Isomeric fatty acids and tumorigenesis: A commentary on recent work

This article critically reviews the existing, although limited, literature concerning trans fatty acids and tumorigenesis. Neither epidemiological nor experimental studies published to date have demonstrated any valid association between trans fatty acid ingestion and tumorigenesis. A recent study showed that under controlled conditions, a fat with a high content of ‘trans fatty acids did not promote the development of mammary tumors induced in rats by 7,12‐dimethylbenz[a]anthracene to any greater extent than did a comparable fat with a high content of as fatty acids. In addition, in this study a high trans fat was less tumor promoting than was a blend of fats that simulated the dietary fat composition of the United States and had a lower level of trans fatty acids. Another study using comparable cis and trans fats demonstrated that the high trans fat did not affect the growth and metastasis of implanted mammary tumors in mice relative to the high cis fat. Also, two recent studies reported no significant difference in the development of induced colon tumors in ratsfed diets high in cis or trans fatty acids. The results of these and other studies are consistent with the conclusion that trans fatty acids are not uniquely related to tumor development.     

Influence of dietary cis and trans fats on DMH-induced colon tumors, steroid excretion, and eicosanoid production in rats prone to colon cancer

The effect of geometrical isomerism of dietary fats on colon tumorigenesis was studied in male and female rats of a strain prone to colon cancer (Wistar-Furth-Osaka). The rats were fed purified diets containing either partially hydrogenated corn oil (trans fat) or high-oleic safflower (cis fat) at the 5% level for one week and received a single oral dose of 1,2-dimethylhydrazine. The difference in the fatty acid composition of dietary fats was confined solely to the geometry of octadecenoate. An appropriate level of linoleic acid (2% of total energy) was supplied. After about 60 weeks, neither fat-type nor sex-dependent differences in the incidence of colon and small intestinal tumors was observed. The fecal excretion of neutral but not acidic steroids was higher in male rats fed the trans fat than in those fed the cis fat, but the composition remained almost unchanged. Aortic production of prostacyclin and the plasma concentration of thromboxane B2 were not influenced by dietary fats, although these were significantly higher in females, irregardless of the fat source. Thus, trans fat behaved much like the cis fat in various parameters, except for steroid excretion.     

Exchanging partially hydrogenated fat for palmitic acid in the diet increases LDL-cholesterol and endogenous cholesterol synthesis in normocholesterolemic women

Partial hydrogenation of oil results in fats containing unusual isomeric fatty acids characterized by cis and trans configurations. Hydrogenated fats containing trans fatty acids increase plasma total cholesterol (TC) and LDL-cholesterol while depressing HDL-cholesterol levels. Identifying the content of trans fatty acids by food labeling is overshadowed by a reluctance of health authorities to label saturates and trans fatty acids separately. Thus, it is pertinent to compare the effects of trans to saturated fatty acids using stable isotope methodology to establish if the mechanism of increase in TC and LDL-cholesterol is due to the increase in the rate of endogenous synthesis of cholesterol. Ten healthy normocholesterolemic female subjects consumed each of two diets containing approximately 30% of energy as fat for a fourweek period. One diet was high in palmitic acid (10.6% of energy) from palm olein and the other diet exchanged 5.6% of energy as partially hydrogenated fat for palmitic acid. This fat blend resulted in monounsaturated fatty acids decreasing by 4.9 % and polyunsaturated fats increasing by 2.7%. The hydrogenated fat diet treatment provided 3.1% of energy as elaidic acid. For each dietary treatment, the fractional synthesis rates for cholesterol were measured using deuterium-labeling procedures and blood samples were obtained for blood lipid and lipoprotein measurements. Subjects exhibited a higher total cholesterol and LDL-cholesterol level when consuming the diet containing trans fatty acids while also depressing the HDL-cholesterol level. Consuming the partially hydrogenated fat diet treatment increased the fractional synthesis rate of free cholesterol. Consumption of hydrogenated fats containing trans fatty acids in comparison to a mixtur e of palmitic and oleic acids increase plasma cholesterol levels apparently by increasing endogenous synthesis of cholesterol.     

