Validation of a dual in vivo-in vitro assay for predicting the digestibility of nutrients in humans

Interactive Effects of Dietary Fiber and Lipid Types Modulate the Predicted Production and Absorption of Cecal and Colorectal Short-Chain Fatty Acids in Growing Pigs

BACKGROUND

High-fiber diets are supplemented with lipids to meet the required energy content, but data on the interactions between dietary fiber (DF) and lipid types on gastrointestinal fermentation in pigs are scant.

OBJECTIVES

This study aimed to use a combination of in vivo and in vitro fermentation methodologies to determine the interactive effects of DF and lipid types on short-chain fatty acid (SCFA) production and absorption and organic matter (OM) fermentability in the cecum and colorectal tract of pigs.

METHODS

Eight ileal- and cecal-cannulated Yorkshire barrows were fed either pectin- or cellulose-containing diets that were supplemented with either corn oil or beef tallow in 2 independent Youden squares with a 2 × 2 factorial arrangement of treatments (n = 6). Ileal and cecal digesta were collected, freeze-dried, and fermented using inoculum from fresh cecal digesta and feces, respectively, to determine individual SCFA production and absorption and fermentability of OM.

RESULTS

Interactions (P < 0.001) between DF and lipid types were observed in which the addition of beef tallow decreased the quantity of cecal and colorectal acetic acid production and cecal acetic absorption, cecal butyric production, predicted cecal OM fermentability, and predicted colorectal propionic acid in pectin diets, but the effects were not observed for cellulose diets. The addition of beef tallow increased (P < 0.001) the production of cecal butyric and propionic acids during in vitro fermentation in cellulose diets and apparent total tract digestibility (ATTD) of OM in pectin diets.

CONCLUSIONS

The interactions between DF and lipids on gastrointestinal fermentation largely depend on the degree of saturation of fatty acids in dietary lipids. The addition of beef tallow selectively decreased the production and absorption of individual SCFAs in pectin and cellulose diets but increased cecal butyric and propionic acid production in cellulose diets and the ATTD of OM in pectin diets.

Validation of a Combined In Vivo/In Vitro Ileal Fermentation Assay in the Growing Pig to be Used as a Model for Adult Humans

BACKGROUND

An in vivo/in vitro ileal fermentation assay using growing pigs has been developed but not yet formally validated.

OBJECTIVES

This study aimed to validate the in vivo/in vitro ileal fermentation assay by comparing in vitro fermentation values with those obtained in vivo in growing pigs. The effect of raising pigs under different environmental conditions was also investigated.

METHODS

Thirty piglets (1.59 ± 0.31 kg body weight, mean ± standard deviation) were subjected to 1 of 3 treatments: artificially reared (AR) (nonfarm, laboratory housing conditions) from postnatal day (PND) 7 (AR group), inoculated orally with human infant fecal extracts from birth until PND 8 and AR (AR+ group), or conventionally reared on a farm (control group). Starting at PND 7, the AR and AR+ pigs received human infant formula for 3 wk, followed by a human-type diet for 5 wk. Control pigs were weaned on the farm and, on PND 63, relocated to the laboratory animal facility. From PND 63, all pigs received a human-type diet. On PND 78, pigs were killed, after which ileal digesta were collected to perform an in vitro ileal fermentation (in vitro organic matter [OM] fermentability and organic acid production) and to determine digesta microbial composition and dietary OM fermentability in vivo.

RESULTS

The rearing regimen resulted in only a few differences in ileal microbial taxonomic composition. The rearing regimen generally did not affect the in vitro production of individual organic acids. The in vivo and in vitro OM fermentability of proximal ileal digesta (19.7 ± 2.04%; mean ± SEM) was similar (P > 0.05) for the AR and control pigs but not for the AR+ pigs.

CONCLUSIONS

The control-rearing regimen was preferred over AR or AR+ because of ease of implementation. The in vitro ileal fermentation assay accurately predicted the in vivo OM fermentability.

Type and Amount of Dietary Fiber Influence the Hindgut Synthesis of Organic Acids from Fermentable Material of Both Total and Nondietary Origin in a Pig Model of the Adult Human

BACKGROUND

Organic acid synthesis by the hindgut microbiota is commonly believed to be mainly of fermentable material of dietary origin.

