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Causes and risk factors
for Cancer
Cancer is 'multifactorial'. Multifactorial means
that there are many factors involved. In other words, there is no
single cause for any one type of cancer.
Carcinogens
A 'carcinogen' is something that can help to cause cancer. Tobacco
smoke is a powerful carcinogen. But not everyone who smokes gets lung
cancer. So there must be other factors at work as well as carcinogens.
Age
Most types of cancer become more common as we get older. This is
because the changes that make a cell become cancerous in the first place
take a long time to develop. There have to be a number of changes to
the genes within a cell before it turns into a cancer cell. These
changes can happen by accident when the cell is dividing. Or they can
happen because the cell has been damaged by carcinogens and the damage
is then passed on to future 'daughter' cells when that cell divides.
The longer we live, the more time there is for genetic mistakes to
happen in our cells.
Genetic make up
There need to be a number of genetic mutations within a cell before it
becomes cancerous. Sometimes we are born with one of these mutations
already. This doesn't mean we will get cancer. But with one mutation
from the outset, it makes it more likely statistically that we will
develop cancer during our lifetime. Doctors call this 'genetic
predisposition'.
The BRCA1 and BRCA2 breast cancer genes are examples of genetic
predisposition. Women who carry one of these faulty genes have a higher
chance of developing breast cancer than women who do not.
The BRCA genes are good examples for another reason. Most women with
breast cancer do not have a mutated BRCA1 or BRCA 2 gene. Less than 5%
of all breast cancer is due to these genes. So although women with one
of these genes are individually more likely to get breast cancer, most
breast cancer is not caused by a high risk inherited gene fault.
This is true of other common cancers where some people have a genetic
predisposition – for example, colon (large bowel) cancer.
Researchers are looking at the genes of people with cancer in a study
called SEARCH.
The immune system
People who have problems with their immune systems are more likely to
get some types of cancer. This group includes people who
- Have had organ transplants and take drugs to
suppress their immune systems to stop organ rejection
- Have HIV or AIDS
- Are born with rare medical syndromes which
affect their immunity
The types of cancers that affect these groups of
people fall into two, overlapping groups
- Cancers that are caused by viruses, such as
cervical cancer and other cancers of the genital or anal area, some
lymphomas, liver cancer and stomach cancer
- Lymphomas
Chronic infections or transplanted organs can
continually stimulate cells to divide. This continual cell division
means that immune cells are more likely to develop genetic faults and
develop into lymphomas.
Day to day environment
By environmental causes we mean what is around you each day that may
help to cause cancer. This could include
- Tobacco smoke
- The sun
- Natural and man made
radiation
- Work place hazards
- Asbestos
Some of these are avoidable and some aren't. Most
are only contributing factors to causing cancers – part of the jigsaw
puzzle that scientists are still trying to put together.
Viruses
Viruses can help to cause some cancers. But this does not mean that
these cancers can be caught like an infection.
These cancers and viruses are linked
- Cervical cancer, and other cancers of the
genital and anal area, and the genital wart virus, HPV
- Primary liver cancer and the Hepatitis B and
C viruses
- Lymphomas and the Epstein-Barr
Virus
- T cell leukaemia in adults and the Human T
cell leukaemia virus
- HPV also probably leads to oropharyngeal
cancer and some non melanoma skin cancers
There will be people with primary liver cancer and
with T cell leukaemia who haven't had the related virus. But
infection increases their risk of getting that particular cancer. With
cervical cancer, scientists now believe that everyone with an invasive
cervical cancer has had an HPV infection beforehand. Vaccine for HPV is
available.
Diet, cancer risk and research
Our diets are made up of hundreds of different types of food, containing
thousands of nutrients and chemicals. Some of these protect us against
cancer, while others increase risk. We all eat different things in
different amounts, which confuses the picture even more. As well as
that, there are more than 200 types of cancer. So trying to find out how
diet affects cancer risk is extremely complicated.
An international research study is going on at the moment across Europe
to try to answer more questions about diet and cancer. The study, called
EPIC, involves 500,000 people in 10 countries. EPIC stands for European
Prospective Investigation into Cancer and Nutrition. Over many years,
the researchers are recording these people's food intake, monitoring
their health records and noting who gets cancer and who doesn't. Then
they may be able to link certain factors in the diet with the risk of
getting particular cancers. It is the largest study into diet and cancer
ever conducted.
The results of EPIC could have huge implications for the prevention of
cancer. To start with, the researchers are looking into common cancers
such as breast, lung, bowel, prostate and stomach cancer. But the size
of this study means that researchers will also be able to look into the
possible role of diet in rarer cancers. The results of the study will
have implications for cancer prevention everywhere. The results will be
presented at international conferences and published in journals
available to researchers and health officials throughout the world.
Being overweight
Overweight or obese people have an increased risk of bowel cancer and
pancreatic cancer and this could be because they tend to have higher
insulin levels.
'Obese' means being more than about 25% overweight. Obesity can also
increase your risk of
* Oesophageal cancer, kidney cancer and gallbladder cancer
* Breast or uterine (womb) cancer if you are a woman
Researchers are not sure exactly why obesity increases the risk of these
cancers. Breast cancer is hormone linked. Its development is affected by
amounts of the sex hormones oestrogen and testosterone. Fatty tissue
produces an enzyme called 'aromatase'. Aromatase affects the balance
between these hormones and this may explain why being obese affects your
risk of breast cancer.
