Use of gas chromatography/isotope ratio-mass spectrometry to study triglyceride metabolism in humans

Final Report on the Safety Assessment of Trilaurin, Triarachidin, Tribehenin, Tricaprin, Tricaprylin, Trierucin, Triheptanoin, Triheptylundecanoin, Triisononanoin, Triisopalmitin, Triisostearin, Trilinolein, Trimyristin, Trioctanoin, Triolein, Tripalmitin, Tripalmitolein, Triricinolein, Tristearin, Triundecanoin, Glyceryl Triacetyl Hydroxystearate, Glyceryl Triacetyl Ricinoleate, and Glyceryl Stearate Diacetate

Triesters of glycerin and aliphatic acids, known generically as glyceryl triesters and specifically as Trilaurin, etc., are used in cosmetic products as occlusive skin-conditioning agents and/or non-aqueous viscosity-increasing agents. Hundreds of glyceryl triesters are used in a wide variety of cosmetic products at concentrations ranging from a few tenths of a percent to 46%. Glyceryl triesters are also known as triglycerides; ingested triglycerides are metabolized to monoglycerides, free fatty acids, and glycerol, all of which are absorbed in the intestinal mucosa and undergo further metabolism. Dermal absorption of Triolein in mice was nil; the oil remained at the application site. Only slight absorption was seen in guinea pig skin. Tricaprylin and other glyceryl triesters have been shown to increase the skin penetration of drugs. Little or no acute, subchronic, or chronic oral toxicity was seen in animal studies unless levels approached a significant percentage of caloric intake. Subcutaneous injections of Tricaprylin in rats over a period of 5 weeks caused a granulomatous reaction characterized by oil deposits surrounded by macrophages. Dermal application was not associated with significant irritation in rabbit skin. Ocular exposures were, at most, mildly irritating to rabbit eyes. No evidence of sensitization or photosensitization was seen in a guinea pig maximization test. Most of the genotoxicity test systems were negative. Tricaprylin, Trioctanoin, and Triolein have historically been used as vehicles in carcinogenicity testing of other chemicals. In one study, subcutaneous injection of Tricaprylin in newborn mice produced more tumors in lymphoid tissue than were seen in untreated animals, whereas neither subcutaneous or intraperitoneal injection in 4-to 6-week-old female mice produced any tumors in another study. Trioctanoin injected subcutaneously in hamsters produced no tumors. Trioctanoin injected intraperitoneally in pregnant rats was associated with an increase in mammary tumors in the offspring compared to that seen in offspring of untreated animals, but similar studies in pregnant hamsters and rabbits showed no tumors in the offspring. One study of Triolein injected subcutaneously in rats showed no tumors at the injection site. As part of an effort to evaluate vehicles used in carcinogenicity studies, the National Toxicology Program conducted a 2-year carcinogenicity study in rats given Tricaprylin by gavage. This treatment was associated with a statistically significant dose-related increase in pancreatic acinar cell hyperplasia and adenoma, but there were no acinar carcinomas, the incidence of mononuclear leukemia was less, and nephropathy findings were reduced, all compared to corn oil controls. Overall, the study concluded that Tricaprylin did not offer significant advantages over corn oil as vehicles in carcinogenicity studies. Trilaurin was found to inhibit the formation of neoplasms initiated by dimethylbenzanthracene (DMBA) and promoted by croton oil. Tricaprylin was not teratogenic in mice or rats, but some reproductive effects were seen in rabbits. A low level of fetal eye abnormalities and a small percentage of abnormal sperm were reported in mice injected with Trioctanoin as a vehicle control. Clinical tests of Trilaurin at 36.3% in a commercial product applied to the skin produced no irritation reactions. Trilaurin, Tristearin, and Tribehenin at 40%, 1.68%, and 0.38%, respectively, in commercial products were also negative in repeated-insult patch tests. Tristearin at 0.32% in a commercial product induced transient, mild to moderate, ocular irritation after instillation into the eyes of human subjects. Based on the enhancement of penetration of other chemicals by skin treatment with glyceryl triesters, it is recommended that care be exercised in using them in cosmetic products. On the basis of the available data, the following 23 glyceryl triesters are considered safe as used in cosmetics: Trilaurin, Triarachidin, Tribehenin, Tricaprin, Tricaprylin, Trierucin, Triheptanoin, Triheptylundecanoin, Triisononanoin, Triisopalmitin, Triisostearin, Trilinolein, Trimyristin, Trioctanoin, Triolein, Tripalmitin, Tripalmitolein, Triricinolein, Tristearin, Triundecanoin, Glyceryl Triacetyl Hydroxystearate, Glyceryl Triacetyl Ricinoleate, and Glyceryl Stearate Diacetate. Some of these are not currently in use, but would be considered safe if used at concentrations similar to those glyceryl triesters that are in use as cosmetic ingredients.

