A gas chromatography/electron ionization-mass spectrometry-selected ion monitoring method for determining the fatty acid pattern in food after formation of fatty acid methyl esters

Effect of Fatty Acid Unsaturation on Phytosteryl Ester Degradation

This study examined the thermo-oxidative degradation of stigmasterol fatty acids esters. Stigmasterol stearate, oleate, linoleate and linolenate were synthesized by chemical esterification and their purity evaluated by ¹H-NMR and GC-MS. The degradation of stigmasterol esters was examined after heating them at 60 and 180 °C for 1, 2, 4, 8 and 12 h. It was established that stigmasterol esters were prone to thermo-oxidative degradation, with time and temperature affecting the degree of degradation. The unsaturation of fatty acids affected the rate of stigmasteryl ester degradation. The kinetics of StS and StO degradation were similar and the additional double bonds in StL and StLn resulted in their faster decomposition. The esters degraded faster at 180 than at 60 °C. The sterol and fatty acid molecules degraded at different rates, such that the fatty acid moiety deteriorated faster than the sterol at both temperatures, independent of the time of heating and the level of unsaturation.

Quantification of fatty acids in the muscle of Antarctic fish Trematomus bernacchii by gas chromatography-mass spectrometry: Optimization of the analytical methodology

This work presents data on the quantification of fatty acids (FAs, in terms of mass unit per tissue weight) in the muscle of Trematomus bernacchii, a key species in Antarctica, often used as bioindicator for contamination studies. Modifications in fatty acids content should be considered a useful biomarker to study how contaminants affect Antarctic biota. Until now, very few studies quantified fatty acids of muscle of T. bernacchii, and only as percentage of a single fatty acid on total lipids. To perform the quantification of fatty acids, we used an analytical method based on a fast microwave-assisted extraction of lipids from a lyophilized sample, a base-catalyzed trans-esterification of lipid extract to obtain Fatty Acids Methyl Esters (FAMEs), and a separation and identification of FAMEs by gas chromatography-mass spectrometry. With the optimized and validated method, a fast and accurate separation of Fatty Acids Methyl Esters was performed in 43 min. The linearity was checked up to about 320 μg mL^-1; limit of detection and limit of quantification are in the range 4-22 μg mL^-1 and 13-66 μg mL^-1, respectively. The optimized method showed a good accuracy and precision. Major fatty acids were 14:0, 16:0, 16:1n7, 18:1n9, 18:1n7, 20:1n9, 20:5n3 and 22:6n3. Quantified FAs compute for about 47 mg g^-1 tissue dry weight (dw), with 9.1 ± 0.1 mg g^-1 dw of saturated FAs, 25.5 ± 0.1 mg g^-1 dw of mono-unsaturated FAs, and 12.2 ± 0.1 mg g^-1 dw of poly-unsaturated FAs.

Unexpected Formation of Low Amounts of (R)-Configurated anteiso-Fatty Acids in Rumen Fluid Experiments

Anteiso-fatty acids (aFA) with odd carbon number are a class of branched-chain fatty acids (BCFA) mainly produced by bacteria. Bacterial sources are also made responsible for their occurrence in the low percent-range in lipids of ruminants (meat and milk) and fish. aFAs are chiral molecules and typically occur predominantly in form of (S)-enantiomers, and their primary precursor has been noted to be isoleucine. Yet, low proportions of (R)-aFAs were also detected in fish and cheese samples. Here we investigated the potential formation of (R)-aFAs by means of incubation experiments with rumen fluid from fistulated cows. Supplementation of rumen fluid with both L- and DL-isoleucine, resulted in a significant (α <0.05) increase of the aFA concentrations but in both cases enantiopure (S)-aFAs were observed. By contrast, incubations without addition of any isoleucine lead to a significant (α <0.05) formation of small proportions of (R)-aFAs similarly to those previously observed in fish and cheese. These results were consistently reproduced in three different years with rumen fluid from different cows fed different diets. All findings point to the existence of a further biosynthesis pathway of aFAs with different stereospecificity than the classic one using isoleucine as primer.

Metabolomics reveals significant variations in metabolites and correlations regarding the maturation of walnuts (Juglans regia L.)

