Caloric intake, body weight, and cancer: a review

Analysis of National Toxicology Program rodent bioassay data for anticarcinogenic effects

We reanalyzed data from 218 two-year rodent carcinogenicity studies carried out by the National Toxicology Program (NTP). These data were originally collected for the purpose of identifying potential human carcinogens. However, the objective of our analysis was to investigate the frequency of possible anticarcinogenic effects in these data, since recurring cases of chemical-associated tumor reductions have been noted in the course of these studies over time. Our analysis reveals that most (>90%) NTP-tested chemicals show at least one statistically significant (p<0.05) decrease in site-specific tumor incidence. Because of the large number of statistical comparisons made in a long-term bioassay, random variability can account for many of these tumor decreases. However, we found that certain tumors (predominantly those with a high spontaneous incidence) show chemically related decreases far more frequently than chance expectation. Many of these decreases, particularly those for pituitary and mammary gland tumors, adrenal pheochromocytoma and uterine polyps in rats and liver and lung tumors in mice, are associated with the reduced body weights frequently observed in the dosed groups. The chemically related decreased incidences of leukemia in rats appear to be related to spleen damage, i.e., chemically related splenic toxicity is evident for most chemicals showing decreased incidences of leukemia. While random variability, associations with body weight and splenic toxicity can account for most of the decreased tumor incidences observed in NTP studies, there are other tumor decreases that could not be totally explained by these factors. Further investigations of possible mechanisms of action are underway. These data are relevant to the concept of chemoprevention as well as to the task of using long-term laboratory animal studies to predict enhanced human environmental-cancer risk for regulatory purposes.

The sensitivity of the NTP bioassay for carcinogen hazard evaluation can be modulated by dietary restriction

Studies were undertaken to compare outcomes when four chemicals were evaluated under typical NTP bioassay conditions as well as by protocols employing dietary restriction. Four chemicals, using three different routes of exposure (in utero [accomplished by feeding the dam dosed feed], dosed feed, and gavage) were used to 1) evaluate the effect of diet restriction on the sensitivity of the bioassay toward chemically-induced chronic toxicity and carcinogenicity; and 2) evaluate the effect of weight-matched control groups on the sensitivity of the bioassays. Control and chemical exposed F344 rats and B6C3F1 mice (50-60/group) were fed NIH-07 diet either ad libitum or at restricted levels such that body weights were approximately 80% of ad libitum control weights. The dietary restricted groups were either sacrificed at the end of two or 3-years. Results consistently show that feed restriction decreased the incidence of neoplastic and non-neoplastic lesions at a variety of anatomic sites in both control and chemical exposed animals. Furthermore, the sensitivity of the bioassay to detect chemical carcinogenic response were altered by dietary restriction: three of the four chemicals were found to increase the incidence of neoplastic lesions at four sites when evaluated under standard ad libitum conditions for 104 weeks. When unexposed and exposed groups were both subjected to dietary restriction, none of these 4 sites were detected as a target for carcinogenesis after two or three years. Rather, two different sites of carcinogenesis were detected. When the top dosed ad libitum fed animals were compared against their weight-matched control groups, a total of 10 sites were identified as targets for carcinogenesis. These included all four sites identified under the ad libitum protocol, both sites identified under the feed restricted protocol, and an additional four sites that were not identified under the other two protocols. These studies show that dietary restriction of all animals can be expected of decrease the sensitivity of carcinogenesis bioassays. However, restricting only unexposed groups (weight matching) of control for non-specific weight loss in chemical exposed groups yielded the most sensitivity among our comparisons.

