(CANCER RESEARCH ISUPPL I 53. 2442s-2445s. May 15. 1993]

Diet, Nutrition, and Cancer: Development of Hypotheses and Their Evaluation inAnimal Studies1

Henry J. Thompson2

AMC Cancer Research Center, Lakenootl, Colorado 80214

Abstract

Research in diet, nutrition, and cancer is multidisciplinar)'. As a result

there are many factors to consider in the conceptualization and design ofexperiments that may not be readily apparent to someone entering thisfield. In this article issues are identified that should be considered in thedevelopment of hypotheses about the effects of diet and nutrition on thecarcinogenic process and the evaluation of these hypotheses using animalmodels. The questions considered are: what comprises an acceptable rationale for diet, nutrition, and cancer experiments in which animal modelsare used; what factors should be considered in the design of these experiments; and what constitutes a mechanistic component in such animalstudies.

Introduction

During the last decade there has been a progressive shift in theemphasis in cancer control research toward prevention (1-3). From

this has emerged an effort to identify possible intervention strategiesthat will reduce cancer prevalence. Interventions defined thus far areprimarily behavioral, chemical, and medical in nature (1-6). Examples of each are, respectively, smoking cessation campaigns, chemo-

prevention. and participation in early detection programs, e.g.. Papsmear and mammography. Potential benefits attributable to these measures are considerable and portend the impact that prevention effortsmay have in reducing cancer prevalence and mortality. Successes inthis direction necessarily prompt the question of what else can bedone.

In 1981 Doll and Peto (7) estimated that dietary modifications couldresult in a 35% reduction of fatal cancers. Despite the uncertainty thatwas noted in making this estimate, their analyses and others kindledinterest in characterizing critical relationships among diet, nutrition,and cancer (7-9). While considerable progress has been made, theresearch now necessary to advance this field is increasingly multidis-

ciplinary. As a result investigators with varied backgrounds must dealwith issues with which they have little or no familiarity, e.g., dietformulation and tumor induction. The purpose of this article is toidentify factors that should be considered in the development of research projects in which animal models are used to investigate diet,nutrition, and cancer hypotheses. Three questions will be addressed:what comprises an acceptable rationale for experiments in whichanimal models are used; what factors should be considered in thedesign of these experiments; and what constitutes a mechanistic component in such studies.

What Constitutes an Acceptable Rationale for Experiments inWhich Animal Models Are Used to Study the Role of Diet andNutrition in Cancer?

In order to answer this question, it is important to establish thesequence of events that are typical of laboratory research in diet,nutrition, and cancer. In general, epidemiological data, in concert withclinical observations, give rise to hypotheses about potentially important diet and cancer relationships. However, these observations arecorrelational in nature and a means is required to determine if a causal

1Presented al the American Cancer Society Research Workshop on Cancer and Nu

trition. July 13 and 14. 1992. Atlanta. GA.2 To whom requests for reprints should he addressed.

relationship exists. While prospective clinical trials could be designedfor this purpose, they are expensive and take years to complete.Moreover, there are limitations on the number of subjects available forsuch studies and it would be considered inappropriate to test certaindiet-cancer associations based on only correlational data. In compar

ison, animal studies provide a direct means by which to determine thecasual nature of an apparent relationship in a timely and cost-effective

manner. This is achieved through the manipulation of key dietaryvariables in a defined tumor model system. Animal experimentationprovides the investigator with a level of control and ability to manipulate critical variables not possible in clinical trials. If a causal relationship is established in an animal experiment, the finding can beevaluated for its potential clinical merit and/or investigated further todetermine the basis of causality at the cellular and molecular level.Frequently these experiments give rise to additional hypotheses abouthow to alter diet to reduce disease occurrence which again must bevalidated in animal studies. From such animal experiments a determination is made of whether a particular hypothesis merits clinicalevaluation. This brief overview identifies the central role animal studies play in evaluating both hypotheses based on epidemiological andclinical data as well as those resulting from laboratory investigations.Animal studies that are designed for these purposes and intended toestablish causality or validate a cause and effect relationship in vivothat is implied by in vitro data are in general considered to be welljustified. However, there are other factors that determine whether ornot the rationale for an animal experiment is compelling.

