasing peristalsis, and pass through more quickly and easily. This
prevents constipation, which affects 10 to 12 per cent of the population.
This figure rises to 20 to 30 per cent of those aged over 60.
Some people think that they have constipation simply
because they don’t realise that their bowel habits are actually
normal. Changing to a high-fibre diet and drinking more fluids can ease
constipation in most people. Laxatives should be used only on medical
advice, as they are not always necessary and their abuse can lead to
other prob-lems, such as loss of muscle tone in the bowel.
A high-fibre diet also helps prevent a common disorder
called diverticular disease. The early stages of this can be detected
in at least 15 per cent of people over 50. Most sufferers have a history
of constipation, which leads to increased pressure in the colon. Straining
to pass hard faeces can stretch the wall of the large intestine, encouraging
the formation of small pouches, called diverticula, which are pushed
outwards from the bowel wall. Inflammation and bacterial overgrowth in
these pouches may cause pain and diarrhoea. A high-fibre diet with plenty
of fluids can relieve the symptoms in most people. Some people, however,
will require treatment with laxatives.
If you
eat a high-fibre diet with plenty of fluids, your faeces will
be bulky. This stimulates the gut wall, increasing peristalsis
and preventing constipation. |
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Bowel cancer
Fibre in the diet is also important in connection with bowel cancer,
the third most common type of cancer. If detected early enough, it has
a very good prognosis, but many people delay seeking medical advice because
of embarrassment. It has been shown that a diet that is low in fibre
increases the risk of developing bowel cancer. This is because, without
fibre, unabsorbed food takes longer to pass through the gut. This means
that the bowel lining is exposed to potentially harmful compounds in
the unab-sorbed food for longer periods.
Straining
to pass hard faeces can stretch the wall of the large intestine,
forming small pouches called diverticula. |
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Case study: Tom
Tom, a widower in his 70s, had suffered from intermittent
diarrhoea and severe pains in his lower abdomen for a couple of weeks.
When his doctor questioned him about his bowel habits, it became clear
that Tom frequently suffered from constipation. Analysis of his faeces
showed that Tom didn’t have an infection. A barium enema, which
gives a picture of the lining of the large intestine, showed that Tom
was suffering from diverticular disease. He was referred to a dietitian.
The dietitian discovered that Tom’s diet didn’t include much
fibre and that he drank only small amounts of fluid. She advised Tom
on how to increase the amount of fibre in his diet while still enjoying
his favourite foods. She also encour-aged him to increase his fluid intake.
By following this advice, Tom was able to relieve many of his symptoms.
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KEY POINTS
- Food is broken down into its building blocks by
enzymes in your stomach and your small intestine
- Nutrients are absorbed into your bloodstream from
your intestine
- Some excess nutrients can be stored in your body,
but others are excreted
- Fibre is essential for the normal movement of
food along your bowel and has an important role in food digestion
and absorption
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