Low density lipoprotein delays clearance of triglyceride-rich lipoprotein by human subcutaneous adipose tissue

Delayed clearance of triglyceride-rich lipoprotein (TRL) by white adipose tissue (WAT) promotes hypertriglyceridemia and elevated apoB-lipoproteins, which are primarily in the form of LDL. This study examines whether LDL promotes delayed clearance of TRL by WAT. Following the ingestion of a (13)C-triolein-labeled high-fat meal, obese women with high plasma apoB (> median 0.93 g/l, N = 11, > 98% as IDL/LDL) had delayed clearance of postprandial (13)C-triglyceride and (13)C-NEFA over 6 h compared with controls. AUC6 h of plasma (13)C-triglyceride and (13)C-NEFA correlated with plasma apoB but not with LDL diameter or adipocyte area. There was no group difference in (13)C-triolein oxidation rate, which suggests lower (13)C-NEFA storage in peripheral tissue in women with high apoB. Ex vivo/in vitro plasma apoB correlated negatively with WAT (3)H-lipid following a 4 h incubation of women's WAT with synthetic (3)H-triolein-TRL. LDL-differentiated 3T3-L1 adipocytes had lower (3)H-TRL hydrolysis and (3)H-NEFA storage. Treatment of women's WAT with their own LDL decreased (3)H-TRL hydrolysis and (3)H-NEFA uptake. Finally, LDL, although not an LPL substrate, reduced LPL-mediated (3)H-TRL hydrolysis as did VLDL and HDL. Exposure to LDL decreases TRL clearance by human WAT ex vivo. This may promote production of apoB-lipoproteins and hypertriglyceridemia through a positive-feedback mechanism in vivo.

13C tracer recovery in human stools after digestion of a fat-rich meal labelled with [1,1,1-13C3]tripalmitin and [1,1,1-13C3]triolein

Lipid metabolism studies focus mainly on oxidation and storage but rarely on faecal elimination, which is needed to assess total lipid distribution during the postprandial period. The purpose of the present work was to set up and validate the analysis of lipid tracers in stools, with an aim of later using this methodology in studies of postprandial lipid tracer metabolism. Eight subjects received a mixture of [1,1,1-(13)C3]tripalmitin and [1,1,1-(13)C3]triolein with a fat-rich meal. The nature and amounts of (13)C lipids excreted in stools during 3 days post-dose were determined by gas chromatography/mass spectrometry (GC/MS) and gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) analysis of fatty acid methyl esters (FAMEs) from total fatty acid (TFA), free fatty acid (FFA) and triacylglycerol (TAG) fractions. The results were expressed as the Cumulative Tracer Recovery of the administered dose (CTR%). The quantities and labelling of FAMEs were higher in FFA than in TAG, indicating that label loss was not due to a lack of digestive lipase activity. The labelling was higher for C16:0 than for C18:1. The CTRs were 7.03 ± 0.77% and 6.87 ± 0.91%, respectively, in TFA and FFA for [1-(13)C] C16:0, while they were 0.60 ± 0.15% and 0.51 ± 0.11% for [1-(13)C] C18:1 (mean ± sem). By studying the kinetics of lipid excretion from subjects, two groups emerged. The first one showed rapid excretion in stool #1, whereas the second showed slower excretion in stools #2-#3. A significant difference was found in the FFA in stool #1 for C16:0 (p < 0.01) and C18:1 (p < 0.05). Individual excretion kinetics showed marked variability. Nevertheless, the CTR over the 3-day study period was substantial and homogenous for all subjects. These results confirm that the assessment of faecal elimination is of great importance when establishing total lipid distribution during the postprandial period and validate the analysis of cumulative tracer loss during 72 h post-tracer ingestion.