As suggested by the discussion above, it is essential that diet,nutrition, and cancer research be hypothesis driven. Since the rationalefor conducting animal experiments will be derived from the hypothesis being tested, the acceptability of the rationale directly relates tothe merit of the hypothesis. This prompts the question of what constitutes a meritorious hypothesis. A list of the sources of data thatgenerally form the basis of diet, nutrition, and cancer hypotheses isshown in Table 1. It can be argued that the strongest hypothesis willbe one for which there is a solid base of support from the sourceslisted. Of these sources, the greatest importance is generally attributedto hypotheses that are based on epidemiological data and clinicalobservations because unlike hypotheses derived from other datasources, the likelihood of human applicability is inherent in suchhypotheses. In developing a hypothesis, it is often found that epidemiological data is contradictory or that certain aspects of the results ofanimal experiments or other laboratory data are not consistent with thehypothesis. In such cases, both the strengths and limitations of all datashould be critically evaluated and a determination should be madeabout whether there is sufficient support to warrant evaluation of thehypothesis. It is common for many hypotheses to be discarded as aresult of this process. Despite inconsistencies among sources of data,certain high potential hypotheses will emerge. For these it is highlydesirable that preliminary data be obtained. Almost without exception,preliminary data demonstrating the feasibility of evaluating a hypothesis in an appropriate animal model is essential in building a strongrationale in support of studies designed to evaluate that hypothesis. Athird factor that determines the strength that a hypothesis is accordedis its potential applicability and relevance to human health issues.There is a high expectation for direct human applicability of results in

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Table 1 Development of hypothèses about relationships among diet, nutrition, and cancer

Source of data Strengths Limitations Comment

Human studiesEpidemiological data/clinical ob

servations

Animal studiesCarcinogen-induced and spontane

ously occurring tumorigenesisdata

In vitro studiesCell and organ culture systems/

morphological, biochemical, andmolecular observations

Population-based source of hypotheses

Human applicability

Associations identified but causality not More weight typically attributed to thisdeterminable source

Data often imprecise and subject to biases

Variables controlled; direct test of causal- Extent to which the data can be extrapo- If this source of data is used, the issue of

ity lated to humans human applicability should be addressed

Simple, well defined models; direct tests Applicability of observation to more com- While powerful in establishing causality,of causality at cellular and molecular plex levels of organization, e.g.. tissue, appropriate caution required in extrapo-level organ, organism lating to higher systems and disease pro

cess

the diet, nutrition, and cancer field. Because of this, it is insufficientto simply obtain knowledge about the effect of a dietary factor on thecarcinogenic process without evidence that such knowledge has potential applicability. While exceptions to this viewpoint exist, thesense of urgency and the need for perceived human applicability tothis area should not be underemphasized.

In summary, it can be argued that a well rationalized experimentalplan in which animal models are used is one designed to test ahypothesis for which there is a strong base of support from categorieslisted in Table 1; for which preliminary data have been obtained whichindicate that evaluation of the hypothesis is feasible in an appropriateanimal model; and for which there is a clear-cut case for human

relevance and in which the dietary factor could have a significantimpact on disease outcome. Unfortunately the number of hypothesesfor which all these conditions will be met is small. This situationmakes it paramount for investigators developing hypotheses aboutspecific dietary factors and cancer to discriminate among the manypossible areas of inquiry that could be pursued and select for studythose hypotheses with the strongest support and to propose experiments designed to investigate causal relationships.

What Factors Should Be Considered in the Design of AnimalStudies for Evaluating Diet, Nutrition, and Cancer Hypotheses?

While there are many facets of experimental design that need to beaddressed in developing a research plan to test diet, nutrition, andcancer hypotheses, three elements of any protocol are particularlycritical. They are shown in Table 2 and discussed below.