Biotechnology and potential nutritional implications for children

The tools of biotechnology have enormous potential to develop new, safe, and nutritious foods and food products that could benefit the immediate and long-term nutritional and health needs of the pediatric population. This is especially true as more emphasis is placed on the prevention, rather than the treatment, of chronic degenerative and metabolic diseases. But the promise of biotechnology for nutritional and health benefits of children's diets must be accepted with cautious optimism. Many advances in food technology and nutritional composition of food animals and plants have already been made through biotechnology, but they represent only the beginning of the necessary research. These advances have been based on relatively little knowledge of basic human nutritional needs, particularly during the dynamic pediatric period of growth and development. More importantly, these advances have been predicated with no understanding of dietary nutrient interactions. Changing nutrient composition of foods through biotechnology may alter nutrient interactions, nutrient-gene interactions, nutrient bioavailability, nutrient potency, and nutrient metabolism. Biotechnology has the potential to produce changes in our foods and in our diet at a pace far greater than our ability to predict the significance of those changes on pediatric nutrition. The Human Genome Project, which relies on biotechnology, will revolutionize science and medicine. Pediatrics will be one of the first medical specialties to benefit from the outcome of this project as recombinant DNA manipulations will replace diet therapies for treating metabolic diseases. Somatic gene therapy eventually may be the ideal means for diagnosis, treatment, and cure of inherited diseases and metabolic disorders; however, many problems exist, especially in situations in which nutrients are involved in the complex regulation of gene expression. DNA and genes themselves do not determine the fate of an individual. The genetic material provides the potential for the individual, but this potential can be modified by environmental factors. The interaction of nutrients with genes is a major determinant in the final outcome of the individual. Biotechnology promises children a more productive and better quality of life, but achieving the full potential of this promise demands a continued diligent search for knowledge of nutrition and nutrient-gene interactions.

Diet and breast cancer: trial and error?

The risk of developing breast cancer in females is determined by characteristics of hormone- and pregnancy-related tissue growth and development and mechanistic interactions between carcinogens and molecular targets and clonal selection of cell lines. Animal models have substantiated the idea that breast tissue may have periods of increased sensitivity to carcinogens and that dietary fat, and particularly energy intake, affect cancer risk. In epidemiological research, ecological studies clearly demonstrated positive associations between dietary fat and breast cancer risk, but individual-based case-control studies and cohort studies did not support a causal interpretation. The debate is now whether the ecological fallacy or biases in analytical epidemiology account for the apparent discrepancy, and the character of the debate is mainly of a methodological nature. Ecological and analytical studies, however, are fundamentally different with respect to the time- and age-axes considered. The lack of association in analytical studies addressing late-stage carcinogenesis may be attributed to exposures being relevant earlier in life. Analytical studies that integrate insights relating to periods of increased tissue susceptibility and mechanistic models of carcinogenic action may be fruitful in producing new outlooks for breast cancer prevention.

Interrelationship between dietary lipids and calories and experimental mammary gland tumorigenesis

The purpose of this communication was to review and critique the studies designed to examine the interrelationship between dietary fat and calories in experimental rodent mammary gland tumorigenesis. The results of these studies clearly show that hyperalimentation of fat, either saturated or unsaturated, significantly stimulates this tumorigenic process. This has been demonstrated in an impressive array of carcinogen-induced, transplantable, spontaneous, and metastatic experimental rodent mammary gland tumor systems. The stimulatory effect of high levels of dietary fat appears to act primarily at the promotional stage of this tumorigenic process. Whether the mammary tumor stimulatory effect of high levels of dietary fat is a result of the metabolic activity of the fat per se or is due to an excessive energy (caloric) intake has been examined. Data obtained from the experimental studies that address this issue support the viewpoint that the mammary tumorigenic-enhancing activities of a high fat diet is, at least in part, through a caloric mechanism.

Diet and adenomatous polyp risk

Studies of risk factors for colonic adenomatous polyps have been reported in greater numbers in 1993 than in all previous years combined. This explosion in interest in polyps has not arisen because polyps themselves cause serious illness. They are in the vast majority of cases asymptomatic. However, adenomas have become the surrogate for colon cancer in a number of phase III dietary intervention trials. These trials were undertaken at a time when very little was known of adenoma risk factors. Data accumulated in the past 18 months in general demonstrate a similarity in risk factors for cancer and polyp. Since it has also been recently established that polypectomy diminishes colon cancer risk, the adenomatous polyp has been established as an ethical and convenient surrogate for cancer of the colon. Prevention of colorectal cancer is the goal of all the above studies, and it is hoped that the dietary intervention trials currently under way will generate the data that will make prevention possible.

A model to increase consumption of fruit and vegetables by implementing the "5-A-Day" initiative

The National Cancer Institute, in conjunction with the Produce for Better Health Foundation, launched a 5-A-Day initiative to increase fruit and vegetable consumption. State departments of health in 24 states were licensed to participate to implement behavior change interventions at the community level aimed at both adults and children. This article describes how schools could use the eight components of the expanded school health program to initiate a 5-A-Day program, and why schools need to participate in the 5-A-Day initiative. Specific strategies to implement in each component of the comprehensive school health program are described.

