807
Dietary Fat and Colon Cancer: Animal Model Studies 1 Bandaru S. Reddy* Division of Nutritional Carcinogenesis, American Health Foundation, Valhalla, New York 10595
Since it was first suggested that high dietary fat is a risk factor in colon cancer, there have been several studies to test this hypothesis. Epidemiologic studies suggested a positive association between dietary fat and colon cancer. L a b o r a t o r y animal model studies demonstrated that n o t o n l y t h e a m o u n t of fat, b u t also types of fat differing in fatty acid composition are important determining factors in colon tumor development. Chemically-induced colon tumor incidence was increased in rats fed the semiparified diets containing 23% corn oil, safflower oil, lard or beef tallow (high-fat) as compared to those fed 5% corn oil, safflower oil, lard or beef tallow diets (low-fat). Diets containing 23% coconut oil, olive oil or fish oil, or high-fat diets containing varying levels of trans fat, had no colon tumor-enhancing effect compared to their respective l o w fat diets. The stage at which the effect of dietary fat is exerted appears to be mostly during the post-initiation phase of colon carcinogenesis. Lack of a c o l o n t u m o r enhancing effect of dietary fish oil is observed both during the initiation and postinitiation phases. The mechanisms by which various dietary fats increase colon carcinogenesis are n o t fully understood. In most instances, however, the high-fat diet appears to enhance tumorigenesis through elevation of agents, such as secondary bile acids, that act as promoters of tumor development. Lack of colon tumor promotion by dietary fish oil and trans fat appears to be mediated through their effect on mucosai ornithine deearboxylase activity, colonic secondary bile acids and/or prostaglandin synthesis. Lipids 27, 807-813 (1992). Cancer of the colon is one of the most common cancers in the western countries, including North America, exhibiting more than a tenfold excess when compared to rural populations in Asia, Africa and certain parts of South America. Studies of cancer incidence in high risk areas such as the United States, and low risk countries, such as Japan and Finland, indicate that rates are increasing faster in low than in high incidence areas of the world. It is the second leading cause of cancer deaths in the United States, with estimated 157,500 new cases and about 60,500 deaths in 1991 (1). Since Wynder et al. (2) and Burkitt (3) first suggested that dietary factors in general, and specifically dietary fat and fiber, might play a role in the etiology of colon cancer, a substantial amount of progress has been made in understanding the relationship between nutritional factors and the development of colon cancer in humans. Continuing
population studies revealed that the diets particularly high in total fat and low in certain dietary fibers are generally associated with an increased risk of developing colon cancer (4). In addition, dietary fat may be a risk factor in the absence of factors that are protective, such as use of high dietary fiber and fibrous foods (4). Several casecontrol studies suggest a positive association between dietary fats, especially saturated fat 15,6). A recent prospective study provided evidence that a high intake of animal fat, but not vegetable fat, increases the risk of colon cancer (7). Animal model studies performed in our laboratory and elsewhere have consistently supported human epidemiologic studies and further provided evidence that the colon tumor-promoting effect of dietary fat depends on the type of fat (8}. These studies also suggested that the main effect of dietary fat in colon carcinogenesis is during the postinitiation phase of carcinogenesis. Unquestionably, there are areas where more research is necessary and, of course, no single dietary factor and mechanism can account for all colon cancer. Since several reviews have already appeared on the relationship between dietary fat and colon carcinogenesis, this report will provide a brief overview of current knowledge as to the relationship between the types of dietary fat and colon carcinogenesis in laboratory animal models with emphasis on r fatty acids {fish oil}. The basis for the latter point is that epidemiologic studies have found that people who traditionally consume {either regularly or frequently} diets containing fish and marine animals have a lower incidence of several chronic diseases, including cancer, than those who do not or rarely consume such foods (9-11).