Selection of Diet. The most typical dietary approach that is encountered in the nutrition and cancer literature is that in which a singleagent is added to either an unrefined or a purified diet formulation.The rationale for this approach is based on the objective of attributingcausality to a specific dietary factor and thus single agent dietaryinterventions best fit this objective. If this approach is used, an initialdecision that needs to be made is whether an unrefined diet or apurified diet is most appropriate for the hypotheses being tested (Table

Table 2 Elements of design to be considered in planning an animal experiment

Selection of dietFormulationFeeding

Selection of animal modelSpecies/strainCarcinogenBiological and molecular characteristics

Experimental designTreatment group configurationAnimal AssignmentEndpoints

3). While there may be justifiable reasons for using an unrefined diet,the use of a purified diet formulation is generally recommended because it affords the investigator complete control over both the qualityof the diet and the ability to manipulate its composition ( 10). When apurified diet formulation is judged to be appropriate for the hypothesisunder investigation, a strong rationale exists for considering the use ofa standard reference diet formulation, especially when an investigatordoes not have expertise in dietary methodology (II). The reasons thatthe selection of a standard reference diet should be considered include:many formulations that are commercially available fail to meet nutrient requirements of the species for which it was intended; nutrientrequirement data for a particular species are difficult to translate intoa nutritionally balanced dietary formulation; and the use of a commondietary formulation permits comparisons of results from differentlaboratories (10-12). The formulation of the diet most widely recommended as a standard for use in rodent studies is AIN-76A (10).

Because of its extensive use, there is a large base of carcinogenesisdata in which it has been fed. Nonetheless, the AIN-76A formulationwas not developed to be optimal for long-term studies which are

common in carcinogenesis research and. under certain conditions, thisformulation should be modified or an alternative formulation shouldbe selected (13). It is important to emphasize that the use of a "reference formulation" does not ensure appropriateness of a diet for the

evaluation of a specific hypothesis nor does the failure to use aspecific formulation necessarily constitute a critical flaw. What isessential is that the process used to select a diet be clearly defined andthat the formulation selected be the most appropriate one for thehypothesis under investigation.

Other issues related to diet formulation also must be considered.Included among these is the effect of the dietary modification underinvestigation on nutrient density of the diet ( 14). If a dietary modification, e.g., addition or deletion of a dietary component, results indiets that will be used in the same experiment and that will differ inenergy content, adjustments are required in the way the diets areformulated. Several approaches can be taken to resolve this problem,the most common of which is to maintain nutrient density/kcal as a

Table 3 Questions that should be considered in selection of a diet

Is the use of a purified diet appropriate to the working hypothesis? if not, why notand what is an appropriate alternative?

If a purified diet is appropriate, is the reference formulation designated AIN-76Asuitable? If not, why not and what is the suitable alternative?

Does the dietary manipulation(s) being used potentially alter the nutrient densityof the diet? If yes, how can the diets be formulated to avoid introduction ofuncontrolled variables?

Does the dietary treatment to be studied have the potential to alter the amount offood consumed? If yes. what controlled feeding technique is most appropriate1.'

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Table 4 Queutions that should be considered in selection of an animal model

Why is the use of an animal model required?

What are the strengths and limitations of the available animal tumor modelsrelative to the human disease process?

What are the advantages and disadvantages of a particular animal tumor model asit relates to the study hypothesis?

Are there alternative animal model systems that offer advantages that are lackingin the animal model that is being considered?

Table 5 Questions that should be considered in the design of an experiment

What are the endpoints to be measured?

What is the variability inherent in the measurement of the endpoint(s|?

What difference in the endpoint(s) is expected to be affected by the experimentaltreatment relative to the control?

With what statistical power is it desirable to detect changes in the endpoint(s)being measured?

constant among treatment groups except for the dietary factor(s) underinvestigation (15). A related question is what feeding methodologyshould be used when it is anticipated that a dietary modification willalter the food intake of animals. Under such circumstances, a controlled feeding procedure is required and the most widely describedmethod is referred to as pair-feeding. However, there are other meth

ods, e.g., restricted feeding which frequently may be more desirablefor controlling food intake in long-term studies. It is important to

emphasize that more than one approach to both nutrient density andcontrolled feeding is available. The key is that the approaches chosenare the most appropriate to the hypothesis being evaluated.