Modulation of oxidative DNA damage levels by dietary fat and calories

Decreased dietary intake of fat and/or calories generally results in a lower incidence of mammary gland tumors in rodents. Feeding of either low-fat or calorie-restricted diets to rats also has been shown to result in decreased levels of oxidative DNA damage. Since oxidative DNA damage is suggested to have a role in carcinogenesis, this may be one mechanism by which dietary change can reduce cancer risk. The effects of calorie-restricted diets on both oxidative DNA damage levels and mammary gland tumor incidence are generally more pronounced than that of low-fat diets. There is, however, some difficulty in defining what amount of fat should be used to prepare 'low-fat' and 'high-fat' rodent diets as well as what a suitable fat intake for control diets should be in studies that examine the effects of dietary fat and/or calories on tumorigenesis. In particular, the promoting effects of dietary fat may be exerted only up to a certain level of fat, above which no further effect is observed. Another difficulty in the interpretation of the results is that there may be a time-dependent effect of high fat diets on oxidative damage, with increased damage resulting only when the diets are fed for longer periods of time. The appropriate experimental approach to model human dietary exposures therefore remains to be determined. Although the effects of caloric intake on mammary gland tumorigenesis appear to be more pronounced than that of fat intake, low-fat diets still may be useful as a preventive measure in human populations to reduce breast cancer risk for individuals who cannot safely reduce their caloric intake.

Dietary guidelines. The rationale for intervention

The recommendations of most dietary guidelines include maintenance of desirable weight; eating a varied diet, including fruits, vegetables, and fiber; and reducing fat intake. After reviewing the influences of dietary components on carcinogenesis, the best current dietary advice would appear to be moderation and variety.

Dietary restriction reduces the incidence of NMU-induced mammary tumors and alters retinoid tissue concentrations in rats

Previous studies suggested a relationship between dietary restriction (DR) effects on mammary carcinogenesis and DR effects on liver retinoids. Therefore, in this study, retinoid concentrations were measured by high-performance liquid chromatography in the plasma, liver, and peripheral organs of DR rats with chemically induced carcinogenesis. Rats were injected with N-methyl-N-nitrosourea (MNU) and maintained on graded levels of DR (reduction of 10-40% from energy ingested by control animals with free access to food). Mammary tumor incidence and multiplicity induced by MNU were reduced in relation to the degree of DR, with virtual prevention occurring at 30% and 40% DR. Total hepatic retinoid concentrations (retinol + retinyl esters) were significantly greater in rats given MNU and subjected to DR, but liver total retinoid content was comparable between the groups. However, plasma retinol concentrations were significantly lower in DR rats than in controls given the carcinogen without DR. Retinoid concentrations were also elevated in adipose tissue, lungs, and intestine of DR rats, while renal concentrations remained unaltered. Retinoid concentrations in mammary glands and mammary tumors were similar in all groups. Thus, in DR rats, vitamin A concentrations in liver and other target tissues are maintained or increased despite decreases in plasma. It remains to be investigated whether these alterations in retinoid content have any relationship to the cancer-preventive effect of DR.

Relation of body size and the risk of lung cancer

Two population-based case-control studies of lung cancer were conducted on the island of Oahu, HI, between 1979 and 1985. Data from these studies were combined to form the basis of this analysis. Interviews were obtained from 518 men and 230 women with lung cancer and 1,102 male and 524 female controls frequency matched to the cases by sex and five-year age group. The interview consisted of a complete tobacco history, information on body size, and other demographic and life-style information. Weight and Quetelet index (kg/m2) 5 years before diagnosis, but not at 20-29 years of age, were inversely related to the risk of lung cancer among men and women. Cases tended to gain less weight during adulthood than did controls. These associations persisted after adjustment for age, ethnicity, tobacco smoking history, and beta-carotene intake. Our results are consistent with reports by several other investigators of an inverse association between body weight and the risk of lung cancer. However, we were unable to rule out the possibility of bias in our findings due to preclinical disease.