AMOUNT OF DIETARY FAT AND COLON CANCER
A variety of compounds, namely 1,2-dimethylhydrazine (DMH}, azoxymethane (AOM), methylazoxymethanol (MAM} acetate, 3,2'-dimethyl-4-aminobiphenyl tDMAB} and methylnitrosourea (MNU), that are carcinogenic for the colon have been used in a number of laboratory animal models to investigate the effect of dietary fat on tumorigenesis at this site (12). Although our early experiments 112-14) and those of others (15-18} did not distinguish between the effect of fat on the initiation and postinitiation phases of carcinogenesis, these studies nonetheless indicated a strong relationship between the amount of dietary fat and colon cancer in animal models. Nigro et al. (15} studied the effect of diets containing 5 and 35% beef fat on AOM-induced intestinal tumors in Sprague-Dawley rats. Animals fed the high beef-fat diet 1Based on a paper presented at the Symposiumon Lipids in Cancer developed more intestinal tumors and more metastases held at the AOCS Annual Meeting, Baltimore, MD, April 1990. in the abdominal cavity, lungs and liver than did the ani*Address correspondence at Div. of Nutritional Carcinogenesis, mals fed the low-fat diet. Howarth and Pihl (17) demonAmerican Health Foundation, 1 Dana Rd., Valhalla, NY 10595. strated that DMH-induced colon tumors were increased Abbreviations: AA, arachidonic acid; AOM, azoxymethane;DHA, in male D/A rats fed a high-fat diet compared with those docosahexaenoicacid; DMAB, 3,2'-dimethyl-4-aminobiphenyl;DMH, 1,2-dimethylhydrazine; DPA, docosapentaenoic acid; EPA, fed a low-fat diet. Studies conducted in our laboratory ineicosapentaenoicacid; MAF, methylformamide;MAM, methylazox- dicate t hat animals fed 20% lard or corn oil diets were ymethanol;MNU, methylnitrosourea;ODC, ornithinedecarboxylase; more susceptible to DMH-induced colon tumors compared PG, prostaglandins. with those fed 5% lard or corn oil diets. The type of fat LIPIDS, Vol. 27, no. 10 (1992)
808
REVIEW appears to be immaterial at the 20% level; however, at the 5% fat level, unsaturated fat (corn oil) induced more colon tumors than did saturated fat (lard} (13). Pence and Buddingh (18) reported that DMH-induced colon tumors were increased in male F344 rats fed a high-fat diet as compared with those fed a low-fat diet. A recent study by Sakaguchi e t al. (19) demonstrated a significantly higher incidence of colon tumors in rats fed 5% linoleic acid diet than in those fed 4.7% stearic acid plus 0.3 linoleic acid diet. Investigations also were carried out to test the effect of diets comprising 20% and 5% beef fat on colon carcinogenesis by a variety of carcinogens which differ in metabolic activation, namely DMH, MAM acetate, DMAB, or MNU (14,20}. In these studies, semipurified diets containing high and low levels of fat were fed to animals befor~ during and after carcinogen treatment to study the specific effects on the initiation and promotion stages of colon carcinogens. Combined results of these two studies (14,20) indicate that, irrespective of the nature of the carcinogens, animals fed the diet containing a high amount of beef fat had a greater incidence of colon tumors than did rats fed a low beef-fat diet. There are studies, however, which did not show an enhancing effect of dietary fat in colon carcinogenesis (21,22). Results from the laboratory of Nauss (21,22} indicated that 20% beef fat or corn oil had no colon tumor enhancing effect. Animals fed a diet which promoted optimal growth had a significant increase in colon tumor incidence, but not in frequency or size This effect was not observed using a second diet, which resulted in slower growth. TYPE AND AMOUNT OF DIETARY FAT DURING INITIATION AND POSTINITIATION PHASES
The pioneering study of Bull e t al. (23} indicated that ingestion of a high amount of beef fat increased the intestinal tumor incidence in rats when fed after AOM treatment, but not during or before the carcinogen administration, suggesting that excess dietary beef fat acts during the postinitiation phase of colon carcinogenesis. We investigated the effect of various levels of polyunsaturated (corn oil) and saturated (lard) fats fed during the initiation or postinitiation phases of colon carcinogenesis (24). When the animals were fed the diets containing 23.5% corn oil during the stage of initiation, there was no increase in the incidence of colon tumors as compared with that of animals on a 5% corn oil diet. When the 23.5% corn oil diet was fed during the postinitiation stage of carcinogenesis, there was a significant increase in colon tumor incidence compared with that of animals fed the 5% corn oil diet. These results suggest that the effect of a high corn oil diet in colon carcinogenesis is observed mainly during the postinitiation stage, rather than during the initiation stage of carcinogenesis. On the other hand, animals fed the 23.5% lard diet during the initiation or postinitiation phase had a higher colon tumor incidence as compared to those fed 5% lard diet, suggesting the importance of type of dietary fat during the initiation and postinitiation stages of colon carcinogenesis. The effect of high-fat diets containing coconut, olive, safflower and corn oil fed during the postinitiation phase of colon carcinogenesis also was investigated in rats (25}. LIPIDS, Vol. 27, no. 10 (1992)