OBJECTIVE

This study aimed to determine the hindgut organic acid synthesis from fermentable material of dietary (mainly fiber) or nondietary origin for different types and amounts of dietary fiber in growing pigs used as a model for adult humans.

METHOD

Seven fiber-containing diets were formulated: 4 fiber types (cellulose, gum acacia, oligofructose, and pectin) at 6% of the diet and 3 (gum acacia, oligofructose, and pectin) at 3% as the sole fiber source. Ileal cannulated female pigs (n = 14; Landrace/Large white) were fed the fiber-containing diets (n = 6 pigs/diet) for 11 days (fiber phase) followed by 3 days on a fiber-free diet (fiber-free phase), using a replicated Youden square. Ileal digesta for each phase were collected and fermented in vitro with a pooled fecal microbial inoculum prepared from feces collected during the fiber phase to determine the organic acids synthesized from fermentable material of dietary (fiber phase) and nondietary (fiber-free phase) origins.

RESULTS

The total amount of each individual organic acid synthesized during in vitro hindgut fermentation differed (P ≤ 0.05) across the types and amounts of dietary fiber intake. For example, the amount of acetate was 3.6-fold higher (P ≤ 0.05) for pigs fed the 6% pectin-containing diet than those fed the 6% oligofructose-containing diet. The nondietary substrate contributed between 36% (hexanoate) and 70% (succinate) to the total hindgut organic acid synthesis. The adaptation to the different fiber-containing diets led to different amounts of some organic acids of nondietary origin.

CONCLUSIONS

The total amount of organic acids synthesized in the hindgut by the resident microbes is influenced by the type and amount of dietary fiber consumed. This study quantifies the interaction between both dietary and nondietary fermentable materials in hindgut fermentation.

Biotransformation of Rutin in In Vitro Porcine Ileal and Colonic Fermentation Models

Quercetin, a polyphenol antioxidant, is widely distributed in food in the form of glycoside rutin, which is not readily absorbed in the gastrointestinal tract. The microbiota of the colon is known to biotransform rutin, generating quercetin aglycones that can be absorbed. We investigated the role of the ileal and colonic microbiota in rutin biotransformation using established in vitro fermentation models. Overall, a higher rate of rutin biotransformation was observed during colonic fermentation compared with ileal fermentation. The colonic microbiome showed higher potential for rutin conversion to quercetin through an increased abundance of α-rhamnosidase- and β-glucosidase-encoding genes compared to the ileal microbiome. Nonetheless, rutin metabolism occurred rapidly during ileal fermentation (∼20% rutin disappearance after 1 h). The appearance of quercetin varied depending on the ileal inoculum and correlated with an increased abundance of Firmicutes, suggesting that quercetin absorption could be improved via modulation of the ileal microbiota.

Sources of Dietary Fiber Affect the SCFA Production and Absorption in the Hindgut of Growing Pigs

Effects of different dietary fiber (DF) sources on short-chain fatty acids (SCFA) production and absorption in the hindgut of growing pigs were studied by an in vivo–vitro (ileal cannulated pigs and fecal inoculum-based fermentation) method. Thirty-six cannulated pigs (body weight: 48.5 ± 2.1 kg) were randomly allocated to 6 treatments containing the same DF content (16.5%), with either wheat bran (WB), corn bran (CB), sugar beet pulp (SBP), oat bran (OB), soybean hulls (SH), or rice bran (RB) as DF sources. Pigs were allowed 15 days for diet adaptation, and then, fresh ileal digesta and feces were collected to determine SCFA concentration which was normalized for food dry matter intake (DMI) and the hindgut DF fermentability. Fecal microbiota was inoculated into the freeze-dried ileal digesta samples to predict the ability of SCFA production and absorption in the hindgut by in vitro fermentation. The SH group had the largest concentration of total SCFA and propionate in ileal digesta and fecal samples of growing pigs (p < 0.05). Nonetheless, the predicted acetate, total SCFA production, absorption in the SBP group were the highest (p < 0.01), but the lowest in the OB group (p < 0.01) among all groups. Even SBP and OB group had a similar ratio of soluble DF (SDF) to insoluble DF (IDF). The CB group had high determined ileal and fecal butyrate concentration but the lowest butyrate production and absorption in the hindgut (p < 0.01). Overall, the source of DF had a great impact on the hindgut SCFA production and absorption, and SBP fiber had a great potential to increase hindgut SCFA production and absorption.