Effects of dietary fatty acids and carbohydrates on the ratio of serum total to HDL cholesterol and on serum lipids and apolipoproteins: a meta-analysis of 60 controlled trials

BACKGROUND: The effects of dietary fats on the risk of coronary artery disease (CAD) have traditionally been estimated from their effects on LDL cholesterol. Fats, however, also affect HDL cholesterol, and the ratio of total to HDL cholesterol is a more specific marker of CAD than is LDL cholesterol. OBJECTIVE: The objective was to evaluate the effects of individual fatty acids on the ratis of total to HDL cholesterol and on serum lipoproteins. DESIGN: We performed a meta-analysis of 60 selected trials and calculated the effects of the amount and type of fat on total:HDL cholesterol and on other lipids. RESULTS: The ratio did not change if carbohydrates replaced saturated fatty acids, but it decreased if cis unsaturated fatty acids replaced saturated fatty acids. The effect on total:HDL cholesterol of replacing trans fatty acids with a mix of carbohydrates and cis unsaturated fatty acids was almost twice as large as that of replacing saturated fatty acids. Lauric acid greatly increased total cholesterol, but much of its effect was on HDL cholesterol. Consequently, oils rich in lauric acid decreased the ratio of total to HDL cholesterol. Myristic and palmitic acids had little effect on the ratio, and stearic acid reduced the ratio slightly. Replacing fats with carbohydrates increased fasting triacylglycerol concentrations. CONCLUSIONS: The effects of dietary fats on total:HDL cholesterol may differ markedly from their effects on LDL. The effects of fats on these risk markers should not in themselves be considered to reflect changes in risk but should be confirmed by prospective observational studies or clinical trials. By that standard, risk is reduced most effectively when trans fatty acids and saturated fatty acids are replaced with cis unsaturated fatty acids. The effects of carbohydrates and of lauric acid-rich fats on CAD risk remain uncertain.     

Nationwide product reformulations to reduce trans fatty acids in Canada: when trans fat goes out, what goes in?

The impact of recent efforts to reduce the content of industrially produced trans fatty acids (TFA) in foods has not been systematically assessed in any country. Concerns exist that food manufacturers/restaurants may replace TFA with saturated fat acids (SFA), rather than cis unsaturated fats, or increase the total fat content. We present findings from a national systematic assessment of grocery and restaurant foods likely to contain TFA in Canada in 2005-2007. Of the total of 221 assessed products, 92 (42%) contained TFA (> or = 5% of fatty acids) on initial assessment. Of an unselected sample assessed more than once, 72% were reformulated during 2005-2007: mean+/-s.d. TFA levels decreased from 26+/-13 to 2+/-4%. Following reformulation, one product had similar TFA+SFA content; all others had decreased TFA+SFA and increased cis unsaturated fat content. The total fat content was generally unaffected. The findings suggest that manufacturers/restaurants generally take advantage of costs and efforts of reformulation to not only reduce TFA but also increase the content of cis unsaturated fats.     

Acides gras trans et cancer

The present article reviews the literature about trans fatty acids and the controversy about their effects on carcinogenesis. Experimental studies on animals are related to mammary tumors, colon tumors and liver tumors: it seems that there is no relation beetween trans fatty acids and mammary tumors, as well as liver tumors ; however, results about trans fatty acids and colon tumors are much debated. Studies on humans are related to breast cancer, prostate cancer and colon tumor: results about trans fatty acids, breast cancer and colon tumor are much debated ; yet, it seems there would be no relation beetween trans fatty acids and prostate cancer. A critical analysis about available data is proposed.     

Modulation of mouse mammary tumor growth and linoleate enhanced metastasis by oleate

This study examines whether oleate may influence the linoleate enhanced metastasis of line 4526 murine mammary tumors. In addition, the in vitro proliferative response of line 4526 to oleate and other selected fatty acids was assessed. Initially, the tumor cells were grown in a defined medium supplemented with palmitate, stearate, oleate, linoleate, linolenate or arachidonate. The unsaturated fatty acids stimulated and the saturated fatty acids inhibited proliferation compared to fatty acid-free medium. Next, we examined the effect of oleate on the linoleate enhanced metastasis of 4526 tumors by substituting oleate for saturated fat in isoenergetic diets containing high or low levels of linoleate. Oleate had no effect on metastasis in mice fed the high linoleate diets but it significantly increased metastasis in mice fed the low linoleate diets. Finally, the fatty acid compositions of tumors and mammary fat pads were compared to diet fatty acid compositions and metastatic frequency. Metastasis corresponded more closely to total unsaturated fatty acids than to total polyunsaturated fatty acids or to any individual fatty acid. These studies suggest that both mono- and polyunsaturated fatty acids may stimulate mammary tumor metastasis. However, the influence of dietary oleate probably depends on the level of linoleate and total unsaturated fatty acids in the diet.     