As noted above, single agent experiments are by far the mostcommon in nutrition and cancer research. However, there is a growingneed for other dietary approaches. Until recently the investigation ofdietary/nutrient interactions that may affect carcinogenesis has beenneglected (16). However, the nutrition literature contains numerousexamples documenting the importance of such interactions in affecting the health and well-being of an organism and work in this area

should be encouraged. In selecting suitable dietary formulations, thesame general principles referenced above underlie the formulation ofa nutritionally adequate diet in which several factors are varied. Whenpossible, existing paradigms for formulation should be considered foradaptation to cancer-specific questions. A major point to consider is

whether evidence exists to indicate that the dietary formulation that isproposed is suitable for the maintenance of animals in long-term

feeding studies.A concern of some investigators working in diet and cancer is that

the purified diets generally used in this field are not representative ofthe way people eat and that a more realistic approach to dietaryformulation is required in order to identify relationships that aredirectly applicable to human populations. While this subject can be thetopic of heated debate, it should be recognized that food-based dietary

formulations that reflect particular eating patterns have been used inother nutrition research areas (17). However, investigators interestedin such an approach must: (a) carefully define the hypothesis that will

be evaluated; (b) identify adequate dietary controls that ensure thevalidity of the experimental approach; and (c) develop methodologyby which the diets that will be used can be reproducibly formulated.Evidence that these diets will support the health and maintenance ofanimals in long-term feeding studies is also required. Because of theinability to specifically identify how various food-based diets differchemically, there are constraints on the extent to which specific cause-and-effect relationships can be investigated. Given that a major ad

vantage of using animal model systems is the ability to study causalrelationships directly, a compelling rationale must be presented for theuse of animal models in food-based research.

Selection of Animal Models for Cancer

Animal models for cancer provide a critical link in the continuumof research necessary to evaluating diet, nutrition, and cancer hypotheses because they permit the investigation of specific cause-and-effect

relationships during defined stages of the disease both prior to andafter the occurrence of clinically detectable lesions. This is an important advantage of animal studies since epidemiological observationson which many diet, nutrition, and cancer hypotheses are based provide only evidence of associations, and it is generally not possible toestablish when during the disease process a particular association ismanifest. For this reason and others, the best animal tumor model isone that closely parallels the biology of the human disease and inwhich the characteristics of the disease process are compatible withthe hypothesis being tested. Some of the questions that need to beaddressed in choosing an animal model are outlined in Table 4.

While numerous model systems exist for the study of cancer(s) atcertain sites, options are limited for other sites. All tumor models haveinherent advantages and shortcomings and, needless to say, perfectmodels of the human disease do not exist. While obviously it isimportant to emphasize that models need not be perfect to be useful,the question of how to overcome deficiencies of a particular model iscritical. Until recently, the principle response to this question wouldhave been to suggest that more than one model system for the cancerof interest be used to study a hypothesis and to choose models that arecomplementary. However, the recent development of new approachesto cancer induction, prominent among which are transgenic modelsand in vitro or in situ transfection/infection models using oncogene/tumor suppressor genes, has opened new avenues for investigators(18-19). The use of these systems should be encouraged when they

are appropriate to the study hypothesis since they afford the opportunity to address directly how dietary/nutritional factors affect thedevelopment of cancer induced by alterations in specific genes. However, these newer approaches should not be considered as substitutesfor other models in which chemical, viral, or physical agents are usedto initiate tumor development. While it is recognized that the newtechnologies now available offer the opportunity to gain insights notpreviously possible when these models are used in appropriate contexts, the perception that such approaches are better than other wellcharacterized tumor models must be questioned. Experience withthese new technologies is restricted and the inherent advantages andlimitations of these new approaches are only beginning to be defined.In this regard, reference is made to the work of Weinberg (19) in

Table 6 What constitutes a mechanistic component in animal studies

Preconceptions Questions

To be of high merit, an experimental plan must have a mechanisticcomponent, and to be cutting edge, research must involve molecularbiology and especially oncogenes/tumor suppressor genes.