The content of walnut metabolites is related to its nutritive value and physiological characteristics, however, comprehensive information concerning the metabolome of walnut kernels is limited. In this study we analyzed the metabolites of walnut kernels at five developmental stages from filling to ripening using GC-MS-based untargeted metabolomics; of a total 252 peaks identified, 85 metabolites were positively identified. Further statistical analysis revealed that these 85 metabolites covered different types of metabolism pathways. PCA scores revealed that the metabolic compositions of the embryo are different at each stage, while the metabolic composition of the endotesta could not be significantly separated into distinct groups. Additionally, 7225 metabolite-metabolite correlations were detected in walnut kernel by a Pearson correlation coefficient approach; during screening of the calculated correlations, 463 and 1047 were determined to be significant with r²≥0.49 and had a false discovery rate (FDR) ≤0.05 in endotesta and embryo, respectively. This work provides the first comprehensive metabolomic study of walnut kernels and reveals that most of the carbohydrate and protein-derived carbon was transferred into other compounds, such as fatty acids, during the maturation of walnuts, which may potentially provide the basis for further studies on walnut kernel metabolism.

Summary: This work provides a comprehensive metabolomic study of walnut kernels and reveals most of the carbohydrate and protein-derived carbons were transferred into other compounds during the maturation of walnuts.

anteiso-Fatty Acids in Brussels Sprouts (Brassica oleracea var. gemmifera L.): Quantities, Enantioselectivities, and Stable Carbon Isotope Ratios

anteiso-Fatty acids (aFAs) are a class of branched-chain fatty acids that are characterized by one methyl branch on the antepenultimate carbon of the straight acyl chain. aFAs are mainly produced by bacteria, and sources in vegetables are scarce. This study reports the concentrations of odd-numbered aFAs (a15:0-a21:0) in Brussels sprout buds. Selective enrichment followed by enantioselective gas chromatography with mass spectrometry in the selected ion monitoring mode revealed that both a15:0 and a17:0 were (S)-enantiopure in Brussels sprout samples. δ(13)C values (‰) of a17:0 in Brussels sprouts were comparable with those of palmitic acid, indicating no different source for both fatty acids.

Detailed analyses of fresh and dried maqui (Aristotelia chilensis (Mol.) Stuntz) berries and juice

In this study, a detailed chemical characterization of nutritionally-relevant, quality-determining constituents in dried and fresh fruits as well as juices of maqui (Aristotelia chilensis (Mol.) Stuntz) is provided. A total of 8 glycosylated anthocyanins was characterized in maqui fruits, being composed of differently substituted cyanidin and delphinidin derivatives. During processing into juice, a substantial loss in total anthocyanin contents (TAC) was observed. TAC values were also reduced after drying of maqui berries. Likewise, the browning index (BI) of fresh fruits increased during processing. Being composed of flavonol glycosides and ellagic acids, 17 non-anthocyanin phenolics were characterized in all maqui samples. Besides characterizing phenolic compounds, antioxidant activities, total phenolics, major sugars, non-volatile organic acids, minerals and trace elements were quantitated. Moreover, total lipid contents and the fruits' mainly unsaturated fatty acid profiles are reported. The presented results indicate the high potential of maqui as so far under-utilized but extremely pigment-rich "superfruit".

Profiling of fatty acids composition in suet oil based on GC-EI-qMS and chemometrics analysis

Fatty acid (FA) composition of suet oil (SO) was measured by precolumn methylesterification (PME) optimized using a Box-Behnken design (BBD) and gas chromatography/electron ionization-quadrupole mass spectrometry (GC-EI-qMS). A spectral library (NIST 08) and standard compounds were used to identify FAs in SO representing 90.89% of the total peak area. The ten most abundant FAs were derivatized into FA methyl esters (FAMEs) and quantified by GC-EI-qMS; the correlation coefficient of each FAME was 0.999 and the lowest concentration quantified was 0.01 μg/mL. The range of recovery of the FAMEs was 82.1%-98.7% (relative standard deviation 2.2%-6.8%). The limits of quantification (LOQ) were 1.25-5.95 μg/L. The number of carbon atoms in the FAs identified ranged from 12 to 20; hexadecanoic and octadecanoic acids were the most abundant. Eighteen samples of SO purchased from Qinghai, Anhui and Jiangsu provinces of China were categorized into three groups by principal component analysis (PCA) according to the contents of the most abundant FAs. The results showed SOs samples were rich in FAs with significantly different profiles from different origins. The method described here can be used for quality control and SO differentiation on the basis of the FA profile.

