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Nutrition & Cancer Research: Sorting Through the Confusion
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Dixon, MPH, MS, RD, Suzanne
Nutrition & Cancer Research: Sorting Through the Confusion
Authored by Suzanne Dixon, MPH, MS, RD
The amount of information that we receive about the role of nutrition in preventing cancer is overwhelming. In recent years, there has been more and more focus on the importance of diet even after a diagnosis of cancer. For most people, including those recently diagnosed with cancer, nutrition research is confusing and often appears to contradict itself over time. However, with just a few basic ideas in mind, it is possible for anyone to gain a better understanding of what these studies mean. This is vitally important, because the confusion arising from nutrition related research, as reported in the media, can be a significant cause of fear and anxiety around a cancer diagnosis. "What should I eat?" This is one of the most frequently asked questions by individuals recently diagnosed with cancer.
The first step to understanding nutrition and cancer research is to make the distinction between correlation and cause & effect. A correlation is a relationship that occurs in a way not expected on the basis of chance alone. On the other hand, a cause and effect relationship must have substantial and definitive evidence that the effect (disease) would not have occurred without the presence of the cause (e.g. diet, smoking, or exercise). It is much more difficult to prove cause and effect than to demonstrate simple correlation.
A good example of a correlation is the relationship between meat intake and colon cancer risk. Early studies compared average meat consumption per person in various countries and the occurrence, or rates, of colon cancer in these countries.1 These early observations led to the belief that high meat consumption causes colon cancer. However, this type of study, a correlational study, is only useful for generating ideas about dietary factors that might be linked with disease risk. These studies cannot determine cause and effect. More current research that has been designed to test cause and effect suggests that certain types of meat, such as processed, cured, smoked, and pickled meats, along with a lack of dietary fiber from a variety of food sources such as fruits, vegetables, whole grains, and legumes (starchy beans), may be more likely causes of colon cancer. Of additional note is research suggesting that fiber from supplements does not appear to decrease colon cancer risk. The fiber must be from a variety of healthy, fibrous foods to produce the risk reducing effect.
To the health professional or researcher, these may seem like obvious distinctions. But to the confused public, the only message that is transmitted from the first type of study is "meat causes colon cancer." This is soon followed by "meat does not cause colon cancer." We now know that certain types of meat and a lack of certain fibrous foods may increase risk of colon cancer. This is a very important difference, because it can allow you to adopt more positive dietary approaches such as focusing on adding important things that are missing from your diet, rather than fixating on the myriad of things that you “shouldn’t” be eating.
Research with the critical goal of determining whether a particular exposure causes, prevents, or helps treat disease is analytic in nature. An example of an analytic study is the Women's Healthy Eating and Living (WHEL) Study currently being conducted at several California Universities.2 This study is examining the effect of a heavily plant based diet on recurrence of breast cancer in breast cancer survivors. Women in the intervention group are eating approximately 10 servings of fruits and vegetables daily, along with moderate fat intake (about 20% of total calories from fat). This intervention is carefully controlled and therefore, cause and effect can be more conclusively determined. Results from this study will not be available for several years, but when they become available, they may resolve many questions about the effect of diet on risk of cancer recurrence.
There are other ideas to consider which may help you better understand the meaning of nutrition and cancer studies. In order to determine whether something is cause and effect, think about the strength of the association, the consistency of available research, and the biology of the relationship. What does this mean?
The strength of an association refers to how much a given factor will increase or decrease disease risk. For example, the association between smoking and lung cancer is very strong. There is a 14-fold increased death rate from lung cancer among smokers of a pack a day or more when compared with non-smokers.3 On the other hand, the association between most dietary factors and cancer risk is much smaller. Often, individuals with a given dietary exposure, for example, low intake of fruits and vegetables, will have only a 1.5 to 2-fold increased risk of a given cancer when compared with individuals who don't have this exposure. The weaker risk does not indicate an absence of cause and effect. However, it is often much more difficult to dismiss an association as cause and effect if the associated risk is very large.