Although obesity increases breast cancer risk if you are postmenopausal,
it lowers breast cancer risk if you are premenopausal. But the risk of
breast cancer increases strongly with age, and being obese increases the
risk of several cancers as well as other diseases. So it is important to
maintain a healthy body weight throughout your life.
Alcohol and cancer
We now know that alcohol can increase your risk of a number of cancers.
It increases the risk of mouth cancer, pharyngeal cancer, laryngeal
cancer, throat cancer, cancer of the foodpipe (oesophagus), liver
cancer, breast cancer and bowel cancer. Even moderate alcohol intake
increases your risk of cancer. A recent UK study showed that women who
drink one or two drinks a day have a slightly increased risk of cancer.
.
Foods that might be protective
There has been a lot of interest in whether eating certain foods might
reduce risk of certain types of cancer. Despite a great deal of
research, we haven't found any convincing links.
There is some evidence that eating plenty of fruit and vegetables can
protect against cancer. The evidence is strongest for cancers of the
upper digestive system, such as the mouth, food pipe (oesophagus) and
stomach.
Fruit and vegetables contain a wide variety of nutrients and are high in
fibre. Scientists are working to find out about which of these nutrients
protect against cancer. So far they are fairly sure that vitamins A and
C, and folate are playing an important role in protecting against
cancer. It's a complicated picture, but as far as we know, it is not
enough to take vitamin supplements. You have to eat the fruit and veg!
There is something about eating fresh vitamin rich foods that is
protective.
We also think that bowel cancer is less common in people who eat lots of
fibre. One EPIC study showed that people who ate the most fibre had a
40% lower risk of bowel cancer than those who ate the least.
Foods that might increase risk
It is also possible that other foods and nutrients might increase risk.
Again it has proved difficult to show any firm links, apart from
alcohol. The strongest evidence we have is probably for high intakes of
red or processed meat increasing risk.
Bowel and stomach cancer are more common in people who eat lots of red
and processed meat. Red meat includes all fresh, minced and frozen beef,
pork, lamb or veal. Processed meats have been preserved in some way
other than freezing and include bacon, ham, salami, sausages, spam,
corned beef, black pudding, pâté and tinned meat.
The way you cook meat may increase cancer risk. Certain chemicals are
made when red and processed meats are cooked at high temperatures, such
as on a barbeque. These chemicals can damage our cells, making them more
likely to become cancerous.
Food additives and cancer
Many different substances are added to commercially prepared foods. But
these are not all bad. Some additives stop food from going off and so
can help to keep us healthy. A good example of this is a toxin called
aflatoxin that comes from a mould. It grows on stored food in hot and
humid countries, especially on peanuts. Aflatoxin is known to help cause
liver cancer so anything that stops the mould from getting into the nuts
is helping to prevent cancer.
Some years ago saccharin was claimed to be a carcinogen. Researchers had
found that when it was fed to rats in huge quantities, the rates of
cancer in the rats increased. We are very unlikely to eat that much
saccharin and so it is unlikely to cause cancer in people, but far fewer
foods contain it now than did a few years ago.
Other additives can be a cancer risk. Pickled foods may increase risk of
cancer of the stomach, particularly if they are very salty. This may
explain why there are such high rates of stomach cancer in Japan, where
salty, pickled foods are popular.
Contaminants
This means chemicals in foods or drinks that are not meant to be there.
For example, they could form as an unwanted effect of the manufacturing
process.
Nitrosamines are chemicals found in cured meats (bacon and ham, for
example) and in very small amounts in beer. Nitrosamines have been shown
to be carcinogenic so the amount in foods has been cut as much as
possible. In brewing, one cause of nitrosamines is a reaction between
pollution in the atmosphere and the malted barley that is a main
ingredient of the beer. The brewing industry has tried to cut the levels
of nitrosamines in beer as much as possible. Levels are now much lower
than they used to be, but it hasn't been possible to get rid of them
completely.
Smoking and barbecuing foods so that they are slightly burnt on the
outside causes chemicals called polycyclic aromatic hydrocarbons to
form. These chemicals are known to help cause cancer. So in theory
barbecuing or cooking over a high heat could increase the cancer risk of
meat and fish. But few research studies have shown this.
Frying and baking meat at high temperatures can create chemicals called
heterocyclic amines. These may increase risk of cancer of the bowel or
gullet (oesophagus), but this isn't certain. Acrylamide, another
chemical that may cause cancer, has been found in foods such as crisps
and chips. But there isn't proof yet that these chemicals cause cancer
in humans and so there are no grounds for telling people to change
cooking or eating habits because of these chemicals.
A healthy diet
Apart from obesity and alcohol, there isn't much specific evidence at
the moment that diet can reduce cancer risk. But a healthy diet may help
and it will also lower your risk of other diseases, such as heart
disease. To eat healthily
* Eat less meat and animal fats (butter, cream, cheese)
* Eat five portions of raw or lightly cooked fruit and vegetables every
day (5 portions is about 400g or 1lb in weight)
* Eat more fibre
* Eat more oily fish (eg salmon, trout, mackerel)
* Eat less salt, salty foods, sugar and sugary foods
* Eat more cereals, bread, pasta and rice
* Don't fry foods and if you use fats in cooking, choose vegetable oils
or olive oil not lard or butter
* Drink less alcohol
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