Emerging importance of omega-3 fatty acids in the innate immune response: molecular mechanisms and lipidomic strategies for their analysis

The beneficial health properties of dietary omega-3 polyunsaturated fatty acids, particularly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) have long been known and their metabolic dysfunction has been linked to a range of diseases including various inflammatory disorders, cardiovascular diseases, and cancer. However, the molecular mechanisms underlying their health benefits have remained unclear. Recent technological advances in lipidomic analytical strategies have resulted in the discovery of a range of bioactive mediators derived from EPA and DHA that possess potent anti-inflammatory and pro-resolving properties and that may be responsible, at least in part, for the beneficial effects observed. These mediators include resolvins, protectins and maresins, as well as EPA derivatives of classical arachidonic acid derived eicosanoids, such as prostaglandin E3 . The aim of this review is to provide an overview of the biosynthetic pathways and biological properties of these omega-3 mediators, with a particular focus on the emerging importance of the counter-regulatory role of omega-3 and -6 fatty acids in the spatial and temporal regulation of the inflammatory response. It will also provide an insight into a range of lipidomic approaches, which are currently available to analyse these fatty acids and their metabolites in biological matrices.

Total Serum Fatty Acid Analysis by GC-MS: Assay Validation and Serum Sample Stability

Analysis of n3 fatty acids in serum samples has clinical applications in supplementation trials, but the analysis can be challenging due to low levels, stability issues and intra-individual variation. This study presents the single laboratory validation of a gas chromatographic-mass spectral (GC-MS) assay for analysis of fatty acid methyl esters (FAME) using sensitive single ion monitoring and provides data on fatty acid stability under different sample handling conditions. Recovery of total fatty acids from serum with Folch extraction was optimized and parallelism tests with spiked samples indicated that the serum matrix did not interfere with mass spectral quantitation. Precision and accuracy of the assay at the lowest limit of quantitation and at low, medium and high levels met with accepted guidelines for single laboratory validation. Several storage conditions that can be encountered with clinical samples also were evaluated for impact on fatty acid levels in serum. Serum from blood that was stored refrigerated for 3 days yielded similar results as serum that was prepared and frozen at -80°C immediately. Serum storage at room temperature for 3-24 hours and serum subjected to one freeze/thaw cycle had minimal effects on fatty acid levels. The intra-individual variability in pregnant women was reasonably small, with significant correlation coefficients ranging from 0.35 to 0.76 for blood drawn between 12-20 weeks versus 34-36 weeks of gestation. These results indicate that GC-MS with single ion monitoring is valid for the analysis of total fatty acids in clinical samples, even when blood processing cannot be performed in a timely manner.

Mass spectra of methyl esters of brominated fatty acids and their presence in soft drinks and cocktail syrups

RATIONALE

Brominated vegetable oil (BVO) is frequently used as a solubility transmitter in soft drinks. Being banned in Europe and Japan but permitted in the United States and Canada, there is a need for analytical methods suitable for use in food control. Brominated fatty acids in BVO are usually determined by gas chromatography (GC) after their conversion into the corresponding methyl esters.

METHODS

GC with mass spectrometry (MS) was used to record the electron ionization (EI) and negative ion chemical ionization (NICI) mass spectra of relevant brominated fatty acid methyl esters synthesized for this purpose. Brominated fatty acids obtained from transesterified BVO from soft drink and syrup samples were also analyzed.

RESULTS

GC/NICI-MS was the most sensitive method for the detection of brominated fatty acids but GC/EI-MS was found to be more suited for quantification due to the formation of more selective fragment ions in the higher mass range. Suitable ions were selected for determination of the methyl esters of brominated fatty acids in the selected ion monitoring (SIM) mode. Artifacts produced by the transesterification of BVO with boron trifluoride were observed and discussed. BVO was also quantified in three syrup samples commercially produced for use in cocktails/long drinks. In one of the syrup samples that tested positive, BVO was not labelled in the ingredient list. Bromination experiments produced evidence that one or more Br2 -18:0 isomers identified as a shoulder peak of threo-9,10-dibromooctadecanoic acid in several soft drink and syrup samples originated from the bromination of partly hydrogenated plant oil.

