Preventing colorectal cancer: What role does lifestyle play?
There’s a lot of advice out there about how to prevent colorectal cancer: The long list of recommendations includes more fiber, less meat, extra vitamins and even medication. But which of the most common claims are backed by scientific evidence?
Healthy people can lower their risk of colorectal cancer by having colorectal cancer screening. Screening is typically done in people over the age of 50 and has been proven to lower the risk of colorectal cancer.
They can also try to avoid getting colorectal cancer by following a healthier lifestyle. But what can be made of all of this advice, especially the tips on diet? Does eating a lot of fruits and vegetables and less red meat really help? And will taking extra vitamins make a difference?
Lifestyle and health: difficult to research
Many of the assumptions about links between lifestyle and health are taken from what are known as observational studies. These studies can be designed in different ways, but ideally they should allow healthy people to document their lifestyle habits over a long period of time. After many years have passed, it may then be possible to tell whether, for instance, people with certain eating habits developed colorectal cancer more often than others.
If, for example, it is observed that colorectal cancer is more common in people who often eat red meat than in those who eat hardly any, it may seem like eating red meat increases the risk of colorectal cancer. But people who eat a lot of red meat might also smoke more, drink more alcohol, or tend to be more overweight. In theory, these “confounding factors” can be considered when interpreting the results of a study. The problem is that people may also differ from one another in ways that aren’t immediately obvious, such as in their genes or their living and working conditions. It is virtually impossible to account for all of that in a study. You would also have to know about all of the factors that influence the development of colorectal cancer, and be able to measure them too. So there is always a risk that observational studies have overlooked something important.
Other types of studies called randomized controlled trials (RCTs) are better for finding out what is causing something. In this kind of study, the participants agree to be assigned to one of two (or more) groups by chance (randomization). One of the groups might then have to follow a diet low in red meat, while the other would eat the same diet as they had before. Randomization ensures that reliable conclusions can be drawn when the two groups are compared with each other. The random allocation not only ensures that the two groups contain similar numbers of men and women, older and younger people, or smokers and non-smokers – it also ensures that the groups contain similar numbers of people with unknown influential factors.
This approach is only rarely used in lifestyle studies, though, so the study results and related recommendations often need to be taken with a grain of salt. This uncertainty is also reflected in the fact that recommendations on “healthy living” are constantly changing.
Can fiber help prevent colorectal cancer?
Generally speaking, dietary fiber is a type of carbohydrate that can’t be digested by the body. It is believed to be good for your health. There are a number of different theories about why fiber might help to prevent colorectal cancer. For instance, fiber helps to reduce the time that stool stays in the colon. Also, carcinogenic (cancer-causing) substances that have entered the body along with food might stick to fiber, and be transported out of the body again in that way.
Many observational studies have found a link between a high-fiber diet and colorectal cancer: People who eat more fiber seem to be somewhat less likely to develop colorectal cancer. But this hasn’t been confirmed in randomized controlled trials: Some of these studies tested whether eating more fiber can prevent colorectal cancer, but they didn’t find this to be true. They concluded that fiber probably has little or no effect.
Fruits and vegetables
Fruits and vegetables contain compounds called flavonoids. These are thought to help prevent cancer. One theory is that they protect the body from certain molecules that can damage the cells, known as “free radicals.” Flavonoids can also be found in seeds and sprouts, tea, cocoa, chocolate and wine.
Researchers from the Cochrane Collaboration (an international research network) analyzed the results of several randomized trials that looked into the link between colorectal cancer and a diet rich in flavonoids. But they found no convincing proof that people who eat a lot of fruits and vegetables are less likely to develop colorectal cancer.
Red meat and processed meats like sausage, or cured or smoked meats are thought to increase the risk of colorectal cancer. Observational studies have in fact shown that there is a link: The more beef, lamb or processed meats people eat, the greater the risk of colorectal cancer. This is currently not believed to be the case for unprocessed poultry and pork, though. But no randomized trials have shown that eating less red and processed meat helps to prevent colorectal cancer.