Ingestion of xylooligosaccharides during the suckling period improve the feed efficiency and hindgut fermentation capacity of piglets after weaning

Fiber ingestion during the suckling period is helpful for gut development and probiotic colonization. Xylooligosaccharides (Xos) and xylan (Xyl) were selected to investigate the effects of different polymerization degree fiber ingestion on the growth performance and microbiota fermentation capacity of pre- and post-weanling piglets. An in vitro fermentation trial was also conducted to verify the microbial fermentation capacity of weanling piglet fecal microbiota. Results showed that Xos and Xyl ingestion had no significant effect on the piglet body weight and D-lactate level in the plasma at 21 d during the suckling period. After weaning, piglets in the Xyl group had a lower average daily gain (ADG) (P < 0.05), vitro dry matter (DM) fermentability (P < 0.05) and activity of xylanase (P < 0.05) than the control and Xos groups. The Xos group had no significant difference in the ADG when compared with the control group, but a significantly lower feed conversion ratio (FCR) (P < 0.05) than the control group, which means a high feed efficiency in the Xos group. The highest carbohydrate digestion and absorption ability of fecal microbiota (P < 0.05) was found in the Xos group. Meanwhile, the Xos group had the highest butyrate production ability (P < 0.05) and activity of xylanase (P < 0.05) during in vitro fermentation. The ingestion of Xyl during the suckling period had negative effects on the feed efficiency and hindgut fermentation capacity of weanling piglets. Xylooligosaccharide ingestion to suckling piglets improves growth performance and feed efficiency after weaning through increasing the fermentation capacity of microbiota and fiber-degrading enzyme secretion.

Adaptation of intestinal fermentation over time in the growing pig is influenced by the amount of kiwi fruit consumed

The effect of kiwi fruit at two dietary levels on the adaptation of intestinal fermentation over time in the growing pig was studied. A semi-synthetic fibre-free diet and two semi-synthetic diets containing kiwi fruit as a model fibre source (133 or 266 g/kg (DM basis); 28 or 48 g fibre/kg) were formulated and the diets contained titanium dioxide as an indigestible marker. A total of fourteen ileal cannulated pigs (41 kg body weight) were fed the fibre-free diet for 7 d followed by either the low or high kiwi fruit-containing diets (n 7/diet) for a further 44 d. Ileal digesta and faeces were collected at five times throughout the study. Ileal digesta were fermented (in vitro) with a standard pooled human faecal inoculum, while fresh pig faeces were used as inocula to ferment in vitro a standard purified fibre. Observations were normalised for diet DM intake using the marker. The 16S ribosomal RNA gene copy number of ileal and total faecal bacteria were high for the high-kiwi fruit level diet (P<0·05). The ileal bacteria tended to decrease over time (P<0·1), while the faecal bacteria increased (P<0·05), at the same rate for both diets. The amounts of crude protein and insoluble dietary fibre entering the hindgut changed over time similarly for both diets, whereas for starch it changed only for the low kiwi fruit-containing diet (P<0·05). Changes over time were also observed for the predicted hindgut valeric acid production and butyric acid absorption (P<0·05). In conclusion, adaptational changes over time of some characteristics of intestinal fermentation depended on the dietary level of kiwi fruit.

Flaxseed meal and oat hulls supplementation: impact on predicted production and absorption of volatile fatty acids and energy from hindgut fermentation in growing pigs