The association of body size, reproductive factors and thyroid cancer

A population-based case-control study of the association of diet and other factors and thyroid cancer was conducted between 1980 and 1987 on Oahu, Hawaii. Study participants included 51 men and 140 women with thyroid cancer, and 113 male and 328 female controls matched on age (+/- 5 years) and sex. A significant, positive monotonic dose-response relation of weight in late adulthood (5 years prior to interview) and the risk for thyroid cancer was found for men and women. A greater than five-fold increase in the risk for thyroid cancer among men, and more than a two-fold increase in risk among women, was found for subjects in the highest compared with the lowest quartile of weight in late adulthood. Height was significantly related to the risk for thyroid cancer among men, but not women. Among men, there was a significant dose-response relation of weight in early adulthood (20-29 years of age) and the odds ratios (ORs) for thyroid cancer, although the trend was not significant after adjustment for height. Among women, there was also a positive relation of adult weight gain and thyroid cancer, with an OR of 2.6 associated with more than a 14% increase in weight. The effects of relative weight and weight gain on thyroid cancer risk were stronger in post-menopausal women than in premenopausal women. There was a significant positive interaction between fertility drug use and early adult weight and the risk for thyroid cancer in women. Odds ratios were also significantly elevated for women above the median weight in early adulthood who experienced a miscarriage or stillbirth at first pregnancy. In summary, these data show an association of weight, particularly in late adulthood, and the risk for thyroid cancer in men and women, and further suggest a positive interaction between weight in young adulthood and fertility drug use on thyroid carcinogenesis in women.

The effect of vagotomy and pyloroplasty on colorectal tumor induction in the rat

Epidemiologic studies have suggested that vagotomy increases subsequent colorectal cancer risk. This hypothesis was investigated in the rat 1,2 dimethylhydrazine (DMH) colorectal carcinogenesis model. Eighty-five rats were divided into four groups having either truncal vagotomy and Heineke-Mickulicz pyloroplasty, pyloroplasty alone, laparotomy alone, and anesthesia alone. After recovery from the procedures, colon tumors were induced with five injections of DMH. Results of carcinogenesis show a trend towards increased incidence and yield of colorectal and duodenal tumors after vagotomy, though this was not statistically significant, perhaps because the high postoperative mortality from vagotomy diminished the power of the study.

Effect of caloric restriction on age-associated cancers

Caloric restriction (CR) without malnutrition in mice and rats reduces the incidence of spontaneous tumors and delays their appearance while increasing maximum life span. These results depend largely on CR per se, and not on low intakes of fat or other nutrients. Although most studies have tested CR imposed early in life, CR started in midadulthood also retards cancer and aging. The way(s) by which CR impedes cancers remain unclear, but possibilities include less cellular oxidative damage, retarded immunologic aging, hormonal changes, less energy available for cell proliferation, reduced exposure to dietary carcinogens and promoters, enhanced DNA repair, and less carcinogen activation. Far less is known about the relationship between caloric intake and cancer incidence in humans; however, recent findings suggest a positive association for certain cancers.

Comparison of two physical activity questionnaires, with a diary, for assessing physical activity in an elderly population

Measurement of physical activity in epidemiological studies is usually achieved by means of a questionnaire. Little work has been done to determine which questionnaire format has greater validity in an elderly population. In this study of elderly subjects, physical activity as reported in two self-administered questionnaires (A and B), which differed in format and length, were compared to activity reported in a 4 day diary. As compared with the diary, moderate/heavy activity was more accurately reported in Questionnaire A (mean difference 5 min), the longer more detailed questionnaire, than B (mean difference 170 min). Light activity was under reported in Questionnaire A (mean difference 68 min) and over reported in B (mean difference 88 min) as compared with the diary. In contrast, time spent sitting was more accurately reported in Questionnaire B (mean difference 40 min) than in A (mean difference 230 min) as compared with the diary. The longer more detailed questionnaire was the more accurate instrument for assessing moderate/heavy activity in this elderly population. The shorter questionnaire was more accurate for assessing time spent sitting.

Medical implications of obesity. Losing pounds, gaining years

Through thousands of years of starvation and poor nutrition, the human body has become adept at storing scarce nutrients. Today, in the United States and Canada particularly, the combination of sedentary habits and excessive intake of calories is imposing a dual burden on a physiologic system that is ill-equipped to handle it. Unable to rid itself of calories, the body's only defense is to store them all. The end result is obesity, with all its deleterious effects on health and longevity.