As expected, the animals fed the high corn oil and safflower oil (23.5%) diets had a higher incidence of AOMinduced colon carcinogenesis than did those fed the diets low in their respective fats 15%). In contrast, high-fat diets containing coconut oil or olive oil had no colon tumor enhancing effect. In another study, we investigated the effect of various levels of t r a n s fat on AOM-induced colon carcinogenesis in female F344 rats {26}. The experimental high-fat diets containing 23.5% corn oil, 5.88% t r a n s fat + 5.88% corn oil + 11.76 oleinate (low t r a n s fat}, 11.76% t r a n s fat + 5.88% oleinate {intermediate t r a n s fat}, and 5.88% corn oil + 17.64% t r a n s fat {high t r a n s fat) were fed during the postinitiation phase of colon carcinogenesis. The results indicate that the high-fat diets with increasing levels of t r a n s fat had no significant effect on colon tumor incidence Animals fed the high-fat diets containing various levels of t r a n s fat have a lower colon tumor incidence as well as multiplicity of adenocarcinomas compared to those fed the diet containing high corn oil {23.5). The lack of colon tumor promoting effect of high dietary olive oil, coconut oil and t r a n s fat in contrast to that of high dietary corn oil, safflower oil, beef fat and lard suggests that the fatty acid composition of a dietary fat is one of the determining factors in colon carcinogenesis. The stage of carcinogenesis at which the effect of dietary fat is exerted appears to be mostly during the promotional phase of carcinogenesis, rather than during the initiation phase. u3 FA'R'Y ACIDS (FISH OIL) AND COLON CANCER
The high levels of highly polyunsaturated co3 fatty acids such as eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; 22:6n-3) present in marine oils make them unique dietary fats. Most common animal and vegetable fats are virtually devoid of these fatty acids, except for perilla off which contains high levels of a-linoleic acid, an r fatty acid 118:3n-3). Most commonly consumed vegetable oils in the United States, such as corn oil and safflower oil, contain high levels of polyunsaturated fatty acids of w6 type, e.g., linoleic acid (18:2n-6) {Table 1). Interest in the potentially beneficial effect of marine lipids originated by a reportedly lower incidence of thrombotic and immunologically-mediated diseases in Greenland Eskimos when compared with mainland Danish population {27}.A low prevalence of cardiovascular disease has been observed in Eskimos, who eat a mostly fish diet, a high source of r fatty acids {27,28}. The Greenland Eskimo diet was found to contain more protein and less carbohydrates than average Danish foods and an almost equal amount of fat {27}. It is also well known that Japanese people have a dietary habit of eating fish as a main source of protein and that incidence of thrombotic cardiovascular disease in Japan is lower than that in western countries. A recent epidemiologic study by Hirai e t al. (10) on the intake of fish and the mortality rate of ischemic heart disease and cardiovascular disease indicates a lower mortality rate in the fishing area of Japan, where the intake of fish was about 260 g/day, than in the farming area of Japan, where the fish intake was about 90 g/day. Interest in marine oils also emerged from the findings that cancer incidence rates are generally low in Eskimos
809
REVIEW TABLE 1 Fatty Acid Composition of Dietary Fats (%)
Fatty acids
Corn oil
4:0 6:0 8:0 10:0 12:0 14:0 16:0 18:0 16:1 18:1n-9 18:2n-6 18:3n-3 20:5n-3 22:5n-3 22:6n-3
. . . . . -11.5 2.2 -22.6 58.7 0.8 ----
Safflower oil . . . . .
. . . . .
Soybean oil
Peanut oil
. . . . .
0.2 9.5 2.6 0.3 12.3 74.0 0.9 0.1 trace --
0.1 12.3 3.9 0.4 21.6 52.6 8.2 . 0.2 0.1
.
of A l a s k a a n d G r e e n l a n d a s c o m p a r e d t o N o r t h A m e r i cans and other western population groups who consume d i e t s c o n t a i n i n g h i g h a m o u n t s of f a t {9,29-31}. I n Greenl a n d a n d I c e l a n d , a r i s e in t h e i n c i d e n c e of b r e a s t c a n c e r followed a p e r i o d w h e n t h e d i e t a r y h a b i t s b e c a m e westernized, c h a n g i n g f r o m a t y p i c a l fish d i e t to one c o n t a i n i n g d e c r e a s e d a m o u n t s of oo3 f a t t y a c i d s {29}. A r e c e n t r e p o r t on p r o s t a t i c c a n c e r in J a p a n i n d i c a t e d t h a t , a m o n g o t h e r factors, t h e r i s k for t h e d e v e l o p m e n t of p r o s t a t e c a n c e r w a s h i g h e r in i n d i v i d u a l s w h o s e d i e t s c o n t a i n e d l i t t l e o r n o s e a f o o d {32}. T h e s e s t u d i e s r e s u l t e d in a n i n t e r e s t in t h e role of fish oil in h e a l t h a n d disease. S t u d i e s c o n d u c t e d in o u r l a b o r a t o r y on t h e r e l a t i o n s h i p of d i e t a r y f i s h oil a n d colon c a n c e r r e v e a l e d t h a t fish oil p r o t e c t s a g a i n s t c h e m i c a l l y - i n d u c e d colon c a r c i n o g e n e s i s {33,34}. I n t h e first study, we t e s t e d t h e effect of d i e t s cont a i n i n g 5% corn oil, 23.5% corn oil, 3% m e n h a d e n off + 1% c o r n oil or 22.5% m e n h a d e n oil + 1% c o r n oil fed d u r i n g p o s t i n i t i a t i o n p h a s e on A O M - i n d u c e d colon c a r c i n o g e n e sis in m a l e F 3 4 4 r a t s {33}. T h e r e s u l t s i n d i c a t e d t h a t t h e colon t u m o r incidence (% a n i m a l s w i t h tumors} a n d multip l i c i t y {number of t u m o r s / a n i m a l ) were s i g n i f i c a n t l y lower in a n i m a l s fed low a n d h i g h m e n h a d e n oil d i e t s t h a n in a n i m a l s fed t h e h i g h c o r n oil diet, s u g g e s t i n g t h a t h i g h i n t a k e of d i e t a r y fish oil h a d n o colon t u m o r p r o m o t i n g effect {Table 2). A l t h o u g h h i g h d i e t a r y f i s h oil i n h i b i t e d colon carcin o g e n e s i s in t h e a b o v e s t u d y (33), l a r g e a m o u n t s of fish oil m a y i n d u c e a v a r i e t y of p a t h o p h y s i o l o g i c a l c o n d i t i o n s a n d h a r m f u l side effects. Therefore, a n o t h e r s t u d y w a s
0.1 9.5 2.3 -45.6 31.0 -. ---
Butter
Lard
Perilla off
Menhaden oil
3.2 1.9 1.1 2.5 2.8 10.1 26.3 12.1 2.3 25.1 2.3 1.4
---0.1 0.5 1.4 23.7 13.0 2.6 40.9 10.0 1.4
---
---
-----0.1 8.3 1.9 0.4 14.3 15.9 58.2 0.3 0.1 0.2
-----8.4 15.2 2.7 11.6 9.5 1.8 1.8 16.0 3.9 10.8
.