The consistency of available research refers to the amount of agreement among existing studies. For example, while the protective effect of fruit and vegetable consumption against cancer is of a smaller magnitude, or strength, than the association between smoking and lung cancer risk, this finding is reasonably consistent among many different studies and in many different populations. This indicates that likely, these foods do offer some protection against various cancers.
The biology of a relationship refers to the fact that if you consider a potential cause and effect relationship, it must make sense based on what is known about the disease and how it develops. Understanding the process of how cancer develops, we know that fruits and vegetables contain many substances that appear to be protective against cancer by intervening in this process at many phases. Research has demonstrated that many phytonutrients (naturally occurring plant compounds) may protect against initiation, promotion, and progression, the three major phases of cancer cell development. Therefore, a finding that fruits and vegetables are protective against cancer fits with what is known about the disease process. Certainly, having a study finding that meets all of these criteria does not guarantee cause and effect. However, if you begin to apply these principles when you think about research, you will have a better understanding of whether a given association is truly cause and effect. This will help you determine how you might use this information to help yourself.
Publication Date: 1/2/2002
An example of a study that did not satisfy these principles, but nonetheless was reported as definitive, is the finding that fruit and vegetable intake does not reduce risk of breast cancer.4 This study compared women who consumed approximately 1 serving of fruits and vegetables daily with women who consumed 2 to 3 servings daily and did not see a difference in the risk of breast cancer between these two groups. This study is not consistent with the preponderance of existing evidence demonstrating that these foods do offer protective effects against many cancers. It is inconsistent with much of available research on the topic. Also, these findings appear to fly in the face of what is known about the biology of the relationship between fruit and vegetable consumption and breast cancer risk.
What does this mean for you? It is important to know that most oncology dietitians do not recommend long term consumption of 2 to 3 servings of fruits and vegetables to protect against cancer and/or cancer recurrence, but rather recommend at least 7 to 9 servings daily, and often even more. The participants in this study certainly didn't even reach the minimum number of these foods to demonstrate a beneficial effect. Remember, if you are in cancer treatment, these recommendations may not be appropriate for you. Consult a qualified oncology dietitian to determine the best diet for you before, during, and after cancer treatment.
With all of these concepts in mind, the next time you hear about nutrition research in the news, think about these results in the context of all of the research that has come before it.
If you would like reliable information about the best type of diet for reducing risk of cancer, consult the American Institute for Cancer Research (AICR).5 This organization provides scientifically sound recommendations on diet and cancer. Pamphlets with tips, recipes, sample menus, serving sizes, and other useful information are available from AICR.
Suzanne Dixon, MPH, MS, RD
University of Michigan Comprehensive Cancer Center
Director of Outpatient Oncology Nutrition Services
Oncology Nutrition Specialist & Epidemiologist
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Works Cited:
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1. Armstrong BK, Doll R. Environmental factors and cancer incidence and mortality in different countries, with special reference to dietary practices. International Journal of Cancer, 1975;15:617.
2. For information on the WHEL study:
http://www-epm.ucdavis.edu/www/Projects/WHEL.htm
http://www.aicr.org/r083100e.htm
3.Doll R and Peto R. Mortality in relation to smoking: Twenty years' observations on male British doctors. British Medical Journal, 1976;2:1525.
4.Smith-Warner SA, Spiegelman D, Yaun SS, Adami HO, Beeson WL, van den Brandt PA, Folsom AR, Fraser GE, Freudenheim JL, Goldbohm RA, Graham S, Miller AB, Potter JD, Rohan TE, Speizer FE, Toniolo P, Willett WC, Wolk A, Zeleniuch-Jacquotte A, Hunter DJ. Intake of fruits and vegetables and risk of breast cancer: a pooled analysis of cohort studies. Journal of the American Medical Association, 2001;285:769-776.
5.To contact AICR:
http://www.aicr.org/aicr.htm
1-800-843-8114
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