Influence of dietary protein sources on putative in vitro and in vivo colon cancer biomarkers
Colon cancer (cancer of the large intestine) is a worldwide problem in especially Western countries. The diet might be responsible for up to 90% of these colon cancer cases. This means that decreasing colon cancer risk should be possible by changing the diet. The research presented in this thesis concerns the question what the influence is of dietary protein sources on colon cancer risk. As described in Chapter1 , casein was compared to other dietary protein sources (mainly soy protein) for its influence on:the carcinogen scavenging capacity of proteins,early stages of colon cancer,fecal colon cancer biomarkers.Chapter2 provides information on various fields of study in the research on dietary proteins and colon cancer, including background information on the methods used. It is explained how dietary proteins can act as carcinogen scavengers by binding to carcinogens in the gastro-intestinal lumen. The carcinogen can be transported along the intestinal tract and be taken up by the body. If a carcinogen is not taken up, it will be excreted with the feces. Furthermore, the influence of the amount of protein intake in the diet on colon cancer risk is discussed. It is concluded that either no or a positive correlation between the amount of protein intake and colon cancer risk, exists. Another important research area is protein digestibility because a low protein digestibility results in a high amount of protein in the colon. Furthermore, the amino acid composition of a protein is important for two reasons. When an essential amino acid is present in a limiting amount, growth of both the individual and the tumor is inhibited. Secondly, some amino acids are needed for specific biological functions. For example, sulfur containing amino acids can stimulate the biotransformational system. The last important factor discussed in the research area of proteins on colon cancer risk is the presence of various non-protein components. As an example, saponins and isoflavones present in many soy proteins and known to be biologically active, are discussed.In Chapter 3 and 4 it is described how dietary proteins can scavenge carcinogens in vitro . In Chapter3 it is shown that carcinogen scavenging is influenced by many factors, such as heat treatment of the protein, degree of protein digestion and presence of bile acids or lipase. Protein type did not have a major influence so no effect of feeding different dietary protein sources on colon cancer risk is expected.In Chapter4 it is shown that the carcinogen scavenging capacity of proteins is also very much dependent on the type of carcinogen. Benzo[a]pyrene, a large non-reactive pro-carcinogen, interacted only with intact dietary proteins, whereas MNNG, as described in Chapter 3, strongly interacted with both intact as well as hydrolyzed proteins. Again no difference between dietary protein sources was shown, suggesting no difference in colon cancer risk.In Chapter 5 the first animal experiments are described. In Study 1, the influence of the addition of 1% methionine to the diet on colonic cell proliferation was tested. Methionine is an important methyl donor in many metabolic routes and is present in different amounts in casein and soy protein preparations. Colonic cell proliferation was used as an indication of colon cancer risk, because cell proliferation is an important phase in the process of colon cancer. The study showed no influence of methionine on colonic cell proliferation. In Study 2 rats were fed different amounts of soy non-nutrients to test the influence of these non-nutrients on several parameters in the feces. Most important fecal parameter was the fecal fat percentage, because earlier studies reported a correlation between high fecal fat and colon tumors. Study 2 showed that fecal fat excretion in rats is dependent on the amount of soy non-nutrients in the diet. However, the difference in fecal fat excretion between casein and soy protein was smaller (factor 2) than reported in literature. Therefore no major effect on colon cancer risk is expected. The influence of soy non-nutrients on processes in the colon was shown in Study 3. In this in vitro study, soy non-nutrients strongly reduced bile acid induced cell damage, suggesting a small protective effect of dietary soy protein.In Chapter 6 two animal studies on the influence of casein and soy protein on colonic cell proliferation are described. The studies showed no difference in colonic cell proliferation between these two protein sources. However different results for the casein and soy protein diets were obtained for several parameters that were measured in the feces of the animals. These fecal parameters were measured because they show how important steps in the process of colon cancer development are affected. A high fecal fat percentage correlates with a high number of tumors in the colon. Fecal alkaline phosphatase (ALP) activity represents damage to cells of the colonic epithelium because epithelial cells exhibited a high ALP activity. Bile acids and free fatty acids can damage the colonic epithelium because of their lytic potential. The cytolytic