c o n d u c t e d to i n v e s t i g a t e t h e efficacy of v a r y i n g a m o u n t s of m e n h a d e n oil a n d c o r n oil on colon c a r c i n o g e n e s i s to d e t e r m i n e t h e o p t i m u m d i e t a r y levels a t w h i c h t h e comb i n a t i o n of t w o s o u r c e s of f a t e l i c i t s m a x i m u m i n h i b i t i o n w h e n fed d u r i n g t h e p o s t i n i t i a t i o n p h a s e of carcinogenesis (34}. I n t h i s s t u d y , in a d d i t i o n t o 5% c o r n oil (5% CO) a n d 1% c o r n oil + 4% m e n h a d e n oil (1% CO + 4% MO) diets, high-fat d i e t s c o n t a i n i n g 23.5% corn oil (23.5% CO), 17.6% c o r n oil + 5.9% m e n h a d e n oil (17.6% CO + 5.9% MO), 11.8% corn oil + 11.8% m e n h a d e n oil (11.8% CO + 11.8% MO) or 5.9% c o r n oil + 17.6% m e n h a d e n oil (5.9% CO + 17.6% MO) were t e s t e d (Table 3). F e e d i n g of highf a t d i e t s c o n t a i n i n g 17.6% CO + 5.9% MO, 11.8% CO + 11.8% M O or 5.9% CO + 17.6% M O s i g n i f i c a n t l y inh i b i t e d t h e i n c i d e n c e of colon t u m o r s a s c o m p a r e d to 23.5% CO. T h e f a t t y a c i d c o m p o s i t i o n of t h e m i c r o s o m a i f r a c t i o n of colonic m u c o s a a n d t u m o r s w a s also m e a s u r e d in t h e a n i m a l s fed t h e e x p e r i m e n t a l diets. T h i s s t u d y d e m o n s t r a t e d t h a t f e e d i n g i n c r e a s i n g levels of m e n h a d e n oil a l t e r e d t h e i n c o r p o r a t i o n of f a t t y a c i d s i n t o t h e micros o m e s of colonic m u c o s a l cells a n d t u m o r s (Tables 4 a n d 5). T h e m o n o u n s a t u r a t e d a n d s a t u r a t e d f a t t y a c i d s in colonic m u c o s a were u n a f f e c t e d b y f e e d i n g v a r i o u s levels of c o r n oil a n d m e n h a d e n oil, w h e r e a s t h e p o l y u n s a t u r a t e d
TABLE 3 o~6 and oo3 Fatty Acid Composition of Experimental Diets and AOM-Induced Colon Tumors
Experimental diets a
co6
co3
Colon tumor incidence {% animals with tumors)
5% CO diet 1% CO + 4% MO diet 5.9% CO + 17.6% MO diet 11.8% CO + 11.8% MO diet 17.6% CO + 5.9% MO diet 23.5% CO diet
2.8 0.6 3.6 6.7 10.0 13.1
0 1.1 4.8 3.2 1.6 0
70b 59 b 63 b 63 b 70 b 93
TABLE 2 Azoxymethane-Induced Colon Carcinogenesis in Male F344 Rats Fed co3 Fatty Acid-Rich Diets During Postinitiation Phase
Experimental diets 5% COa 23.5% CO 4% MO + 1% CO 22.5% MO + 1% CO
% Animals with colon tumors 54 b 92 50 b 33 b
a c o , corn oil; MO, menhaden oil. bSignificantly different from the 23.5% CO diet, P < 0.05.