Fatty acids in confectionery products

The content of fat and fatty acids in 144 different confectionery products purchased on the market in Warsaw region during 1997-1999 have been investigated. In examined confectionery products considerable variability of both fat and fatty acids content have been found. The content of fat varied from 6.6% (coconut cookies) up to 40% (chocolate wafers). Saturated fatty acids were present in both cis and trans form. Especially trans fatty acids reach (above 50%) were fats extracted from nut wafers, coconuts wafers.     

The interaction of dietary fat and antiestrogen treatment on DMBA‐induced mammary tumors in the rat

This study was designed to determine whether an estrogenic mechanism is in volved in dietary fat‐modulated tumor development and growth. Female Sprague‐Dawley rats were placed on a semipurified low‐fat (2% fat), high‐saturated fat (20% fat), or high‐polyunsaturatedfat (20% fat) diet at 21 days of age. A single dose of7,12‐dimethylbenz[at]anthracene (DMBA, 10 mg) was administered intragastrically at 50 days of age. Two studies were performed. One tested the effectiveness of antiestrogen treatment (either tamoxifen or analog II) on tumor development when it was given one week prior to and one week after DMBA treatment in animals consuming a high‐polyunsaturated fat diet. The second six‐week study tested the antiestrogen effectiveness in arresting tumor growth and in producing regressions of established DMBA‐induced tumors in rats consuming various levels and types of fat.

The results of these studies indicate that both antiestrogens employed reduced the rate of growth and increased the number of regressions of established DMBA‐induced tumors. In general, this was true in animals fed diets with a high content of either saturated or polyunsaturated fats and to a lesser extent in animals fed a low‐fat diet. Tamoxifen produced a somewhat greater reduction in the growth of established tumor than did analog II. However, analog II, which is a more biologically “pure” antiestrogen, reduced the incidence of animals with mammary tumors and total tumor burden when administered one week beforeandone week after DMBA dosing. Tamoxifen, which is a partial estrogen‐agonist, did not alter tumor incidence, but it did reduce the total tumor burden under these same experimental conditions. We concluded that estrogens may be partially responsible for the observed dietary fat enhancement of breast tumor development.     

Reversal of the promotional effect of high-fat diet on mammary tumorigenesis by subsequent lowering of dietary fat

Female Sprague-Dawley rats were given 7,12-dimethylbenz(a)anthracene at 50 days of age to induce mammary tumors, and beginning one week later were fed a high-fat, semipurified diet containing 20% sunflowerseed oil to promote tumor development. After another 7 weeks, when one third of the rats had palpable mammary tumors, the rats were randomly assigned to five groups of 31 animals each, with the same number of tumor-bearers in each group. One group was continued on the high-fat diet, another was given a fat-free diet, and the three remaining groups were fed diets containing 10% lard, butter, or coconut oil, respectively. During the next 29 weeks, rats fed the diets containing 0% or 10% fat developed significantly fewer tumors than those continued on the 20% fat diet. The diets containing 10% fat suppressed tumorigenesis at least as effectively as the fat-free diet. Rats fed the 10% butter and 10% lard diets had growth rates comparable to those fed the 20% sunflowerseed-oil diet throughout, and evidence of essential fatty acid deficiency was seen only in rats on the fat-free diet. These results provide additional evidence that high-fat diets promote development of mammary cancer and suggest that reducing the level of dietary fat might help to prevent the development and recurrence of breast cancer in humans.     

Beef tallow increases the potency of conjugated linoleic acid in the reduction of mouse mammary tumor metastasis

Animal studies consistently show that dietary conjugated linoleic acid (CLA) reduces mammary tumorigenesis including metastasis. Relatively low concentrations of CLA are required for those effects, and a threshold level exists above which there is no added reduction. We reasoned that the concentration of CLA required to effectively alter mammary tumor metastasis may be dependent on the type of dietary fat because select fatty acids can enhance or suppress normal or malignant cell growth and metastasis. For this study, the diets (a total of 12 different groups) differed in fatty acid composition but not in energy from fat (40%). In experiments involving spontaneous metastasis, mice were fed for 11 wk; in experiments in which mice were injected i.v. with tumor cells, they were fed for 7 wk. Mice were then assessed for the effect of CLA concentration on mammary tumorigenesis. Mammary tumor growth was not altered, but metastasis was significantly decreased when beef tallow (BT) replaced half of a defined vegetable fat blend (VFB). That blend reflects the typical fat content of a Western diet. In addition, that same VFB:BT diet lowered the concentration of CLA required to significantly decrease mammary tumor metastasis from 0.1% of the diet to 0.05%. A diet in which corn oil replaced half of the VFB did not lower the threshold from 0.1 to 0.05%. In vitro, the main fatty acid in vegetable oil, linoleic acid, reduced the efficacy of CLA toxicity on mammary tumor cells in culture. Alternatively, fatty acids normally found in BT, such as oleic, stearic, and palmitic acids, either did not change or enhanced the cytolytic effects of CLA isomers on mouse mammary tumor cells in culture. These data provide evidence that dietary BT, itself with negligible levels of CLA, may increase the efficacy of dietary CLA in reducing mammary tumorigenesis.     

