The VFA, also known as short-chain fatty acids, are produced in the gastrointestinal tract by microbial fermentation of carbohydrates and endogenous substrates, such as mucus. This can be of great advantage to the animal, since no digestive enzymes exist for breaking down cellulose or other complex carbohydrates. The VFA are produced in the largest amounts in herbivorous animal species and especially in the forestomach of ruminants. The VFA, however, also are produced in the lower digestive tract of humans and all animal species, and intestinal fermentation resembles that occurring in the rumen. The principal VFA in either the rumen or large intestine are acetate, propionate, and butyrate and are produced in a ratio varying from approximately 75:15:10 to 40:40:20. Absorption of VFA at their site of production is rapid, and large quantities are metabolized by the ruminal or large intestinal epithelium before reaching the portal blood. Most of the butyrate is converted to ketone bodies or CO2 by the epithelial cells, and nearly all of the remainder is removed by the liver. Propionate is similarly removed by the liver but is largely converted to glucose. Although species differences exist, acetate is used principally by peripheral tissues, especially fat and muscle. Considerable energy is obtained from VFA in herbivorous species, and far more research has been conducted on ruminants than on other species. Significant VFA, however, are now known to be produced in omnivorous species, such as pigs and humans. Current estimates are that VFA contribute approximately 70% to the caloric requirements of ruminants, such as sheep and cattle, approximately 10% for humans, and approximately 20-30% for several other omnivorous or herbivorous animals. The amount of fiber in the diet undoubtedly affects the amount of VFA produced, and thus the contribution of VFA to the energy needs of the body could become considerably greater as the dietary fiber increases. Pigs and some species of monkey most closely resemble humans, and current research should be directed toward examining the fermentation processes and VFA metabolism in those species. In addition to the energetic or nutritional contributions of VFA to the body, the VFA may indirectly influence cholesterol synthesis and even help regulate insulin or glucagon secretion. In addition, VFA production and absorption have a very significant effect on epithelial cell growth, blood flow, and the normal secretory and absorptive functions of the large intestine, cecum, and rumen. The absorption of VFA and sodium, for example, seem to be interdependent, and release of bicarbonate usually occurs during VFA absorption.(ABSTRACT TRUNCATED AT 400 WORDS)

Energy contributions of volatile fatty acids from the gastrointestinal tract in various species

For the characterization of organic substances in gas chromatography a number termed «retention index» is proposed. A simple relation exists between the retention index of a compound on a non-polar stationary phase and its boiling point.

Gas‐chromatographische Charakterisierung organischer Verbindungen. Teil 1: Retentionsindices aliphatischer Halogenide, Alkohole, Aldehyde und Ketone

https://web.iitd.ac.in/~arunku/files/CEL311_Y13/Adsorption%20Theory%20to%20practice_Dabrowski.pdf

Adsorption — from theory to practice

Volatile Fatty Acid Analyses of Blood and Rumen Fluid by Gas Chromatography

Solvophobic interactions in liquid chromatography with nonpolar stationary phases

Industry has long used charcoal or other solid absorbents in columns for cleaning gas streams or for solvent recovery, and more recently Turner (1943), Claesson (1946), Glueckauf, Barker & Kitt (1949), Phillips (1949) and Turkel'taub (1950) have used charcoal in gas chromatograms for the analysis of hydrocarbons and esters. Gas-liquid scrubbing columns have been used in industry for many years but, though Martin & Synge (1941) suggested the use of gas-liquid partition chromatograms for analytical purposes, no work has been reported along these lines. This paper presents modifications necessary to the theory of Martin & Synge (1941) to allow for the compressibility of the mobile phase and describes the application ofthe gas-liquid partition chromatogram to the separation of volatile fatty acids. The separations obtainable by this method are essentially parallel to those obtainable by distillation, but good separations can be achieved much more easily and it is possible to work with very much smaller quantities. In fact, the lower limit of quantity of material used is determined only by the efficiency of detection. In general, far less trouble from azeotrope formation is to be expected, since the concentrations of the substances to be separated in the liquid phase are always low and it may be possible to choose a liquid phase which associates only with one component of the azeotrope. Work on a preparative scale, though theoretically possible, is likely to be inconvenient because of the bulk of the apparatus. The method of detection described here is acid-base titration, but many methods of detecting changes in the composition ofa gas stream could be used and it is intended in future publications to explore some of these, which should extend the range of application to all substances capable of being distilled at the pressure of a few mm. of mercury. In suitable cases the gas-liquid partition column has two principal advantages over the ordinary liquid-liquid partition column: (a) the low viscosity of the mobile phase allows relatively longer columns to be used with a corresponding gain in efficiency and (b) in general it is easier to detect changes in composition of a gas than of a liquid stream. A THEORY OF GAS-LIQUID PARTITION CHROMATOGRAPHY

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Gas-liquid partition chromatography: the separation and micro-estimation of volatile fatty acids from formic acid to dodecanoic acid

A new form of chromatogram employing two liquid phases: A theory of chromatography. 2. Application to the micro-determination of the higher monoamino-acids in proteins.