Listen to nature. The challenge of lifestyle medicine

Unlike the days of Robert Koch when infectious diseases were the principal contributors to morbidity and mortality, today's illness and death are most often caused by noncommunicable diseases that have the special characteristics of resulting largely from one's own lifestyle, especially tobacco use and nutritional excesses. By "listening to nature", we can detect and identify risk factors for various types of cancer, explore their mechanisms of action, and execute preventive strategies leading to their reduction or modification, thereby, decreasing the incidence and mortality of disease. An example of the role of metabolic overload in carcinogenesis is the impact of an excessive intake of dietary fat on the development and progression of breast cancer. For the general pathogenesis of cancer, the risks associated with metabolic overloads are contrasted with those of low-level exposures. To broaden the impact of preventive medicine beyond factorial nutritional education in Germany, we recommend that (a) every medical school have a department of preventive medicine with emphasis on epidemiology and health promotion, and (b) all schools beginning in first grade have a comprehensive school health education program coordinated by a full-time health education teacher.

Effect of dietary calorie and fat restriction on mammary tumor growth and hepatic as well as tumor glutathione in rats

Effect of dietary calorie restriction and fat reduction on growth of established mammary carcinoma in rats and on glutathione levels in liver and tumor tissue was investigated. Reduced (GSH) and oxidized (GSSG) glutathione were determined enzymatically. Female Sprague-Dawley rats were injected with 25 mg/kg methylnitrosourea (MNU) on day 50 of life for tumor induction, and subsequently fed a diet containing 50 kcal/day with 45% (energy %) fat. When tumors reached approximately 1 cm3, the diet was changed for 10 +/- 2 weeks. Four dietary groups were formed: two high calorie groups (50 kcal/day) with 45% or 25% fat and two calorie restricted groups (35 kcal/day) with 45% or 25% fat, respectively. Tumor growth was significantly inhibited by the 30% calorie restriction, and the inhibition was most effective in the calorie restricted group with low fat level. However, reduction of fat, alone, had no significant inhibitory effect. GSSG levels in both liver and tumor showed no differences among the groups. Hepatic GSH levels tended to be lower in the calorie-restricted groups, and showed no difference between isocaloric groups with different fat levels. In contrast, GSH in tumor tissue tended to be lower in the low fat groups, independently of calorie levels.

Relation of body size to prognosis in lung cancer patients

A survival analysis was conducted among a sample of 463 men and 212 women with histologically confirmed lung cancer. Interview information from these patients was obtained from two population-based case-control studies of lung cancer conducted on the island of Oahu, Hawaii, between 1979 and 1985. The interview consisted of a complete tobacco history, and other demographic and lifestyle information. Records from the Hawaii Tumor Registry were reviewed for data on stage, histology, and follow-up status of these patients. Height, weight in early adulthood, and weight five years prior to diagnosis were inversely related to survival among women, after adjustment for age at diagnosis, stage, histology, ethnicity, and smoking status. No relationship of these variables to survival was found among men. The association of body size and the relative risk of death among women was strongest for patients with small cell cancer, although the association was not restricted to this cell type. The covariate-adjusted median survival times for female never-smokers below the 50th percentile of weight in early- and late-adulthood was at least twice as long as that for any other group (46 cf 17-23 months). There was no evidence for an interaction between weight in early adulthood and weight five years prior to diagnosis on the risk of death among women. Stage at diagnosis and education did not modify the association of the anthropometric variables and survival. These data suggest that weight and height may be associated with prognosis for women with lung cancer.

Nutrition and breast cancer

International comparisons have provided striking correlations between fat consumption and risk of breast cancer, but these comparisons do not often consider variations in life style. Case-control studies carried out in several countries showed no real association between fat intake and breast cancer. There is some evidence that vitamin A or carotenoid intake may exert a protective effect. Alcohol intake, on the other hand, seems to be positively associated with breast cancer risk. Elevated body weight, body mass, stature, and frame size have been found to be associated as risk factors for breast cancer in women. Animal studies found that caloric restriction inhibits growth of spontaneous and induced mammary tumors, an observation that held up even when the calorie-restricted animals ingest more fat than the ad-libitum-fed controls. College women who exercise have a lower incidence of breast cancer than their more sedentary classmates. Exercise is another means of reducing caloric availability.