Use of stable isotopes to evaluate the functional effects of nutrients

PURPOSE OF REVIEW

Lifestyle, particularly dietary habits, plays a major role in the increasing prevalence of obesity, type 2 diabetes and atherosclerosis. Understanding how diet and specific foods can modify important functions of the body, in a beneficial or detrimental way, is therefore important. This review presents recent advances in the use of stable isotopes to investigate how nutrients can influence pathways of glucose, lipids and protein metabolism, and also intestinal absorption, body composition and fat mass turnover.

RECENT FINDINGS

Recent developments have focused mainly on the measurement of intratissular metabolic pathways, particularly in lipids and proteins. Advances in the field of cell proliferation rates, body composition and energy expenditure have also been made. Many of these developments rely on the use of deuterated water to trace multiple metabolic pathways.

SUMMARY

Stable isotopes can now be used to measure the kinetics and oxidation rate of circulating metabolites and also body composition, colonic fermentation, intestinal absorption, and important intratissular metabolic pathways, such as gluconeogenesis, lipogenesis, cholesterol synthesis, the turnover rate of triglycerides, individual protein synthesis and cell proliferation. Deuterated water has emerged as a powerful and versatile tool, allowing the simultaneous investigation of several aspects of protein, lipid and glucose metabolism.

Contribution of hepatic de novo lipogenesis and reesterification of plasma non esterified fatty acids to plasma triglyceride synthesis during non-alcoholic fatty liver disease

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) is frequently observed in insulin-resistant subjects and can lead to liver fibrosis and cirrhosis. The abnormalities of lipid metabolism behind this development of excess hepatic TG stores are poorly understood.

METHODS

To clarify these mechanisms we measured triglyceride secretion rate and the contributions of hepatic lipogenesis and reesterification of non-esterified fatty acids (NEFA) to this secretion in healthy subjects and in patients with clear evidence of NAFLD. All subjects were studied in the post-absorptive state. Hepatic lipogenesis was measured with deuterated water. NEFA turnover rate, triglyceride secretion rate and the contribution of NEFA reesterification to this secretion were determined with [1-(13)C] palmitate infusion.

RESULTS

NAFLD patients had higher NEFA concentrations (p<0.05) but normal NEFA turnover rates (5.23 +/- 0.80 vs 5.91 +/- 0.97 micromol.kg(-1).min(-1) in control subjects, ns). Despite a trend for higher plasma triglyceride levels in patients (p<0.10), triglyceride turnover rates were not increased (0.11 +/- 0.01 micromol.kg(-1).min(-1) in patients vs 0.14 +/- 0.01 in controls, ns). However the contribution of hepatic lipogenesis to triglyceride secretion was largely increased in patients (14.9 +/- 2.7 vs 4.6 +/- 1.1% p<0.01) while that of NEFA reesterification was reduced (25.1 +/- 2.9 vs 52.8 +/- 6.2% p<0.01).

CONCLUSION

Enhanced lipogenesis appears as a major abnormality of hepatic fatty metabolism in subjects with NAFLD. Therapeutic measures aimed at decreasing hepatic lipogenesis would therefore be the most appropriate in order to reduce hepatic TG synthesis and content in such patients.

Use of gas chromatography-combustion-isotope ratio mass spectrometry in nutrition and metabolic research

Linking gas chromatography via an on-line combustion interface to isotope ratio mass spectrometry has opened the door to high-precision compound-specific isotope analysis. For this reason, gas chromatography-combustion-isotope ratio mass spectrometry is now increasingly employed in metabolic and nutritional research because it offers a reliable and risk-free alternative to the use of radioactive tracers.