The interaction of dietary fat and antiestrogen treatment on DMBA-induced mammary tumors in the rat

This study was designed to determine whether an estrogenic mechanism is involved in dietary fat-modulated tumor development and growth. Female Sprague-Dawley rats were placed on a semipurified low-fat (2% fat), high-saturated fat (20% fat), or high-polyunsaturated fat (20% fat) diet at 21 days of age. A single dose of 7,12-dimethylbenz[a]anthracene (DMBA, 10 mg) was administered intragastrically at 50 days of age. Two studies were performed. One tested the effectiveness of antiestrogen treatment (either tamoxifen or analog II) on tumor development when it was given one week prior to and one week after DMBA treatment in animals consuming a high-polyunsaturated fat diet. The second six-week study tested the antiestrogen effectiveness in arresting tumor growth and in producing regressions of established DMBA-induced tumors in rats consuming various levels and types of fat. The results of these studies indicate that both antiestrogens employed reduced the rate of growth and increased the number of regressions of established DMBA-induced tumors. In general, this was true in animals fed diets with a high content of either saturated or polyunsaturated fats and to a lesser extent in animals fed a low-fat diet. Tamoxifen produced a somewhat greater reduction in the growth of established tumor than did analog II. However, analog II, which is a more biologically "pure" antiestrogen, reduced the incidence of animals with mammary tumors and total tumor burden when administered one week before and one week after DMBA dosing. Tamoxifen, which is a partial estrogen-agonist, did not alter tumor incidence, but it did reduce the total tumor burden under these same experimental conditions.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dimethylbenzanthracene mammary tumorigenesis in sprague-dawley rats fed diets differing in content of beef tallow or rapeseed oil

Female Sprague-Dawley rats fed purified diets containing 5% or 20% fat as beef tallow or rapeseed oil plus corn oil were given 2.5 mg 7,12-dimethylbenz(a)anthracene intragastrically at age 55 days. Mammary tumorigenesis was not significantly increased in rats fed diets high in beef tallow or rapeseed oil compared to rats fed 5% fat. In earlier studies in rats fed similar diets containing corn oil or lard, mammary tumorigenesis was enhanced by the high fat diets. These results indicate that dietary fats vary in their ability to increase mammary tumorigenesis in rats.In addition, body weight, feed efficiency ratios, sexual maturation, and estrous cycles were examined. High erucic acid rapeseed oil did not support growth as well as beef tallow, and feed efficiency ratios were decreased in animals fed the higher amount of the oil. There was no significant dietary fat effect on age at vaginal opening or regularity or duration of estrous cycles.     

Trans fatty acids– Metabolic and nutritional significance

'Trans' fatty acids are unsaturated acids with special structural features that occur naturally in dietary fats from animal and plant sources and in fats processed by catalytic hydrogenation. They are readily metabolized by the human body. Thus, although when consumed in the diet they are incorporated into body fat (including depot and milk fats), they are subject to rapid 'turnover'. In physical properties, trans monounsaturatedfatty acids are intermediate between cis-monounsaturated and saturated acids, and they tend to be treated either as saturated or cis-monounsaturated acids in metabolic pathways. The author argues in this article that any adverse effects on health or metabolism that may have been observed can be ascribed to an imbalance between the intake of trans and essential fatty acids. Such imbalances, could also occur with non-essential fatty acids other than trans fats. Normally, the amounts eaten in average diets would not pose serious problems and only when products have excessively high trans contents and make a significant contribution to the diet need trans acids be highlighted on labels.     