Regulation of food-related carcinogens in Japan

Use of hazardous environmental substances in Japan is regulated, based on their risk assessment, by laws of various ministries, depending upon the chemicals concerned. Food additives and food contaminants are controlled by the Food Sanitation Law of the Ministry of Health and Welfare. When manufacture or import of new food additives is intended, their physicochemical and biological data are evaluated by the Food Sanitation Investigation Council. When new data become available on the carcinogenicity of food additives that have been approved and are in use, these compounds are reevaluated. Between 1974 and 1986, twelve chemicals were proved to be carcinogenic in Japan by task groups for evaluation of the carcinogenicity of food additives, medical drugs and pesticides, which were organized by the Ministry of Health and Welfare. Of these compounds, 2-(2-furyl)-3-(5-nitro-2-furyl)acrylamide was banned from use, but use of hydrogen peroxide and potassium bromate was permitted, provided that these compounds were decomposed or removed from the final products. The use of butylated hydroxyanisole within fixed limits in some foodstuffs was also permitted. Decisions on the use of these compounds were based on the prevailing opinion on carcinogens at the time. This paper reports these facts with a consideration of future problems on the regulation of food carcinogens.

International differences in body height and weight and their relationship to cancer incidence

The relationship between body size (adult height and weight) and cancer incidence was investigated in an international ecological study of 24 populations. Site-specific and total cancer incidence rates (age standardized) from 1973 to 1977 were correlated with body size data generally obtained between 1954 and 1974. All-sites cancer incidence was highly correlated with height among both men (r = 0.50; p less than or equal to 0.01) and women (r = 0.70; p less than or equal to 0.001). Among men, there were significant correlations between height and cancers of the central nervous system (r = 0.72), prostate (r = 0.66), bladder (r = 0.65), pancreas (r = 0.59), lung (r = 0.47), and colon (r = 0.46). Significant correlations were observed for cancers of the rectum (r = 0.76), pancreas (r = 0.75), ovary (r = 0.73), central nervous system (r = 0.68), breast (r = 0.65), uterine corpus (r = 0.50), and bladder (r = 0.48) in women. Adjustment for weight altered these correlations only minimally. Weight was significantly correlated to all-sites cancer only among women (r = 0.44; p less than 0.05), and site-specific correlations were significant for the same sites as for height, but the magnitude of the correlation coefficients was somewhat diminished. In addition, adjustment for height greatly reduced the correlations with weight. These findings support previously observed associations between height and specific cancers (e.g., breast and colon) and identify several additional cancer sites that may be similarly related.

A proxy approach to the determination of total caloric intake for use in cancer epidemiology

In the absence of complete dietary information on a cohort of 472 early-stage breast cancer patients a caloric prediction technique was developed. Deriving an accurate caloric denominator would enable examination of individual nutrient effects by controlling for potential confounding by calories. Surrogate measures of mean daily caloric intake were generated from estimates of basal metabolic rate (BMR) predicted from age, height, weight, and physical activity. A validation study was undertaken to test the relationship between the BMR proxy terms and mean daily energy intake obtained from four-day food diaries for 51 subjects. Pearson correlation coefficients between the diary-derived and predicted values of caloric intake were computed (r = 0.43, p = 0.001). The results are in the range of what many researchers found when comparing self-reports of total caloric intake in validation and reliability studies of various dietary assessment tools. Because of the large heterogeneity of results across the ages represented in this study (25-77 yrs), it is recommended that small validation studies of this type be carried out in the subpopulations of interest.

Health risks of obesity

Evidence implicating obesity as a risk-factor disease is critically reviewed. Possible reasons for the many conflicting findings are addressed. The classification of obesity, based upon the site of body fat distribution, and possible biologic mechanisms associating regional adiposity with morbidity, are discussed.

A comparison of dietary fish oil and corn oil in experimental colorectal carcinogenesis

Rats fed either fish oil (n = 16) or corn oil (n = 16) in calorically and nutritionally balanced diets were injected with 1,2-dimethylhydrazine, which is a colorectal specific carcinogen; differential colorectal tumor induction was then measured. In addition, plasma peroxide concentrations were measured in rats in each dietary group as well as in a group receiving a low-fat diet, either with or without prior carcinogen treatment (n = 3 for each of the 6 groups). Tumor incidence did not differ between groups fed corn oil and fish oil. Tumor yield in the left colon was significantly lower in rats fed fish oil (p = 0.0499). Total colorectal tumors induced were also fewer in the rats fed fish oil (p = 0.065). Plasma peroxide concentrations were difficult to interpret because of the wide variation within groups. The data on tumor yield in the left colon support the hypothesis that a diet rich in n-3 fatty acids, which are found in fish oil, may be less supportive of colorectal tumor development than a diet rich in n-6 fatty acids, which is found in corn oil. However, the mechanism by which fish oil decreases tumor induction is still unknown.