People are considered to be overweight if they have a body mass index (BMI) of between 25 and 30. Overweight men might have a slightly increased risk of colorectal cancer, but being overweight is less of a risk factor in women. In terms of overall health, being somewhat overweight isn’t likely to be a problem. Studies haven’t shown whether losing a bit of weight helps to prevent colorectal cancer.
This is different for men and women who are extremely overweight (obese), with a BMI of over 30. They have a much higher risk of colorectal cancer and other diseases. But there haven’t been any randomized trials on whether losing weight can lower the risk of colorectal cancer in obese men and women.
People who are physically active seem to have a lower risk of colorectal cancer. The effect of exercise on the risk of colorectal cancer hasn’t yet been studied in randomized trials, though, so it’s not clear whether getting more exercise really does prevent colorectal cancer.
But physical activity helps many people to feel better, and improves your overall fitness.
People who have more than one alcoholic drink per day are more likely to develop colorectal cancer. “One drink” is the equivalent of a bottle of beer (0.33 liters) or a small glass of wine (0.1 liters). This risk increases much more in people who have more than four alcoholic drinks per day.
Regardless of whether alcohol has any effect on the risk of colorectal cancer, there are many other reasons to avoid drinking too much alcohol. Drinking too much alcohol usually has a lot of health, emotional and social consequences, such as problems in relationships or the family, or trouble at work. Too much alcohol can also cause permanent liver damage. And it may aggravate emotional problems or illnesses such as depression. Last but not least, the risk of being in an accident, falling or injuring yourself increases too.
Not smoking is probably one of the best things you can do for your health. Smokers have a much higher risk of many different types of cancer, especially lung, throat and laryngeal cancer. Smoking is also a common cause of chronic lung disease and makes other illnesses more likely, including cardiovascular disease. The risk of colorectal cancer is probably also somewhat higher in smokers, but the link is a lot weaker than the link with diseases like lung cancer.
According to advertising, dietary supplements have all sorts of positive effects on our health. But the only really valid reason for taking dietary supplements is if you have a deficiency – or if they are prescribed as treatment for a disease. And even then, research should be done to see whether they can actually prevent the illness or relieve symptoms.
Many different dietary supplements have already been tested in randomized trials to see whether they can prevent colorectal cancer. These include calcium, folic acid, selenium, and vitamins A, C and E. Some of these have been better studied than others, but it's clear that none of them have been proven to prevent colorectal cancer. Some studies even suggest that certain dietary supplements could increase the risk of colorectal cancer or cancer in general, or may shorten your life expectancy. High doses of the following supplements have been shown to have a potentially harmful effect:
- Vitamin A
- Vitamin E
- Folic acid products
Just to be clear about the results of the studies: The body needs vitamins and minerals, and can usually get enough from a balanced diet. If this isn't the case, or there are special circumstances where more is needed (such as folic acid before and during pregnancy), it may be a good idea to use dietary supplements.
But taking too many of some dietary supplements (overdosing) for no good reason can be quite risky.
Certain medications are sometimes claimed to be effective in preventing colorectal cancer. But using medications to prevent cancer isn’t necessarily a good idea: In order to provide any protection, they need to be taken regularly over a very long time – sometimes even decades. This also increases the risk of side effects.
The medications that may possibly reduce the risk of colorectal cancer include acetylsalicyclic acid (ASA, the drug in medications like “Aspirin”). This medication is often used as a painkiller or blood thinner. In studies on preventing complications in people with heart problems, it was observed that ASA may also provide some protection from colorectal cancer. But there aren’t many studies specifically in this area, so it’s difficult to draw any reliable conclusions about the effectiveness of ASA in the prevention of colorectal cancer. One thing is clear: The medication has to be taken for at least ten years to have a preventive effect, if at all.
The side effects of ASA include stomach ulcers and bleeding, especially in the gastrointestinal tract. Scientific societies in Germany and other countries currently do not advise people to use ASA for the prevention of colorectal cancer.
Is there anything at all I can do to prevent colorectal cancer?
If you don't have any specific risk factors, you aren’t very likely to develop colorectal cancer. It's difficult to know for sure whether changing your lifestyle or diet, or taking medication, can lower the risk. Some dietary supplements may even be harmful. The best studied way to prevent colorectal cancer is by going to colorectal cancer screening, which involves a stool test and colonoscopy.
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