Background: Diets that are high in protein but reduced in carbohydrate contents provide a common approach for achieving weight loss in obese humans. However, the effect of such diets on microbiota-derived metabolites that influence colonic health has not been established.

Epidemiological studies have identified increased colorectal cancer (CRC) risk with high red meat (HRM) intakes, whereas dietary fibre intake appears to be protective. In the present study, we examined whether a HRM diet increased rectal O⁶-methyl-2-deoxyguanosine (O⁶MeG) adduct levels in healthy human subjects, and whether butyrylated high-amylose maize starch (HAMSB) was protective. A group of twenty-three individuals consumed 300 g/d of cooked red meat without (HRM diet) or with 40 g/d of HAMSB (HRM+HAMSB diet) over 4-week periods separated by a 4-week washout in a randomised cross-over design. Stool and rectal biopsy samples were collected for biochemical, microbial and immunohistochemical analyses at baseline and at the end of each 4-week intervention period. The HRM diet increased rectal O⁶MeG adducts relative to its baseline by 21 % (P< 0·01), whereas the addition of HAMSB to the HRM diet prevented this increase. Epithelial proliferation increased with both the HRM (P< 0·001) and HRM+HAMSB (P< 0·05) diets when compared with their respective baseline levels, but was lower following the HRM+HAMSB diet compared with the HRM diet (P< 0·05). Relative to its baseline, the HRM+HAMSB diet increased the excretion of SCFA by over 20 % (P< 0·05) and increased the absolute abundances of theClostridium coccoidesgroup (P< 0·05), theClostridiumleptumgroup (P< 0·05),Lactobacillusspp. (P< 0·01),Parabacteroides distasonis(P< 0·001) andRuminococcus bromii(P< 0·05), but loweredRuminococcus torques(P< 0·05) and the proportions ofRuminococcus gnavus,Ruminococcus torquesandEscherichia coli(P< 0·01). HRM consumption could increase the risk of CRC through increased formation of colorectal epithelial O⁶MeG adducts. HAMSB consumption prevented red meat-induced adduct formation, which may be associated with increased stool SCFA levels and/or changes in the microbiota composition.

Deoxynivalenol (DON) is a potent mycotoxin produced by Fusarium molds and affects intestinal nutrient absorption and barrier function in experimental and farm animals. Free DON and the plant metabolite DON-3-β- d -glucoside (D3G) are frequently found in wheat and maize. D3G is stable in the upper human gut, but some human intestinal bacteria release DON from D3G in vitro . Furthermore, some bacteria derived from animal digestive systems degrade DON to a less toxic metabolite, deepoxy-deoxynivalenol (DOM-1). The metabolism of D3G and DON by the human microbiota has not been fully assessed. We therefore conducted in vitro batch culture experiments assessing the activity of the human fecal microbiota to release DON from D3G. We also studied detoxification of DON to DOM-1 by the microbiota and its potential effect on urinary DON excretion in humans. Fecal slurry from five volunteers was spiked with DON or D3G and incubated anaerobically (from 1 h to 7 days), and mycotoxins were extracted into acetonitrile. Mycotoxins were detected in fecal extracts and urine by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The fecal microbiota released DON from D3G very efficiently, with hydrolysis peaking after 4 to 6 h. The fecal microbiota from one volunteer transformed DON to DOM-1. Urine from the same volunteer also contained DOM-1 (4.7% of DON), whereas DOM-1 was not detectable in urine from other volunteers. Our results confirm that the fecal microbiota releases DON from its glycosylated form, hence increasing the toxic burden in exposed individuals. Furthermore, this is first evidence that the human fecal microbiota of one volunteer detoxifies DON, resulting in the appearance of DOM-1 in urine.

In this study, the modulation of aflatoxin B ₁ (AFB ₁ ) uptake in rats by administration of the probiotic Lactobacillus rhamnosus GG was demonstrated. Fecal AFB ₁ excretion in GG-treated rats was increased via bacterial AFB ₁ binding. Furthermore, AFB ₁ -associated growth faltering and liver injury were alleviated with GG treatment.

The probiotic Lactobacillus rhamnosus GG is able to bind the potent hepatocarcinogen aflatoxin B ₁ (AFB ₁ ) and thus potentially restrict its rapid absorption from the intestine. In this study we investigated the potential of GG to reduce AFB ₁ availability in vitro in Caco-2 cells adapted to express cytochrome P-450 (CYP) 3A4, such that both transport and toxicity could be assessed. Caco-2 cells were grown as confluent monolayers on transmembrane filters for 21 days prior to all studies. AFB ₁ levels in culture medium were measured by high-performance liquid chromatography. In CYP 3A4-induced monolayers, AFB ₁ transport from the apical to the basolateral chamber was reduced from 11.1% ± 1.9% to 6.4% ± 2.5% ( P = 0.019) and to 3.3% ± 1.8% ( P = 0.002) within the first hour in monolayers coincubated with GG (1 × 10 ¹⁰ and 5 × 10 ¹⁰ CFU/ml, respectively). GG (1 × 10 ¹⁰ and 5 × 10 ¹⁰ CFU/ml) bound 40.1% ± 8.3% and 61.0% ± 6.0% of added AFB ₁ after 1 h, respectively. AFB ₁ caused significant reductions of 30.1% ( P = 0.01), 49.4% ( P = 0.004), and 64.4% ( P < 0.001) in transepithelial resistance after 24, 48, and 72 h, respectively. Coincubation with 1 × 10 ¹⁰ CFU/ml GG after 24 h protected against AFB ₁ -induced reductions in transepithelial resistance at both 24 h ( P = 0.002) and 48 h ( P = 0.04). DNA fragmentation was apparent in cells treated only with AFB ₁ cells but not in cells coincubated with either 1 × 10 ¹⁰ or 5 × 10 ¹⁰ CFU/ml GG. GG reduced AFB ₁ uptake and protected against both membrane and DNA damage in the Caco-2 model. These data are suggestive of a beneficial role of GG against dietary exposure to aflatoxin.