Fatty acids (%)
aModified AIN-76 diet; CO, corn oil; MO, menhaden oil. bSignificantly different from the 23.5% CO diet, P < 0.05. LIPIDS, Vol. 27, no. 10 (1992)
810 REVIEW
0
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+1 +1 +1 -H -H +1 +1 -H +1 +1 ~
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5
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8 @
8
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5III
+1 +1 +1 +1 +1 +t +1 +1 +1 +1 ~
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+1 +1 +1 +1 +1 +1 +1 +1 +1 +1 ~ 0
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Dietary Fat and Colon Cancer: Animal Model Studies 1 Bandaru S. Reddy* Division of Nutritional Carcinogenesis, American Health Foundation, Valhalla, New York 10595
Since it was first suggested that high dietary fat is a risk factor in colon cancer, there have been several studies to test this hypothesis. Epidemiologic studies suggested a positive association between dietary fat and colon cancer. L a b o r a t o r y animal model studies demonstrated that n o t o n l y t h e a m o u n t of fat, b u t also types of fat differing in fatty acid composition are important determining factors in colon tumor development. Chemically-induced colon tumor incidence was increased in rats fed the semiparified diets containing 23% corn oil, safflower oil, lard or beef tallow (high-fat) as compared to those fed 5% corn oil, safflower oil, lard or beef tallow diets (low-fat). Diets containing 23% coconut oil, olive oil or fish oil, or high-fat diets containing varying levels of trans fat, had no colon tumor-enhancing effect compared to their respective l o w fat diets. The stage at which the effect of dietary fat is exerted appears to be mostly during the post-initiation phase of colon carcinogenesis. Lack of a c o l o n t u m o r enhancing effect of dietary fish oil is observed both during the initiation and postinitiation phases. The mechanisms by which various dietary fats increase colon carcinogenesis are n o t fully understood. In most instances, however, the high-fat diet appears to enhance tumorigenesis through elevation of agents, such as secondary bile acids, that act as promoters of tumor development. Lack of colon tumor promotion by dietary fish oil and trans fat appears to be mediated through their effect on mucosai ornithine deearboxylase activity, colonic secondary bile acids and/or prostaglandin synthesis. Lipids 27, 807-813 (1992). Cancer of the colon is one of the most common cancers in the western countries, including North America, exhibiting more than a tenfold excess when compared to rural populations in Asia, Africa and certain parts of South America. Studies of cancer incidence in high risk areas such as the United States, and low risk countries, such as Japan and Finland, indicate that rates are increasing faster in low than in high incidence areas of the world. It is the second leading cause of cancer deaths in the United States, with estimated 157,500 new cases and about 60,500 deaths in 1991 (1). Since Wynder et al. (2) and Burkitt (3) first suggested that dietary factors in general, and specifically dietary fat and fiber, might play a role in the etiology of colon cancer, a substantial amount of progress has been made in understanding the relationship between nutritional factors and the development of colon cancer in humans. Continuing
population studies revealed that the diets particularly high in total fat and low in certain dietary fibers are generally associated with an increased risk of developing colon cancer (4). In addition, dietary fat may be a risk factor in the absence of factors that are protective, such as use of high dietary fiber and fibrous foods (4). Several casecontrol studies suggest a positive association between dietary fats, especially saturated fat 15,6). A recent prospective study provided evidence that a high intake of animal fat, but not vegetable fat, increases the risk of colon cancer (7). Animal model studies performed in our laboratory and elsewhere have consistently supported human epidemiologic studies and further provided evidence that the colon tumor-promoting effect of dietary fat depends on the type of fat (8}. These studies also suggested that the main effect of dietary fat in colon carcinogenesis is during the postinitiation phase of carcinogenesis. Unquestionably, there are areas where more research is necessary and, of course, no single dietary factor and mechanism can account for all colon cancer. Since several reviews have already appeared on the relationship between dietary fat and colon carcinogenesis, this report will provide a brief overview of current knowledge as to the relationship between the types of dietary fat and colon carcinogenesis in laboratory animal models with emphasis on r fatty acids {fish oil}. The basis for the latter point is that epidemiologic studies have found that people who traditionally consume {either regularly or frequently} diets containing fish and marine animals have a lower incidence of several chronic diseases, including cancer, than those who do not or rarely consume such foods (9-11).
