Red meat and colon cancer: the cytotoxic and hyperproliferative effects of dietary heme.

The intake of a Western diet with a high amount of red meat is associated with a high risk for colon cancer. We hypothesize that heme, the iron carrier of red meat, is involved in diet-induced colonic epithelial damage, resulting in increased epithelial proliferation. Rats were fed purified control diets, or purified diets supplemented with 1.3 micromol/g of hemin (ferriheme), protoporphyrin IX, ferric citrate, or bilirubin (n = 8/group) for 14 days. Feces were collected for biochemical analyses. Fecal cytotoxicity was determined from the degree of lysis of erythrocytes by fecal water. Colonic epithelial proliferation was measured in vivo using [3H]thymidine incorporation into colonic mucosa. The colonic epithelial proliferation in heme-fed rats was significantly increased compared to control rats [55.2 +/- 5.8 versus 32.6 +/- 6.3 dpm/microg DNA (mean +/- SE); P < 0.05]. The fecal water of the heme group was highly cytotoxic compared to the controls (90 +/- 2% versus 2 +/- 1%; P < 0.001), although the concentrations of cytotoxic bile acids and fatty acids were significantly lower. Organic iron was significantly increased compared to the controls (257 +/- 26 versus 80 +/- 21, microM; P < 0.001). Spectrophotometric analyses suggest that this organic iron is heme-associated. Thiobarbituric acid-reactive substances were greatly increased in the fecal water of heme-fed rats compared to the controls (177 +/- 12 versus 59 +/- 7 microM; P < 0.05). Heme itself could not account for the increased cytotoxicity because the addition of heme to the fecal water of the control group, which was equimolar to the organic iron content of the fecal water of the heme group, did not influence the cytotoxicity. Hence, an additional heme-induced cytotoxic factor is involved, which may be modulated by the generation of luminal-reactive oxygen species. Protoporphyrin IX, ferric citrate, and bilirubin did not increase proliferation and cytotoxicity. In conclusion, dietary heme leads to the formation of an unknown, highly cytotoxic factor in the colonic lumen. This suggests that, in heme-fed rats, colonic mucosa is damaged by the intestinal contents. This results in a compensatory hyperproliferation of the epithelium, which supposedly increases the risk for colon cancer.

Quercetin glucuronides but not glucosides are present in human plasma after consumption of quercetin-3-glucoside or quercetin-4'-glucoside.

The nature of quercetin conjugates present in blood after consumption of quercetin glucosides is still unclear. In this study, we analyzed plasma of volunteers that had consumed 325 micromol of either quercetin-3-glucoside or quercetin-4'-glucoside as an oral solution. Quercetin metabolites were extracted with acetonitrile/phosphoric acid and these extracts were analyzed using a high performance liquid chromatography with Coularray detection that distinguishes between the glucuronidated and the glucosylated forms of quercetin. No intact quercetin glucosides and only trace amounts of aglycone were found in human plasma, irrespective of the glucoside ingested. This was confirmed by spiking the plasma with glucoside standards. The major components in plasma had the same retention time as quercetin glucuronide standards. These plasma components disappeared after treatment of the plasma with bovine liver beta-glucuronidase, under reformation of quercetin, and showed the same oxidation pattern as the glucuronides. These results suggest that after consumption of quercetin glucosides, quercetin glucuronides are major metabolites in plasma.

Red meat and colon cancer: dietary haem-induced colonic cytotoxicity and epithelial hyperproliferation are inhibited by calcium.

High intake of red meat is associated with increased colon cancer risk. We have shown earlier that this may be due to the high haem content of red meat, because dietary haem increased cytolytic activity of faecal water and colonic epithelial proliferation. Dietary calcium inhibits diet-induced epithelial hyperproliferation. Furthermore, it has been shown that supplemental calcium inhibited the recurrence of colorectal adenomas. Therefore, we studied whether dietary calcium phosphate can exert its protective effects by inhibiting the deleterious effects of haem. In vitro, calcium phosphate precipitated haem and inhibited the haem-induced cytotoxicity. Subsequently, rats were fed diets, differing in haem (0 or 1.3 micromol/g) and calcium phosphate content only (20 or 180 micromol/g). Faeces were collected for biochemical analyses. Cytolytic activity of faecal water was determined from the degree of lysis of erythrocytes by faecal water. Colonic epithelial proliferation was measured in vivo using [(3)H]thymidine incorporation. In rats fed low calcium diets, dietary haem increased cytolytic activity of faecal water (98 +/- 1 versus 1 +/- 1%, P < 0.001) and the concentration of cations in faeces (964 +/- 31 versus 254 +/- 20 micromol/g), when compared with controls. This indicates that dietary haem increased colonic mucosal exposure to luminal irritants. Colonic epithelial proliferation was increased compared with controls (70 +/- 4 versus 48 +/- 8 d.p.m./microg DNA, P < 0.001). This was accompanied by metabolism of the ingested haem and solubilization of haem compounds in the faecal water. A high calcium diet largely prevented this metabolism and solubilization. It also inhibited the haem-induced cytolytic activity of faecal water and increase in faecal cation concentration. In accordance, the haem-induced colonic epithelial hyperproliferation was prevented. We therefore suggest that dietary calcium phosphate acts as a chemopreventive agent in colon carcinogenesis by inhibiting the cytolytic and hyperproliferative effects of dietary haem.

Red meat and colon cancer: dietary haem, but not fat, has cytotoxic and hyperproliferative effects on rat colonic epithelium.

High intake of red meat is associated with an increased risk of colon cancer. It has been suggested that fat from red meat is responsible, because high fat intake increases the concentration of cytotoxic lipids in the colon. Experimental studies have not unequivocally supported such a role for fat, however. Recently, we showed that dietary haem, which is abundant in red meat, increased colonic cytotoxicity and epithelial proliferation. In this study, we wanted to clarify whether dietary fat affects colon cancer risk by itself or by modulating the detrimental effects of haem on the colonic epithelium. Rats were fed control or haem-supplemented diets with 10%, 25% or 40% of the energy derived from fat for 14 days. Faeces were collected for biochemical analyses. Colonic cytotoxicity was determined from the degree of lysis of erythrocytes by faecal water. Colonic epithelial proliferation was measured in vivo using [(3)H]thymidine incorporation. Increasing the fat content of the control diets stimulated faecal disposal of both fatty acids and bile acids. It also increased the concentration of fatty acids, but not that of bile acids, in faecal water in control rats. The cytolytic activity of faecal water and colonic epithelial proliferation were unaffected. Dietary haem increased faecal cation content and cytolytic activity of faecal water at all fat levels, suggesting that the colonic mucosa was exposed to high amounts of luminal irritants. This effect was smaller in rats on the low-fat diet. Dietary haem also increased colonic epithelial proliferation at all fat levels. The haem-induced effects were independent of fatty acids or bile acids in the faecal water. In western societies, 30-40% of ingested energy is supplied by dietary fat, so our results suggest that the association between consumption of red meat and risk of colon cancer is mainly due to its haem content, and is largely independent of dietary fat content.