Preventing colorectal cancer: What role does lifestyle play?

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. These are all factors that increase the likelihood of getting different types of cancer. 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 almost 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's 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 diets” are constantly changing, for instance.

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 move stool more quickly through the bowel. 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.

Observational studies suggest that there’s 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. These results aren't all that reliable either, though, because many of the people participating in the studies dropped out early.

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, cured meats or smoked meats are thought to increase the risk of colorectal cancer. Some observational studies have shown 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. 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). The risk of developing colorectal cancer is a lot higher if you have a BMI of 30 or more – and the more you weigh, the higher the risk. But there are no 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 many people find that physical activity helps them to feel better, it improves their level of fitness and has other health benefits.

Studies have shown that there is a link between alcohol and colorectal cancer: The more alcohol you drink, the higher your risk of developing colorectal cancer.

Regardless of whether alcohol has any effect on your risk of colorectal cancer, there are many other reasons to drink alcohol only in moderation. Drinking too much alcohol usually has a lot of health, emotional and social consequences, such as problems in relationships and 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 as well.

The German Centre for Addiction Issues (Deutsche Hauptstelle für Suchtfragen) recommends that healthy adults who don’t have an alcohol problem should have at least two alcohol-free days per week. The recommendations on low-risk consumption of alcohol is different for men and women.

Not smoking is one of the best things you can do for your health. If you smoke, you have a much higher risk of many different types of cancer, especially lung, throat and laryngeal cancer. Smoking is a common cause of chronic lung disease and makes other illnesses more likely, including cardiovascular disease. The risk of colorectal cancer is also somewhat higher in people who smoke, but this link is a lot weaker than the link to 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, you should check whether they 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, D and E. Not all of them have been subjected to the same amount or quality of testing, 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
• Beta-carotene
• 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 can be a good idea to use dietary supplements.

But taking too many of some dietary supplements (overdosing) over the long term for no good reason can be quite risky.

Certain medications are sometimes claimed to prevent 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 reduce the risk of colorectal cancer include acetylsalicylic acid (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 also observed to 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 acetylsalicylic acid ("Aspirin") in the prevention of colorectal cancer. One thing is clear: The medication has to be taken for at least ten years to have any kind of preventive effect.

The side effects of acetylsalicylic acid include stomach ulcers and bleeding, especially in the gastrointestinal tract (stomach and bowel). Scientific societies in Germany and other countries currently advise people not to use this drug for the prevention of colorectal cancer.

If you don't have any specific risk factors, you aren’t very likely to develop colorectal cancer. It's not possible to know for sure whether taking medication or changing your lifestyle or diet can lower the risk. Some dietary supplements can even be harmful.

Screening for colorectal cancer is the best way to lower your risk. This can be done using two approaches: the fecal occult blood test (FOBT) and colonoscopy.