| Nutrition &
Cancer Research: Sorting Through the Confusion |
Dixon, MPH, MS, RD, Suzanne
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.
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.
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| Works
Cited: |
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:
1-619-534-4977
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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