AMOUNT OF DIETARY FAT AND COLON CANCER
A variety of compounds, namely 1,2-dimethylhydrazine (DMH}, azoxymethane (AOM), methylazoxymethanol (MAM} acetate, 3,2'-dimethyl-4-aminobiphenyl tDMAB} and methylnitrosourea (MNU), that are carcinogenic for the colon have been used in a number of laboratory animal models to investigate the effect of dietary fat on tumorigenesis at this site (12). Although our early experiments 112-14) and those of others (15-18} did not distinguish between the effect of fat on the initiation and postinitiation phases of carcinogenesis, these studies nonetheless indicated a strong relationship between the amount of dietary fat and colon cancer in animal models. Nigro et al. (15} studied the effect of diets containing 5 and 35% beef fat on AOM-induced intestinal tumors in Sprague-Dawley rats. Animals fed the high beef-fat diet 1Based on a paper presented at the Symposiumon Lipids in Cancer developed more intestinal tumors and more metastases held at the AOCS Annual Meeting, Baltimore, MD, April 1990. in the abdominal cavity, lungs and liver than did the ani*Address correspondence at Div. of Nutritional Carcinogenesis, mals fed the low-fat diet. Howarth and Pihl (17) demonAmerican Health Foundation, 1 Dana Rd., Valhalla, NY 10595. strated that DMH-induced colon tumors were increased Abbreviations: AA, arachidonic acid; AOM, azoxymethane;DHA, in male D/A rats fed a high-fat diet compared with those docosahexaenoicacid; DMAB, 3,2'-dimethyl-4-aminobiphenyl;DMH, 1,2-dimethylhydrazine; DPA, docosapentaenoic acid; EPA, fed a low-fat diet. Studies conducted in our laboratory ineicosapentaenoicacid; MAF, methylformamide;MAM, methylazox- dicate t hat animals fed 20% lard or corn oil diets were ymethanol;MNU, methylnitrosourea;ODC, ornithinedecarboxylase; more susceptible to DMH-induced colon tumors compared PG, prostaglandins. with those fed 5% lard or corn oil diets. The type of fat LIPIDS, Vol. 27, no. 10 (1992)
808
REVIEW appears to be immaterial at the 20% level; however, at the 5% fat level, unsaturated fat (corn oil) induced more colon tumors than did saturated fat (lard} (13). Pence and Buddingh (18) reported that DMH-induced colon tumors were increased in male F344 rats fed a high-fat diet as compared with those fed a low-fat diet. A recent study by Sakaguchi e t al. (19) demonstrated a significantly higher incidence of colon tumors in rats fed 5% linoleic acid diet than in those fed 4.7% stearic acid plus 0.3 linoleic acid diet. Investigations also were carried out to test the effect of diets comprising 20% and 5% beef fat on colon carcinogenesis by a variety of carcinogens which differ in metabolic activation, namely DMH, MAM acetate, DMAB, or MNU (14,20}. In these studies, semipurified diets containing high and low levels of fat were fed to animals befor~ during and after carcinogen treatment to study the specific effects on the initiation and promotion stages of colon carcinogens. Combined results of these two studies (14,20) indicate that, irrespective of the nature of the carcinogens, animals fed the diet containing a high amount of beef fat had a greater incidence of colon tumors than did rats fed a low beef-fat diet. There are studies, however, which did not show an enhancing effect of dietary fat in colon carcinogenesis (21,22). Results from the laboratory of Nauss (21,22} indicated that 20% beef fat or corn oil had no colon tumor enhancing effect. Animals fed a diet which promoted optimal growth had a significant increase in colon tumor incidence, but not in frequency or size This effect was not observed using a second diet, which resulted in slower growth. TYPE AND AMOUNT OF DIETARY FAT DURING INITIATION AND POSTINITIATION PHASES
The pioneering study of Bull e t al. (23} indicated that ingestion of a high amount of beef fat increased the intestinal tumor incidence in rats when fed after AOM treatment, but not during or before the carcinogen administration, suggesting that excess dietary beef fat acts during the postinitiation phase of colon carcinogenesis. We investigated the effect of various levels of polyunsaturated (corn oil) and saturated (lard) fats fed during the initiation or postinitiation phases of colon carcinogenesis (24). When the animals were fed the diets containing 23.5% corn oil during the stage of initiation, there was no increase in the incidence of colon tumors as compared with that of animals on a 5% corn oil diet. When the 23.5% corn oil diet was fed during the postinitiation stage of carcinogenesis, there was a significant increase in colon tumor incidence compared with that of animals fed the 5% corn oil diet. These results suggest that the effect of a high corn oil diet in colon carcinogenesis is observed mainly during the postinitiation stage, rather than during the initiation stage of carcinogenesis. On the other hand, animals fed the 23.5% lard diet during the initiation or postinitiation phase had a higher colon tumor incidence as compared to those fed 5% lard diet, suggesting the importance of type of dietary fat during the initiation and postinitiation stages of colon carcinogenesis. The effect of high-fat diets containing coconut, olive, safflower and corn oil fed during the postinitiation phase of colon carcinogenesis also was investigated in rats (25}. LIPIDS, Vol. 27, no. 10 (1992)