Triglyceride oxidation in cystic fibrosis: a comparison between different 13C-labeled tracer substances

BACKGROUND

For indirect evaluation of pancreatic lipase activity in cystic fibrosis, different 13C-labeled triglycerides may be used.

METHODS

Triglyceride oxidation in patients with cystic fibrosis was investigated after administration of different 13C-labeled triglycerides by comparing 13CO2 breath exhalation. In the comparative study, five patients with cystic fibrosis (age, 8-15 years; body weight, 22.5-39.8 kg) were treated with Pangrol (individual dosages: 1-3 capsules per morning meal; Berlin-Chemie, Berlin, Germany). [1,1,1-13C3]Glyceryl tripalmitate and [1,1,1-13C3]glyceryl trioleate were administered as a single oral pulse at 8:00 A.M. (dosage, 4 mg/kg each) with the standard diet Fresubin (dosage, 10 ml/kg; Fresenius, Bad Homburg, Germany). Alternately, the same subjects were given the synthetic mixed triglyceride 1,3-distearyl, 2[13C]octanoyl glycerol (dosage, 12.5 mg/kg) contained in the standard diet Nutri-Mix (dosage, 10 ml/kg; Nutricia, Zoetemeer, The Netherlands). Breath samples were taken in 15- and 30-minute intervals over 8 hours. The 13CO2 enrichment was measured by continuous-flow isotope ratio mass spectrometry.

RESULTS

After administration of the 13C-labeled tripalmitin-triolein mixture and the mixed triglyceride, mean maximum 13CO2 enrichments were 4.70 and 7.37 delta over baseline, occurring at 7.0 and 3.5 hours, respectively. The corresponding percentage cumulative 13CO2 exhalations were 12.25% and 29.19%, respectively, and differed significantly in the five paired subjects (p = 0.003).

CONCLUSIONS

After using different 13C-labeled triglycerides the resultant 13CO2 exhalation reflected the triglyceride hydrolysis and subsequent oxidation. It is concluded that the different cumulative 13CO2 exhalations were mainly caused by the rate-limiting step of triglyceride hydrolysis to free fatty acids and 2-monoglycerides and by fat deposition. Noninvasive 13C breath tests using different 13C-labeled triglycerides can be used for evaluation of pancreatic lipase activity before and during enzyme supplementation.

Applied gas chromatography coupled to isotope ratio mass spectrometry

Compound-specific isotope analysis (CSIA) by isotope ratio mass spectrometry (IRMS) following on-line combustion (C) of compounds separated by gas chromatography (GC) is a relatively young analytical method. Due to its ability to measure isotope distribution at natural abundance level with great accuracy and high precision, GC-C-IRMS has increasingly become the method of choice in authenticity control of foodstuffs and determination of origin in archaeology, geochemistry, and environmental chemistry. In combination with stable isotope labelled compounds, GC-C-IRMS is also used more and more in biochemical and biomedical application as it offers a reliable and risk-free alternative to the use of radioactive tracers. The literature on these topics is reviewed from the advent of commercial GC-C-IRMS systems in 1990 up to the beginning of 1998. Demands on sample preparation and quality of GC separation for GC-C-IRMS are discussed also.