CONCLUSIONS

BVO was determined for the first time in syrup samples. Attention should be paid to the problem of BVO occurring unlabeled in soft drinks and cocktail syrups imported from North America.

A comprehensive method for determination of fatty acids in the initial oral biofilm (pellicle)

The acquired pellicle is a tenacious organic layer covering the surface of teeth, protecting the underlying dental hard tissues. Lipids account for about one quarter of the pellicle's dry weight and are assumed to be of considerable importance for their protective properties. Nevertheless, only preliminary information is available about the nature of lipids in the pellicle. Gas chromatography coupled with electron impact ionization mass spectrometry was used to establish a convenient analytical protocol in order to obtain a qualitative and quantitative characterization of a wide range of FAs (C(12)-C(22)). In situ biofilm formation was performed on bovine enamel slabs mounted on individual splints carried by 10 subjects. A modified Folch extraction procedure was adopted to extract the lipids from the detached pellicle, followed by transesterification to fatty acid methyl esters using methanol and concentrated hydrochloric acid. Tridecanoic and nonadecanoic acid were used as internal standards suitable and reliable for robust, precise and accurate measurements. The present study demonstrates, for the first time, a procedure based on a combination of innovative specimen generation and convenient sample preparation with sensitive GC-MS analysis for the determination of the fatty acid profile of the initial oral biofilm.

Analysis of fatty acids in 12 Mediterranean fish species: advantages and limitations of a new GC-FID/GC-MS based technique

When fatty acids in fish are analyzed, results in percentage form (profile analysis) are mostly reported. However, the much more useful results expressed as mg/100 g (absolute analysis) is the main information required. Absolute methods based on calibration curves are of good accuracy but with a high degree of complexity if applied to a great number of analytes. Procedures based on the sequence profile analysis-total FA determination-absolute analysis may be suitable for routine use, but suffer from a number of uncertainties that have never been really resolved. These uncertainties are mainly related to the profile analysis. In fact, most profile analyses reported in the literature disagree about the number and type of fatty acids monitored as well as about the total percentage to assign to their sum so leading to possible inaccuracies; in addition the instrumental response factor for all FAME (fatty acid methyl esters) is often considered as a constant, but this is not exactly true. In this work, a set of 24 fatty acids was selected and studied on 12 fish species in the Mediterranean area (variable in lipid content and month of sampling): in our results, and in these species, this set constitutes, on average, 90 ± 3 % of the total fatty acid content. Moreover the error derived from the assumption of a unique response factor was investigated. Two different detection techniques (GC-FID and GC-MS) together with two capillary columns (different in length and polarity) were used in order to acquire complementary data on the same sample. With the protocol here proposed absolute analyses on the 12 cited species are easily achievable by the total FA determination procedure. The accuracy of this approach is good in general, but in some cases (DHA for example) is lower than the accuracy of calibration-based methods. The differences were evaluated on a case by case basis.

Quantitation of cis- and trans-monounsaturated fatty acids in dairy products and cod liver oil by mass spectrometry in the selected ion monitoring mode

Monounsaturated fatty acids (MUFAs) are important constituents of dietary fats. While cis-configurated isomers belong to the classic fatty acids of food, trans-fatty acids are suspected to pose a risk to human health. In either case, a thorough assessment of both positional and geometrical isomers of MUFAs is an important task in food and life sciences. For this purpose, a method suitable for routine analysis was developed. After lipid extraction and conversion of fatty acids into corresponding fatty acid methyl esters, cis and trans isomers of MUFAs were separated on silver-ion-impregnated cartridges. Fractions containing either cis- or trans-MUFAs were determined by gas chromatography-electron ionization mass spectrometry in the selected ion monitoring (GC/EI-MS-SIM) mode using [M-32](+) as quantification ions and [M-74](+) as well as M(+) as qualifier ions. A total of 14 MUFAs were available as reference standards, but a total of 40 MUFAs (22 cis and 18 trans isomers) were identified with high selectivity in samples of cheese, goat fat, human milk, and cod liver oil. The 18:1 and 16:1 isomers contributed most to both the cis- and trans-MUFAs. Application of internal standards allowed for the quantification of MUFAs only in the food samples. The amount of trans-fatty acids was determined to be 0.9- 4.3 g/100 g, with the lowest levels in human milk fat and the highest levels in Roquefort cheese. After exclusion of oleic acid, the concentrations of trans- and cis-MUFAs were at the same level in samples from ruminants and human milk fat.