A combination of in vivo (ileal cannulated pigs) and in vitro (fecal inoculum-based incubation) methodologies were used to predict the effects of dietary supplementation with soluble or insoluble dietary fiber on hindgut VFA production and absorption. Energy contribution from hindgut VFA and apparent ileal (AID) and total tract (ATTD) digestibility of energy and DM was also investigated. Twelve ileal cannulated Genesus barrows (initial BW: 35.1 ± 0.44 kg) were allocated to 1 of the 3 corn-soybean meal-based diets without (control), or with flaxseed meal (FM) or oat hulls (OH) in a 2-period cross-over design. Flaxseed meal and oat hulls were used as sources of soluble and insoluble fiber, respectively. In each period, 4 pigs were offered 1 of the 3 diets, for 12 d followed by fecal (day 13 and 14) and ileal digesta collection (day 15 and 16) (n = 8). Ileal digesta were collected, freeze-dried, and subjected to in vitro fermentation using fecal inoculum, to predict production and absorption of VFA and energy production, and digestibility of DM and energy. The quantity of acetic, propionic, butyric, and valeric acids produced by in vitro fermentation was higher (P < 0.05) for the diet containing flaxseed meal compared with the control and OH diets. The predicted quantity of VFA produced and absorbed in the hindgut was greater (P < 0.05) in pigs that consumed the FM diet than those fed the control or OH diet. Pigs fed the control diet had greater (P < 0.05) AID and ATTD of DM than pigs offered the OH or FM diet. The determined disappearance of DM was lower (P < 0.05) in pigs fed the control and OH diets than in pigs that consumed the FM diet. The quantity of digested energy in the upper gut was reduced (P < 0.05) more in pigs fed the OH diet than in pigs fed the FM diet. The consumption of the FM diet increased (P < 0.05) the quantity of digested energy, energy produced and absorbed from VFA in the hindgut, and the percentage contribution of VFA from fermentation to total tract digestible energy, compared with the control and OH diets. In conclusion, dietary supplementation with insoluble fiber from oat hulls reduced ileal digested energy more than soluble fiber from flaxseed meal. Addition of soluble fiber to pig diets increased the energy contribution from VFA produced by hindgut fermentation to the total tract digestible energy, compared with dietary addition of insoluble fiber.

Development of an In Vivo and In Vitro Ileal Fermentation Method in a Growing Pig Model

Background

Substantial microbial fermentation may occur mainly in the lower small intestine (SI) of human adults, but there is no established methodology to determine this.

Objective

The study aimed to develop a combined in vivo and in vitro methodology for ileal fermentation based on the pig as an animal model for digestion in human adults.

Methods

Several aspects of a combined in vivo/in vitro ileal fermentation assay were evaluated. Male 9-wk-old pigs (n = 30; mean ± SD body weight: 23 ± 1.6 kg) were fed a human-type diet (143, 508, 45, 49, and 116 g/kg dry matter diet of crude protein, starch, total lipid, ash, and total dietary fiber) for 15 d. On day 15, pigs were killed, and the last third of the SI was collected to prepare an ileal digesta-based inoculum. Terminal jejunal digesta (last 50 cm of the second third of the SI) were collected as substrate for the assay to test the form of substrate (fresh or freeze-dried), origin (location in jejunum or SI) of the substrate, storage of the inoculum, incubation time (1.2-6.8 h), pH of the medium, and inoculum concentration (6-26 mg inoculum/100 mg substrate).

Results

The group of donor pigs used to prepare the inoculum, form of the substrate, origin of the substrate, origin of the inoculum (location in the SI), storage of the inoculum, incubation time, and inoculum concentration did not influence the in vitro ileal organic matter (OM) fermentability (P > 0.05). The in vitro ileal OM fermentability decreased when the pH of the medium increased from 5.5 to 7.5 (31% to 28%; P ≤ 0.05). Predicted (in vivo/in vitro) apparent ileal OM digestibility was similar to the value measured in vivo.

Conclusions

Thirty-percent of the terminal jejunal digesta OM was fermented in the ileum. Fiber fermentation in the ileum can be studied using the optimized in vivo/in vitro ileal fermentation method.

Ileal Digesta Nondietary Substrates from Cannulated Pigs Are Major Contributors to In Vitro Human Hindgut Short-Chain Fatty Acid Production

BACKGROUND

It has been assumed that short-chain fatty acids (SCFAs) in the colon originate mainly from dietary fiber fermentation. However, SCFAs in the colon are also produced from the fermentation of nondietary material.

OBJECTIVES

We aimed to predict the relative contributions of dietary and nondietary substrates in the production of SCFAs with the use of a human fecal inoculum for diets containing kiwifruit as a model fiber.