Gas-liquid chromatography: the separation and identification of the methyl esters of saturated and unsaturated acids from formic acid to n-octadecanoic acid

In a previous paper (PopjAk, French & Folley, 1951) some results of an experiment carried out on a lactating goat with the aid of CH314CO2Na were described. It was found that, in the origin of milk fat, synthesis within the udder from acetate played an outstanding part. This conclusion was based on a comparison of the specific activities of four crude fatty-acid fractions obtained from milk glycerides with those of the plasma fatty acids after a single injection of CH314CO2Na. During the first 12 hr. of the experiment the steam-volatile acids insoluble in water (consisting mainly of C8-C12 acids) had the highest specific activities, the next in order being the water-soluble steam-volatile acids (C4-C8 acids). These were followed by the specific activity of the non-volatile solid and liquid acids, the latter having the lowest specific activities. From 12 hr. onwards, however, the specific activity-time curve of the non-volatile solid acids gradually crossed over the specific activity-time curves of the two steam-volatile fractions so that by the 23rd hour of the experiment the specific activities of the four fractions were in the following order: non-volatile solid acids> waterinsoluble steam-volatile acids> water-soluble steamvolatile acids> non-volatile liquid acids. This same order was maintained until the end ofthe experiment (48 hr.) (cf. Popjak et al. 1951, Fig. 3). The specific activity-time curves of all four fractions, after reaching their maxima 3-4 hr. after the injection of labelled acetate, decayed smoothly with an average half-life of about 4 hr. The similarity of the half-lives suggested that the different specific activities ofthe fractions were in somewayconnected with the biochemical mechanism of fatty-acid synthesis rather than with any other factor (e.g. differences in turnover time, dilution with nonisotopic blood fat). Since the specific activities of these mixtures of fatty-acid samples bore no simple relationship to the average chain length of their component acids, it was pointed out that resolution into individual acids was required in order to interpret more fully the role of acetate in milk-fat synthesis. In this paper the results of complete fractionations of the fatty-acid samples obtained in the previously * Member of the staff of the National Institute for Research in Dairying, University of Reading, at present working at the National Institute for Medical Research. described experiment are reported together with the chemical degradations of acetic, butyric and caproic acids, in which the position of the '4C-label in individual carbon atoms of the fatty-acid chain has been established. It would have been a gargantuan task to resolve into individual acids the four crude fatty-acid fractions from twenty-two milk samples. If the specific activities ofthe four fractions hadmaintained the same order relative to one another throughout the experiment, it would have been justifiable to pool the samples from the entire experiment for the isolation of the individual acids. However, because of the change in the relative position of the specific activity-time curves at 12 hr. after the injection, it was decided to divide the acids into two main groups, (1) those obtained up to 12 hr. and (2) those between 12 and 48 hr. of the experiment. Thus during the first period the fatty acids had their highest specific activities and during the second period the specific activities of all the fractions were already declining. In addition to these two main parts of the experiment, some results obtained on two small samples of the water-soluble volatile acids, one representing 0-7 hr. and the other approximately the entire experimental period, are also presented. It isnow apparent that, although acetate is amajor source of carbon for milk fatty-acid synthesis, the results strongly suggest that another precursor, which we believe to be a C4 compound, derived from the blood, must also be involved. As a result of the participation of this second precursor of low isotope content, it has been possible to show by chemical degradations that the synthesis of fatty acids must occur by the stepwise elongation of a shorter acid at the carboxyl end with a C2 compound and not by the simultaneous condensation of C2 units. Some of these results have been published in a preliminary form (French, Hunter, Martin & Popjak, 1951).

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Mode of formation of milk fatty acids from acetate in the goat

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A. J. P. Martin

Papers

Gas-liquid partition chromatography: the separation and micro-estimation of volatile fatty acids from formic acid to dodecanoic acid

A new form of chromatogram employing two liquid phases: A theory of chromatography. 2. Application to the micro-determination of the higher monoamino-acids in proteins.

Gas-liquid chromatography: the separation and identification of the methyl esters of saturated and unsaturated acids from formic acid to n-octadecanoic acid

Mode of formation of milk fatty acids from acetate in the goat

Gas-liquid partition chromatography: the separation and micro-estimation of ammonia and the methylamines.