Enantioselective analysis of 2- and 3-hydroxy fatty acids in food samples

2-Hydroxy fatty acids (2-OH-FAs) and 3-hydroxy fatty acids (3-OH-FAs) were recently identified at trace levels in dairy products and other food samples (vegetable oils and animal brains). Due to the asymmetric carbon bearing the hydroxy group, they are chiral. This study focused on the enantioselective determination of 2- and 3-OH-FAs in food. For this purpose, extracted saponifiable lipids were converted into methyl esters, and the resulting fatty acid methyl esters (FAMEs) were separated into OH-FAMEs (minor fraction) and non-OH-FAMEs (bulk fraction). OH-FAMEs were then derivatized with (R)-(-)-alpha-methoxy-alpha-trifluoromethylphenylacetyl chloride [(R)-(-)-MTPA-Cl, Mosher's reagent] to produce the corresponding MTPA-O-FAMEs. MTPA-O-FAME diastereomers were then analyzed by gas chromatography with electron capture negative-ion mass spectrometry (GC/ECNI-MS) in the selected ion monitoring (SIM) mode. In the food samples, both (S)- and (R)-enantiomers of 2- and 3-OH-FAs were detected, with the (R)-enantiomer being enantiopure or predominant with one exception. Especially 2- and 3-OH-16:0 were found to contain relevant proportions of the (S)-enantiomer. The differences in enantiomeric composition of 2- and 3-OH-16:0 detected for cheese samples were proposed as markers for authenticity controls.

Gas chromatography/electron-capture negative ion mass spectrometry for the quantitative determination of 2- and 3-hydroxy fatty acids in bovine milk fat

2- and 3-hydroxy fatty acids (2- and 3-OH-FAs) are bioactive substances reported in sphingolipids and bacteria. Little is known of their occurrence in food. For this reason, a method suitable for the determination of OH-FAs at trace levels in bovine milk fat was developed. OH-FAs (and conventional fatty acids in samples) were converted into methyl esters and the hydroxyl group was derivatized with pentafluorobenzoyl (PFBO) chloride to give PFBO- O-FA methyl esters. These derivatives with strong electron affinity were determined by gas chromatography interfaced to mass spectrometry using electron-capture negative ion in the selected ion monitoring mode (GC/ECNI-MS-SIM). This method proved to be highly sensitive and selective for PFBO-O-FA methyl esters. For the analysis of samples, two internal standards were used. For this purpose, 9,10-dideutero-2-OH-18:0 methyl ester (ISTD-1) from 2-OH-18:1(9 c) methyl ester as well as the ethyl ester of 3-PFBO-O-12:0 (ISTD-2) was synthesized. ISTD-1 served as a recovery standard whereas ISTD-2 was used for GC/MS measurements. The whole-sample cleanup consisted of accelerated solvent extraction of dry bovine milk, addition of ISTD 1, saponification, conversion of fatty acids into methyl esters by use of boron trifluoride, separation of the methyl esters of OH-FAs from nonsubstituted FAs on activated silica, conversion of OH-FAs methyl esters into PFBO-O-FA methyl esters, addition of ISTD-2, and measurement by GC/ECNI-MS-SIM. By this method, ten OH-FAs were quantified in bovine milk fat with high precision in the range from 0.02 +/- 0.00 to 4.49 +/- 0.29 mg/100 g of milk fat.

Stable carbon isotope ratios of methyl-branched fatty acids are different to those of straight-chain fatty acids in dairy products