METHODS

Terminal ileal digesta were collected from ileal-cannulated male pigs [n = 7; mean ± SD: 41.4 ± 2.98 kg body weight] adapted (44-d feeding) to diets containing either 25 g/kg dry matter (DM) of kiwifruit fiber (KFf) (25 KFf) or 50 g/kg DM of KFf (50 KFf) and then again after receiving a fiber-free diet (n = 14) for a further 7 d. Pigs were used as a model for adult humans for digestion in the upper digestive tract (mouth to the terminal ileum). The ileal digesta (either unfractionated or fractionated into crude mucin and microbial fractions) were fermented in vitro for 24 h with the use of a fresh human fecal inoculum to predict SCFA production in the human hindgut.

RESULTS

SCFAs of nondietary origin were the main source (65%) of total SCFAs predicted to be produced in the human hindgut. The contribution of SCFAs from KFf was only 26% of the total SCFAs, and that from total dietary material was 35%. The higher contribution of nondietary material to total predicted SCFA production was observed at both dietary fiber concentrations. Predicted SCFA intake from dietary fiber was 76 and 105 mmol/kg diet DM intake for the diets containing 25 KFf and 50 KFf, respectively, and from the nondietary substrates it was 178 and 280 mmol/kg diet DM intake, respectively.

CONCLUSION

A considerable proportion of the SCFAs produced in the human hindgut seems to be derived from the fermentation of nondietary substrates.

Kiwifruit fibre level influences the predicted production and absorption of SCFA in the hindgut of growing pigs using a combined in vivo-in vitro digestion methodology

Combined in vivo (ileal cannulated pig) and in vitro (faecal inoculum-based fermentation) digestion methodologies were used to predict the production and absorption of SCFA in the hindgut of growing pigs. Ileal and faecal samples were collected from animals (n 7) fed diets containing either 25 or 50 g/kg DM of kiwifruit fibre from added kiwifruit for 14 d. Ileal and faecal SCFA concentrations normalised for food DM intake (DMI) and nutrient digestibility were determined. Ileal digesta were collected and fermented for 38 h using a fresh pig faecal inoculum to predict SCFA production. The predicted hindgut SCFA production along with the determined ileal and faecal SCFA were then used to predict SCFA absorption in the hindgut and total tract organic matter digestibility. The determined ileal and faecal SCFA concentrations (e.g. 8·5 and 4·4 mmol/kg DMI, respectively, for acetic acid for the low-fibre diet) represented only 0·2-3·2 % of the predicted hindgut SCFA production (e.g. 270 mmol/kg DMI for acetic acid). Predicted production and absorption of acetic, butyric and propionic acids were the highest for the high-fibre diet (P0·05). In conclusion, determined ileal and faecal SCFA concentrations represent only a small fraction of total SCFA production, and may therefore be misleading in relation to the effect of diets on SCFA production and absorption. Considerable quantities of SCFA are produced and absorbed in the hindgut of the pig by the fermentation of kiwifruit.

A proposed food breakdown classification system to predict food behavior during gastric digestion

The pharmaceutical industry has implemented the Biopharmaceutics Classification System (BCS), which is used to classify drug products based on their solubility and intestinal permeability. The BCS can help predict drug behavior in vivo, the rate-limiting mechanism of absorption, and the likelihood of an in vitro-in vivo correlation. Based on this analysis, we have proposed a Food Breakdown Classification System (FBCS) framework that can be used to classify solid foods according to their initial hardness and their rate of softening during physiological gastric conditions. The proposed FBCS will allow for prediction of food behavior during gastric digestion. The applicability of the FBCS framework in differentiating between dissimilar solid foods was demonstrated using four example foods: raw carrot, boiled potato, white rice, and brown rice. The initial hardness and rate of softening parameter (softening half time) were determined for these foods as well as their hypothesized FBCS class. In addition, we have provided future suggestions as to the methodological and analytical challenges that need to be overcome prior to widespread use and adoption of this classification system. The FBCS gives a framework that may be used to classify food products based on their material properties and their behavior during in vitro gastric digestion, and may also be used to predict in vivo food behavior. As consumer demand increases for functional and "pharma" food products, the food industry will need widespread testing of food products for their structural and functional performance during digestion.