Conjugated linoleic acid isomers and cancer

We reviewed the literature regarding the effects of conjugated linoleic acid (CLA) preparations enriched in specific isomers, cis9, trans11-CLA (c9, t11-CLA) or trans10, cis12-CLA (t10, c12-CLA), on tumorigenesis in vivo and growth of tumor cell lines in vitro. We also examined the potential mechanisms by which CLA isomers may alter the incidence of cancer. We found no published reports that examined the effects of purified CLA isomers on human cancer in vivo. Incidence of rat mammary tumors induced by methylnitrosourea was decreased by c9, t11-CLA in all studies and by t10, c12-CLA in just a few that included it. Those 2 isomers decreased the incidence of forestomach tumors induced by benzo (a) pyrene in mice. Both isomers reduced breast and forestomach tumorigenesis. The c9, t11-CLA isomer did not affect the development of spontaneous tumors of the intestine or mammary gland, whereas t10, c12-CLA increased development of genetically induced mammary and intestinal tumors. In vitro, t10, c12-CLA inhibited the growth of mammary, colon, colorectal, gastric, prostate, and hepatoma cell lines. These 2 CLA isomers may regulate tumor growth through different mechanisms, because they have markedly different effects on lipid metabolism and regulation of oncogenes. In addition, c9, t11-CLA inhibited the cyclooxygenase-2 pathway and t10, c12-CLA inhibited the lipooxygenase pathway. The t10, c12-CLA isomer induced the expression of apoptotic genes, whereas c9, t11-CLA did not increase apoptosis in most of the studies that assessed it. Several minor isomers including t9, t11-CLA; c11, t13-CLA; c9, c11-CLA; and t7, c11-CLA were more effective than c9, t11-CLA or t10, c12-CLA in inhibiting cell growth in vitro. Additional studies with purified isomers are needed to establish the health benefit and risk ratios of each isomer in humans.     

Oxidized fat reduces milk triacylglycerol concentrations by inhibiting gene expression of lipoprotein lipase and fatty acid transporters in the mammary gland of rats

Feeding oxidized fats to lactating rats causes a strong reduction of triacylglycerol concentration in the milk. The reason for this, however, has not yet been elucidated. Pregnant Sprague-Dawley rats were assigned to 2 groups of 11 rats each and fed diets containing either fresh fat (FF group) or an oxidized fat (OF group) from d 1 to d 20 of lactation. Concentrations of triacylglycerols and long-chain fatty acids in the milk and weight gain of suckling pups were lower in the OF group than in the FF group (P < 0.05). Concentrations of medium-chain fatty acids in the milk and messenger RNA (mRNA) abundance of lipogenic enzymes in the mammary gland did not differ between the 2 groups of rats. However, the OF group had a lower concentration of triacylglycerols and nonesterified fatty acids (NEFA) in plasma and lower mRNA concentrations of lipoprotein lipase and fatty acid transporters in the mammary gland than the FF group (P < 0.05). Moreover, the OF group had higher mRNA concentrations of hepatic lipase, fatty acid transporters, and several genes involved in fatty acid oxidation in the liver than the FF group (P < 0.05). The present findings suggest that a dietary oxidized fat lowers the concentration of triacylglycerols in the milk by a reduced uptake of fatty acids from triacylglycerol rich-lipoproteins and NEFA into the mammary gland. The study, moreover, indicates that an oxidized fat impairs normal metabolic adaptations during lactation, which promote the utilization of metabolic substrates by the mammary gland for the synthesis of milk.     

Effect of trans fatty acids on calcium influx into human arterial endothelial cells.

BACKGROUND: A recent task force of The American Society for Clinical Nutrition and American Society for Nutritional Sciences recommended in a position paper on trans fatty acids that models be developed to assess the effects of changes in fat intake on disease risk. OBJECTIVE: The objective was to investigate, using human arterial endothelial cells as a model, the influence of trans fatty acids and magnesium on cell membrane composition and on calcium influx into arterial cells, a hallmark of atherosclerosis. DESIGN: Endothelial cells were cultured for 3 d in media with high (adequate) or low (inadequate) amounts of magnesium plus various concentrations of trans,trans linoelaidic; cis,cis linoleic; trans elaidic; oleic; or stearic acids. The cells were then harvested and the fatty acid composition and the amount of (45)Ca(2+) incorporated into the cell was determined. RESULTS: The percentage of fatty acids incorporated into the endothelial cells was proportional to the amount added to the culture medium. Adequate magnesium was crucial in preventing calcium influx into endothelial cells. Without an adequate amount of magnesium in the culture medium, linoelaidic and elaidic acids, even at low concentrations, increased the incorporation of (45)Ca(2+) into the cells, whereas stearic acid and oleic acid did not (P < 0.05). CONCLUSION: Our model indicated that a diet inadequate in magnesium combined with trans fat may increase the risk of calcification of endothelial cells.     