Methyl-branched fatty acids (MBFAs) are the dominant form of fatty acid found in many bacteria. They are also found at low levels in a range of foodstuffs, where their presence has been linked to bacterial sources. In this study we evaluated the potential of compound-specific isotope analysis to obtain insights into the stable carbon isotope ratios (delta(13)C values in per thousand) of individual MBFAs and to compare them to the stable carbon isotope ratios of straight-chain fatty acids in food. Due to their low abundance in foodstuffs, the MBFAs were enriched prior to gas chromatography coupled to isotope ratio mass spectrometric (GC-IRMS) analysis. After transesterification, urea complexation was used to suppress the 16:0 and 18:0 methyl esters that were dominant in the samples. Following that, silver-ion high performance liquid chromatography was used to separate the saturated from the unsaturated fatty acids. The resulting solutions of saturated fatty acids obtained from suet, goat's milk, butter, and human milk were studied by GC-IRMS. The delta(13)C values of fatty acids with 12-17 carbons ranged from -25.4 per thousand to -37.6 per thousand. In all samples, MBFAs were most depleted in carbon-13, followed by the odd-chain fatty acids 15:0 and 17:0. 14:0 and 16:0 contained the highest proportions of carbon-13. The results from this study illustrate that MBFAs have distinctive delta(13)C values and must originate from other sources and/or from very different substrates. These measurements support the initial hypothesis that delta(13)C values can be used to attribute MBFAs to particular sources.

Enantioselective Determination of Anteiso Fatty Acids in Food Samples

Anteiso fatty acids (aFAs)-long-chain carboxylic acids with a methyl branch on the (n - 2)-carbon-are among the most simple fatty acids that are chiral. The most frequently occurring aFAs in food are 12-methyltetradecanoic acid (a15:0) and 14-methylhexadecanoic acid (a17:0), structures where the asymmetric carbon is more than 10 carbons separated from the polar head group. Previously, only enantioseparation of 4-methyl-substituted carboxylic fatty acids has been reported by gas chromatography. Here we present the first direct partial enantioresolution of synthesized racemic a15:0-a17:0 on a capillary column coated with 50% heptakis(6-O-tert-butyldimethylsilyl-2,3-di-O-methyl)-beta-cyclodextrin diluted in OV1701. Synthesized (S)-(+)-enantiomers were used to demonstrate that the elution order was (R)- prior to (S)-enantiomers. Using this system, food samples (butter, goat's milk fat, suet, human milk, seal oil, cod liver oil) known to contain aFAs were analyzed. Prior to the enantioselective gas chromatography, unsaturated fatty acids were preseparated by urea complexation, silver ion high performance liquid chromatography (Ag+-HPLC), or both from food samples. The fractions of the food samples enriched with methyl-branched fatty acids were then analyzed by GC/MS in the SIM mode. The measurements confirmed that the (S)-enantiomer of a15:0 (ee >96%), a16:0, and a17:0 (ee >90%, respectively) dominated in all samples. While the (R)-enantiomers could not be identified in samples from ruminants and human milk, their presence could be established in cod liver and seal oil (ee <86%).

Application of ethyl esters and d3-methyl esters as internal standards for the gas chromatographic quantification of transesterified fatty acid methyl esters in food

Ethyl esters (FAEE) and trideuterium-labeled methyl esters (d3-FAME) of fatty acids were prepared and investigated regarding their suitability as internal standards (IS) for the determination of fatty acids as methyl esters (FAME). On CP-Sil 88, ethyl esters of odd-numbered fatty acids eluted approximately 0.5 min after the respective FAME, and only coelutions with minor FAME were observed. Depending on the problem, one or even many FAEE can be added as IS for the quantification of FAME by both GC-FID and GC-MS. By contrast, d3-FAME coeluted with FAME on the polar GC column, and the use of the former as IS requires application of GC-MS. In the SIM mode, m/z 77 and 90 are suggested for d3-methyl esters of saturated fatty acids, whereas m/z 88 and 101 are recommended for ethyl esters of saturated fatty acids. These m/z values give either no or very low response for FAME and can thus be used for the analysis of FAME in food by GC-MS in the SIM mode. Fatty acids in sunflower oil and mozzarella cheese were quantified using five saturated FAEE as IS. Gravimetric studies showed that the transesterification procedure could be carried out without of loss of fatty acids. GC-EI/MS full scan analysis was suitable for the quantitative determination of all unsaturated fatty acids in both food samples, whereas GC-EI/MS in the SIM mode was particularly valuable for quantifying minor fatty acids. The novel GC-EI/MS/SIM method using fatty acid ethyl esters as internal standards can be used to quantify individual fatty acids only, that is, without determination of all fatty acids (the common 100% method), although this is present. This was demonstrated by the exclusive quantification of selected fatty acids including methyl-branched fatty acids, erucic acid (18:1n-9trans), and polyunsaturated fatty acids in cod liver oil and goat's milk fat.