activity measures this damaging capacity, because it measures the lytic potential of the colonic contents towards cells in an in vitro test. The pH of the fecal contents was measured because a low pH represents a healthy colonic environment. Fecalb-glucuronidase andb-glucosidase activities were measured because they represent the capacity of the colonic contents to liberate toxicants in the colonic lumen that were earlier detoxified by the liver.Results on cytolytic activity confirmed the lack of difference between casein and soy on colonic cell proliferation because there was no difference between diets. Fecal fat excretion was doubled after feeding soy protein compared to casein. After feeding soy protein, fecal water bile acid concentrations and ALP activity were decreased. However,b-glucuronidase andb-glucosidase were increased 10-100 fold. Overall, no consistent conclusion on fecal parameters was possible.In Chapter 7 the influence of casein, soy protein isolate and soy flour on colonic aberrant crypt foci (ACF) is tested. ACFs are considered to be early stages of colonic tumors. Overall no consistent difference of casein and soy protein on aberrant crypt foci was detected. Againb-glucuronidase,b-glucosidase and ALP activities were measured in feces.b-Glucuronidase andb-glucosidase activities were significantly increased after feeding soy protein suggesting an increased risk after soy feeding.In Chapter 8 the influence of casein, soy protein isolate, soy protein flour and red meat is tested on the occurrence of intestinal polyps in the Apc Minmodel. In this model, mice with a defect Apc gene spontaneously develop polyps in both small and large intestine. The four diets showed no difference in the occurrence of polyps. Again several differences were observed in the fecal parameters.b-Glucuronidase andb-glucosidase were strongly increased after soy protein compared to the other diets. The pH of the colonic contents was decreased after soy flour feeding indicating fermentation of fibers present in soy flour preparations has a major influence. Furthermore it was shown that heme in the form of meat protein is not nearly as lytic as heme added to the diet in the pure form. No differences between the diets were found for free fatty acids. The concentrations of bile acids in fecal water were decreased for especially the soy protein diets. Overall, fecal parameters showed marked differences, but again no consistent protective or risk inducing effects.In Chapter 9 all studies performed are compared and discussed with an emphasis on the animal studies. Overall, fecal fat excretion was consistently doubled after feeding soy protein.b-Glucuronidase andb-glucosidase, able to release toxicants, were consistently increased after feeding soy protein. Bile acids in fecal water, able to damage colonic epithelium, were consistently decreased after soy protein feeding. Other fecal parameters either showed no difference between diets (pH, cytolytic activity) or variable results (ALP, free fatty acids). It was shown that fecal magnesium excretion has no predictive value for colonic cell proliferation. Fecalb-glucuronidase,b-glucosidase, ALP, bile acids and free fatty acids showed large differences in absolute values between animal experiments. Possibly the effect caused by some protein sources on fecal parameters, is also partly dependent on interaction with other dietary constituents such as fat and fiber or non-protein components. Because no differences on cell proliferation, aberrant crypt foci and intestinal polyps were found, a difference in colon cancer risk after consumption of casein or soy protein is unlikely. Correlation studies revealed that none of the fecal parameters tested consistently predicted the outcome of the colonic parameters tested, stressing the need for further research in this area. Because of the variable composition and results obtained with soy, it was concluded that one should be very cautious concerning the interpretation of studies in which soy protein preparations are used.The main conclusion from the studies performed is that many significant differences occurred between casein and other dietary protein sources such as soy. Differences specifially occurred on parameters related to the carcinogen scavenging capacity of proteins and on fecal parameters such as fecal bile acid, ALP andb-glucuronidase excretion. However, no consistent in vivo protective effect of casein occurred on colon cancer, based on markers as colonic cell proliferation, aberrant crypt foci formation and polyp formation. Therefore results do not support an advice on consuming either more or less casein or soy protein containing products.
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| Format: | Doctoral thesis biblioteca |
| Language: | English |
| Subjects: | biological indicators, caseinates, colorectal cancer, dietary protein, mice, rat feeding, soya protein, biologische indicatoren, caseïnaten, colorectaal kanker, muizen, rattenvoeding, sojaeiwit, voedingseiwit, |
| Online Access: | https://research.wur.nl/en/publications/influence-of-dietary-protein-sources-on-putative-in-vitro-and-in- |
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