As expected, the animals fed the high corn oil and safflower oil (23.5%) diets had a higher incidence of AOMinduced colon carcinogenesis than did those fed the diets low in their respective fats 15%). In contrast, high-fat diets containing coconut oil or olive oil had no colon tumor enhancing effect. In another study, we investigated the effect of various levels of t r a n s fat on AOM-induced colon carcinogenesis in female F344 rats {26}. The experimental high-fat diets containing 23.5% corn oil, 5.88% t r a n s fat + 5.88% corn oil + 11.76 oleinate (low t r a n s fat}, 11.76% t r a n s fat + 5.88% oleinate {intermediate t r a n s fat}, and 5.88% corn oil + 17.64% t r a n s fat {high t r a n s fat) were fed during the postinitiation phase of colon carcinogenesis. The results indicate that the high-fat diets with increasing levels of t r a n s fat had no significant effect on colon tumor incidence Animals fed the high-fat diets containing various levels of t r a n s fat have a lower colon tumor incidence as well as multiplicity of adenocarcinomas compared to those fed the diet containing high corn oil {23.5). The lack of colon tumor promoting effect of high dietary olive oil, coconut oil and t r a n s fat in contrast to that of high dietary corn oil, safflower oil, beef fat and lard suggests that the fatty acid composition of a dietary fat is one of the determining factors in colon carcinogenesis. The stage of carcinogenesis at which the effect of dietary fat is exerted appears to be mostly during the promotional phase of carcinogenesis, rather than during the initiation phase. u3 FA'R'Y ACIDS (FISH OIL) AND COLON CANCER
The high levels of highly polyunsaturated co3 fatty acids such as eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; 22:6n-3) present in marine oils make them unique dietary fats. Most common animal and vegetable fats are virtually devoid of these fatty acids, except for perilla off which contains high levels of a-linoleic acid, an r fatty acid 118:3n-3). Most commonly consumed vegetable oils in the United States, such as corn oil and safflower oil, contain high levels of polyunsaturated fatty acids of w6 type, e.g., linoleic acid (18:2n-6) {Table 1). Interest in the potentially beneficial effect of marine lipids originated by a reportedly lower incidence of thrombotic and immunologically-mediated diseases in Greenland Eskimos when compared with mainland Danish population {27}.A low prevalence of cardiovascular disease has been observed in Eskimos, who eat a mostly fish diet, a high source of r fatty acids {27,28}. The Greenland Eskimo diet was found to contain more protein and less carbohydrates than average Danish foods and an almost equal amount of fat {27}. It is also well known that Japanese people have a dietary habit of eating fish as a main source of protein and that incidence of thrombotic cardiovascular disease in Japan is lower than that in western countries. A recent epidemiologic study by Hirai e t al. (10) on the intake of fish and the mortality rate of ischemic heart disease and cardiovascular disease indicates a lower mortality rate in the fishing area of Japan, where the intake of fish was about 260 g/day, than in the farming area of Japan, where the fish intake was about 90 g/day. Interest in marine oils also emerged from the findings that cancer incidence rates are generally low in Eskimos
809
REVIEW TABLE 1 Fatty Acid Composition of Dietary Fats (%)
Fatty acids
Corn oil
4:0 6:0 8:0 10:0 12:0 14:0 16:0 18:0 16:1 18:1n-9 18:2n-6 18:3n-3 20:5n-3 22:5n-3 22:6n-3
. . . . . -11.5 2.2 -22.6 58.7 0.8 ----
Safflower oil . . . . .
. . . . .
Soybean oil
Peanut oil
. . . . .
0.2 9.5 2.6 0.3 12.3 74.0 0.9 0.1 trace --
0.1 12.3 3.9 0.4 21.6 52.6 8.2 . 0.2 0.1
.
of A l a s k a a n d G r e e n l a n d a s c o m p a r e d t o N o r t h A m e r i cans and other western population groups who consume d i e t s c o n t a i n i n g h i g h a m o u n t s of f a t {9,29-31}. I n Greenl a n d a n d I c e l a n d , a r i s e in t h e i n c i d e n c e of b r e a s t c a n c e r followed a p e r i o d w h e n t h e d i e t a r y h a b i t s b e c a m e westernized, c h a n g i n g f r o m a t y p i c a l fish d i e t to one c o n t a i n i n g d e c r e a s e d a m o u n t s of oo3 f a t t y a c i d s {29}. A r e c e n t r e p o r t on p r o s t a t i c c a n c e r in J a p a n i n d i c a t e d t h a t , a m o n g o t h e r factors, t h e r i s k for t h e d e v e l o p m e n t of p r o s t a t e c a n c e r w a s h i g h e r in i n d i v i d u a l s w h o s e d i e t s c o n t a i n e d l i t t l e o r n o s e a f o o d {32}. T h e s e s t u d i e s r e s u l t e d in a n i n t e r e s t in t h e role of fish oil in h e a l t h a n d disease. S t u d i e s c o n d u c t e d in o u r l a b o r a t o r y on t h e r e l a t i o n s h i p of d i e t a r y f i s h oil a n d colon c a n c e r r e v e a l e d t h a t fish oil p r o t e c t s a g a i n s t c h e m i c a l l y - i n d u c e d colon c a r c i n o g e n e s i s {33,34}. I n t h e first study, we t e s t e d t h e effect of d i e t s cont a i n i n g 5% corn oil, 23.5% corn oil, 3% m e n h a d e n off + 1% c o r n oil or 22.5% m e n h a d e n oil + 1% c o r n oil fed d u r i n g p o s t i n i t i a t i o n p h a s e on A O M - i n d u c e d colon c a r c i n o g e n e sis in m a l e F 3 4 4 r a t s {33}. T h e r e s u l t s i n d i c a t e d t h a t t h e colon t u m o r incidence (% a n i m a l s w i t h tumors} a n d multip l i c i t y {number of t u m o r s / a n i m a l ) were s i g n i f i c a n t l y lower in a n i m a l s fed low a n d h i g h m e n h a d e n oil d i e t s t h a n in a n i m a l s fed t h e h i g h c o r n oil diet, s u g g e s t i n g t h a t h i g h i n t a k e of d i e t a r y fish oil h a d n o colon t u m o r p r o m o t i n g effect {Table 2). A l t h o u g h h i g h d i e t a r y f i s h oil i n h i b i t e d colon carcin o g e n e s i s in t h e a b o v e s t u d y (33), l a r g e a m o u n t s of fish oil m a y i n d u c e a v a r i e t y of p a t h o p h y s i o l o g i c a l c o n d i t i o n s a n d h a r m f u l side effects. Therefore, a n o t h e r s t u d y w a s
0.1 9.5 2.3 -45.6 31.0 -. ---
Butter
Lard
Perilla off
Menhaden oil
3.2 1.9 1.1 2.5 2.8 10.1 26.3 12.1 2.3 25.1 2.3 1.4
---0.1 0.5 1.4 23.7 13.0 2.6 40.9 10.0 1.4
---
---
-----0.1 8.3 1.9 0.4 14.3 15.9 58.2 0.3 0.1 0.2
-----8.4 15.2 2.7 11.6 9.5 1.8 1.8 16.0 3.9 10.8
.