To be "molecular" is of "higher merit" than to be "organismic."

What is meant by mechanistic research?

At what stage of hypothesis testing in animal studies is a cellular/molecular research component appropriate?

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which some of the limitations in the use of transgenic animals aredefined. While many issues can be debated about the usefulness ofanimal models in general and what models are most appropriate, twoprinciples invariably emerge from such discussions, that animal models play a central role in diet, nutrition, and cancer research; and thatthe nature of the hypothesis being tested should be the key determinant in selection of the most appropriate animal model(s).

Experimental Protocol

While the selection of the dietary formulation and tumor model fordiet and cancer studies is critical, equal importance should be accorded to the other aspects of designing an experiment (Table 5). Forexample, the arrangement of treatment groups within an experimentshould take into consideration the appropriateness of using more thanone dose of carcinogen and/or multiple concentrations of the dietaryagent being investigated. Another important question is whether theuse of time series comparisons among treatment groups is warranted.Once an experimental design is defined, powers calculations shouldbe used to determine the number of animals to assign per treatmentgroup. This necessarily requires consideration of the endpoints thatwill be measured and their sensitivity, the level of variation observedin the endpoint(s) among animals, and the magnitude of change in theendpoint(s) that it is desirable to detect. From these considerationsemerge not only the number of animals that should be assigned to eachtreatment group but also the plan for data tabulation and statisticalevaluation. It is important to recognize that failure to address suchissues at the outset of planning an experiment raises numerous questions, prominent among which is will the experimental design allow adefinitive test of the hypothesis under investigation given the nature ofthe data that will be obtained.

What Comprises a Mechanistic Component in Animal Studies?

Perhaps more than in other areas of investigation, there is a considerable danger in diet, nutrition, and cancer research to engage in theconduct of what is referred to as "descriptive" rather than "mechanistic" research. This is an important issue because scientific merit is

frequently equated with the extent to which a research plan has amechanistic component. Common perceptions about descriptive versus mechanistic research are summarized in Table 6.

There is a natural tendency to characterize research as descriptive ifit involves earlier stages of hypothesis testing in which animal experiments are proposed to establish that a relationship suggested byepidemiológica!, clinical, or laboratory data can be validated in anappropriate tumor model system. Later stages of testing the samehypothesis that are targeted at the cellular or molecular level aregenerally defined as mechanistic. However, such arbitrary definitionsof descriptive versus mechanistic research obscure the critical question. At issue is whether the use of the word "mechanistic" properly

refers to whether cellular or molecular approaches are used in aresearch plan or to the extent to which experiments are designed toestablish causality. It is argued that experiments designed to establishcausality are mechanistic and the level at which causality is addressedneeds to be consistent with what is understood about the hypothesisbeing evaluated. The point is that animal carcinogenesis studies designed to establish a cause-and-effect relationship between a dietary

factor and cancer outcome are mechanistic as are studies targeted atthe cellular or molecular level that are designed to establish the basis

of causality of an effect(s) observed in a carcinogenesis experiment.Conversely, studies at the animal, cellular, or molecular level that arenot designed to investigate causality are descriptive irrespective of thetypes of techniques that are used. Thus, a goal for investigators working in this area is to develop experiments that investigate causalityrather than describe phenomena and to advance to the cellular andmolecular level of causality as is feasible.

Conclusion

Experiments in which animal models are used play a key role in theevaluation of diet, nutrition, and cancer hypotheses. The developmentof well rationalized experiments requires that they be hypothesisdriven and have a good likelihood of applicability to the humandisease. This is considered essential to the continued progress of diet,nutrition, and cancer research. Because of the nature of the variablesstudied and the complexity of dietary and nutrient interactions, it iscritical that investigators working in this field be reminded of theimportance of investigating causal relationships in the experimentsthey design and that they be encouraged to use, to the extent that ispossible, cellular and molecular approaches that enhance the opportunity to determine causality directly and with the greatest precisionpossible.

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