Non-esterified fatty acid metabolism and postprandial lipaemia

Non-esterified fatty acids (NEFA, or free fatty acids) are an important metabolic fuel. Both the concentration of NEFA and their flux through the circulation vary widely from hour to hour, reflecting nutritional state and physical activity. Inappropriately elevated plasma NEFA concentrations may have a number of adverse effects on both carbohydrate and lipid metabolism. These adverse effects are likely to be most marked in the postprandial period, when NEFA release from adipose tissue is usually suppressed. Although the regulation of NEFA release in the postabsorptive state is well understood in molecular terms, the predominant pathway for release of NEFA in the postprandial state is the action of lipoprotein lipase (LPL) in adipose tissue capillaries on chylomicron-triacylglycerol (TG). Fatty acids released by LPL may either be sequestered in the adipocytes by esterification, or released as NEFA into the plasma. The regulation of this branch-point, which may be of crucial significance for postprandial metabolism, is not well understood. Factors stimulating tissue retention of fatty acids include insulin and acylation stimulating protein. There is considerable indirect evidence that impaired regulation of this step occurs in insulin resistance and other conditions collectively recognised by an elevated concentration of apolipoprotein B (hyper-apo B). Inappropriate release of NEFA in the postprandial period is likely both to reduce the sensitivity of glucose metabolism to insulin and to accentuate postprandial lipaemia. Further study of the regulation of this pathway is much needed.

Role of human liver lipogenesis and reesterification in triglycerides secretion and in FFA reesterification

To measure 1) the contribution of hepatic de novo lipogenesis (DNL) and plasma free fatty acid (FFA) reesterification to plasma triglyceride (TG) secretion and 2) the role of oxidation and hepatic and extrahepatic reesterification in FFA utilization, five normal subjects drank deuterate water and were infused (postabsorptive state) with [1-13C]palmitate and [1,2,3-2H5]glycerol. Total lipid oxidation (Lox) was measured by indirect calorimetry. FFA oxidation (2.76 +/- 0.65 mumol.kg.-1.min-1) accounted for 45% of FFA turnover rate (Rt) (1.04 mumol.kg-1.min-1) and 91% of Lox; FFA reesterification was 3.27 +/- 0.54 mumol.kg-1.min-1. Fractional and absolute TG Rt were 0.21 +/- 0.02 h-1 and 0.11 +/- 0.05 mumol.kg-1.min-1. DNL accounted for 3.9 +/- 0.9% of TG secretion, and hepatic FFA reesterification accounted for 49.4 +/- 5.7%; this last process represented a utilization of FFA of 0.16 +/- 0.02 mumol.kg-1.min-1. We conclude that, in the postabsorptive state, 1) DNL and FFA reesterification account for only 50-55% of TG secretion, the remaining presumably being provided by stored lipids or lipoproteins taken up by liver, 2) most reesterification occurs in extrahepatic tissues, and 3) oxidation and reesterification each contribute about one-half to FFA utilization; FFA oxidation accounts for almost all Lox.

Stable-isotope method for determining the gastrointestinal handling of [1-13C]palmitic acid

The 13C enrichment in individual fatty acids extracted from human feces following the oral administration of [1-13C]palmitic acid has been determined using a novel approach based upon gas chromatography-isotope ratio mass spectrometry. The method was established and tested for precision and repeatability. Analytical precision was determined from 10 repeated injections of a sample containing 16:0 and 18:0 with levels of delta 13C abundance measured at -34.01 +/- 0.60 and -23.62 +/- 0.95 delta per mil (parts per thousand) (/1000), respectively (mean +/- SD). For the repeatability study, measurement of enrichment of the same mixture of unlabeled fatty acid methyl ester (FAME) standards (13:0, 14:0, 16:0, and 18:0) was found to have standard deviations (0.45, 0.56, 1.46 and 1.54/1000, respectively). When labeled [1-13C]palmitic acid was serially diluted with naturally enriched palmitic acid, a linear relationship was obtained to a dilution of 10% enriched compound (530/1000). FAME were prepared from two fecal samples from a normal healthy adult; the first, a baseline specimen, containing no added label and the second, followed a single oral dose of [1-13C]palmitic acid and was enriched. Enrichment in 13C was confined to the solvent-soluble fraction following lipid extraction, and was only identified with prior acidification. The enrichments were measured in triplicate, baseline sample -32.66 +/- 0.5/1000, enriched sample +268.61 +/- 8.0/1000. Enrichment was restricted to the labeled species consumed, 16:0. The methodology described here allows for the separation of compounds prior to the determination of enrichment and can be utilized to contribute to a more complete description of the gastrointestinal handling of labeled substrates than previously obtained.