c o n d u c t e d to i n v e s t i g a t e t h e efficacy of v a r y i n g a m o u n t s of m e n h a d e n oil a n d c o r n oil on colon c a r c i n o g e n e s i s to d e t e r m i n e t h e o p t i m u m d i e t a r y levels a t w h i c h t h e comb i n a t i o n of t w o s o u r c e s of f a t e l i c i t s m a x i m u m i n h i b i t i o n w h e n fed d u r i n g t h e p o s t i n i t i a t i o n p h a s e of carcinogenesis (34}. I n t h i s s t u d y , in a d d i t i o n t o 5% c o r n oil (5% CO) a n d 1% c o r n oil + 4% m e n h a d e n oil (1% CO + 4% MO) diets, high-fat d i e t s c o n t a i n i n g 23.5% corn oil (23.5% CO), 17.6% c o r n oil + 5.9% m e n h a d e n oil (17.6% CO + 5.9% MO), 11.8% corn oil + 11.8% m e n h a d e n oil (11.8% CO + 11.8% MO) or 5.9% c o r n oil + 17.6% m e n h a d e n oil (5.9% CO + 17.6% MO) were t e s t e d (Table 3). F e e d i n g of highf a t d i e t s c o n t a i n i n g 17.6% CO + 5.9% MO, 11.8% CO + 11.8% M O or 5.9% CO + 17.6% M O s i g n i f i c a n t l y inh i b i t e d t h e i n c i d e n c e of colon t u m o r s a s c o m p a r e d to 23.5% CO. T h e f a t t y a c i d c o m p o s i t i o n of t h e m i c r o s o m a i f r a c t i o n of colonic m u c o s a a n d t u m o r s w a s also m e a s u r e d in t h e a n i m a l s fed t h e e x p e r i m e n t a l diets. T h i s s t u d y d e m o n s t r a t e d t h a t f e e d i n g i n c r e a s i n g levels of m e n h a d e n oil a l t e r e d t h e i n c o r p o r a t i o n of f a t t y a c i d s i n t o t h e micros o m e s of colonic m u c o s a l cells a n d t u m o r s (Tables 4 a n d 5). T h e m o n o u n s a t u r a t e d a n d s a t u r a t e d f a t t y a c i d s in colonic m u c o s a were u n a f f e c t e d b y f e e d i n g v a r i o u s levels of c o r n oil a n d m e n h a d e n oil, w h e r e a s t h e p o l y u n s a t u r a t e d
TABLE 3 o~6 and oo3 Fatty Acid Composition of Experimental Diets and AOM-Induced Colon Tumors
Experimental diets a
co6
co3
Colon tumor incidence {% animals with tumors)
5% CO diet 1% CO + 4% MO diet 5.9% CO + 17.6% MO diet 11.8% CO + 11.8% MO diet 17.6% CO + 5.9% MO diet 23.5% CO diet
2.8 0.6 3.6 6.7 10.0 13.1
0 1.1 4.8 3.2 1.6 0
70b 59 b 63 b 63 b 70 b 93
TABLE 2 Azoxymethane-Induced Colon Carcinogenesis in Male F344 Rats Fed co3 Fatty Acid-Rich Diets During Postinitiation Phase
Experimental diets 5% COa 23.5% CO 4% MO + 1% CO 22.5% MO + 1% CO
% Animals with colon tumors 54 b 92 50 b 33 b
a c o , corn oil; MO, menhaden oil. bSignificantly different from the 23.5% CO diet, P < 0.05.
Fatty acids (%)
aModified AIN-76 diet; CO, corn oil; MO, menhaden oil. bSignificantly different from the 23.5% CO diet, P < 0.05. LIPIDS, Vol. 27, no. 10 (1992)
810 REVIEW
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