Dietary butyrate inhibits NMU-induced mammary cancer in rats

Butyrate has been proposed as an antineoplastic agent, leading to the inhibition of tumorigenesis. The purpose of this study was to examine butyrate, supplied as tributyrin (Tbn) or as a natural component of anhydrous milk fat (AMF), on the development of nitrosomethylurea-induced mammary tumors in female Sprague-Dawley rats. Diets were 1) semipurified rodent diet (AIN-93) with high fat [20% sunflower seed oil (SSO), control], 2) SSO diet with Tbn added at 1%, 3) SSO diet with Tbn added at 3%, and 4) 19% AMF with 1% SSO diet, which contained butyrate equivalent to the 1% Tbn diet. These diets were fed ad libitum from weaning at 21 days of age, and at 24 days of age each rat was injected with nitrosomethylurea (50 mg/kg body wt i.p.). At any one period, there was a relative risk increase of 88% (p < 0.05) that rats in the SSO diet group would develop a mammary tumor compared with those in the AMF diet group. The addition of 1% and 3% Tbn to SSO diets reduced the tumor incidence by 20% and 52%, respectively, in comparison to SSO alone (p < 0.05). There was a linear inverse relationship between Tbn concentration and rats developing a tumor. From 89 days to the end of the experiment, rats fed the diet containing 3% Tbn showed a significantly lower multiplicity of palpable tumors (50% less at Day 118, p < 0.05) than SSO-fed rats. These results indicate that although the AMF diet was effective, particularly early in reducing mammary tumorigenesis, the 3% Tbn diet produced a sustained reduction of tumor multiplicity relative to the control (SSO) group. An inhibitory influence of butyrate on mammary tumorigenesis against a background of high polyunsaturated fat diet has been demonstrated in this animal model of breast cancer.     

Influence of dietary cis- and trans-fat on 1,2-dimethylhydrazine-induced colon tumors and fecal steroid excretion in Fischer 344 rats

To investigate the effects of the difference in the geometry of dietary fatty acids on colon tumorigenesis, male rats were fed semipurified diets containing either partially hydrogenated corn oil (trans-monoene fat) or olive oil (cis-monoene fat) at the 10% level and received a single oral dosage of 1,2-dimethylhydrazine (DMH). The difference in the fatty acid composition of dietary fats was confined essentially to the geometrical isomerism of octadecenoate, and the linoleic acid content was made equivalent (2% of total energy). After about 15 mo of feeding, colon tumor incidence in DMH treated rats was nearly the same in both fat groups. Fecal neutral steroid excretion was higher, while the transformation of cholesterol to coprostanol was lower in rats given the trans-fat. There were no marked differences in the excretion and composition of fecal bile acids between two fat groups. Serum cholesterol and tocopherol levels of rats given trans-fat diets tended to be low. The results suggested that the trans-monoene behaves much like the cis-monoene in the incidence of DMH-induced colon tumors, although there were characteristic differences in metabolic events in the intestine.     

Inhibitory effect of a fat‐free diet on mammary carcinogenesis in rats

Young female Sprague‐Dawley rats were administered 7,12‐dimethylbenz(a)‐anthracene, and a week later the rats were transferred from commercial feed to a semipurified diet containing 20% corn oil. Eight weeks after receiving the carcinogen, half of the rats were changed to a fat‐free diet to determine the effects on mammary tumor growth and development. After another 20 weeks, rats fed the fat‐free diet had significantly fewer tumors per tumor‐bearing rat and the tumors were smaller than those of rats that continued on the high‐fat diet. Rats fed the fat‐free diet weighed somewhat less, but showed no physical evidence of essential fatty acid deficiency. Tumors regressed in about half of the rats on the fat‐free diet and in some cases became nonpalpable. After 28 weeks on this diet, the remaining rats were transferred back to the high‐fat diet and subsequently showed a marked stimulation in tumor growth and development. This continued even after the rats were returned to the fat‐free diet 8 weeks later, indicating that the tumors were no longer susceptible to the deprivation of dietary fat. The results of this study provide further evidence that dietary fat affects the promotional stage of mammary carcinogenesis.