Medium‐Chain Triglycerides Increase Energy Expenditure and Decrease Adiposity in Overweight Men

Objective: The objectives of this study were to compare the effects of diets rich in medium‐chain triglycerides (MCTs) or long‐chain triglycerides (LCTs) on body composition, energy expenditure, substrate oxidation, subjective appetite, and ad libitum energy intake in overweight men. Research Methods and Procedures: Twenty‐four healthy, overweight men with body mass indexes between 25 and 31 kg/m² consumed diets rich in MCT or LCT for 28 days each in a crossover randomized controlled trial. At baseline and after 4 weeks of each dietary intervention, energy expenditure was measured using indirect calorimetry, and body composition was analyzed using magnetic resonance imaging. Results: Upper body adipose tissue (AT) decreased to a greater extent (p < 0.05) with functional oil (FctO) compared with olive oil (OL) consumption (?0.67 ± 0.26 kg and ?0.02 ± 0.19 kg, respectively). There was a trend toward greater loss of whole‐body subcutaneous AT volume (p = 0.087) with FctO compared with OL consumption. Average energy expenditure was 0.04 ± 0.02 kcal/min greater (p < 0.05) on day 2 and 0.03 ± 0.02 kcal/min (not significant) on day 28 with FctO compared with OL consumption. Similarly, average fat oxidation was greater (p = 0.052) with FctO compared with OL intake on day 2 but not day 28. Discussion: Consumption of a diet rich in MCTs results in greater loss of AT compared with LCTs, perhaps due to increased energy expenditure and fat oxidation observed with MCT intake. Thus, MCTs may be considered as agents that aid in the prevention of obesity or potentially stimulate weight loss.     

The Effects of Long‐ or Medium‐Chain Fat Diets on Glucose Tolerance and Myocellular Content of Lipid Intermediates in Rats

Accumulation of triacylglycerols (TAGs) and acylcarnitines in skeletal muscle upon high‐fat (HF) feeding is the resultant of fatty acid uptake and oxidation and is associated with insulin resistance. As medium‐chain fatty acids (MCFAs) are preferentially β‐oxidized over long‐chain fatty acids, we examined the effects of medium‐chain TAGs (MCTs) and long‐chain TAGs (LCTs) on muscle lipid storage and whole‐body glucose tolerance. Rats fed a low‐fat (LF), HFLCT, or an isocaloric HFMCT diet displayed a similar body weight gain over 8 weeks of treatment. Only HFLCT increased myocellular TAG (42.3 ± 4.9, 71.9 ± 6.7, and 48.5 ± 6.5 µmol/g for LF, HFLCT, and HFMCT, respectively, P < 0.05) and long‐chain acylcarnitine content (P < 0.05). Neither HF diet increased myocellular diacylglycerol (DAG) content. Intraperitoneal (IP) glucose tolerance tests (1.5 g/kg) revealed a significantly decreased glucose tolerance in the HFMCT compared to the HFLCT‐fed rats (802 ± 40, 772 ± 18, and 886 ± 18 area under the curve for LF, HFLCT, and HFMCT, respectively, P < 0.05). Finally, no differences in myocellular insulin signaling after bolus insulin injection (10 U/kg) were observed between LF, HFLCT, or HFMCT‐fed rats. These results show that accumulation of TAGs and acylcarnitines in skeletal muscle in the absence of body weight gain do not impede myocellular insulin signaling or whole‐body glucose intolerance.     

Triglyceride with medium-chain fatty acids increases the activity and expression of hormone-sensitive lipase in white adipose tissue of C57BL/6J mice

We have previously shown that medium-chain triglyceride (MCT) resulted in significantly less body fat mass than long-chain triglyceride (LCT) did in hypertriglyceridimic subjects. The possible mechanism for this was investigated by measuring and analyzing changes in the body fat, blood lipid profile, enzymatic level and activity of hormone-sensitive lipase (HSL) and its mRNA expression, and levels of cyclic adenosine monophosphate (cAMP) and protein kinase A (PKA) in white adipose tissue (WAT) of C57BL/6J mice fed for 16 weeks on an MCT or LCT diet. MCT induced lower body weight and body fat, and an improved blood lipid profile than LCT did. The enzymatic level and activity of HSL and its mRNA expression, and the levels of cAMP and PKA were significantly higher in WAT of mice fed with the MCT diet. No significant differences in the levels of lipoprotein lipase and peroxisome proliferator-activated receptor-γ in WAT were apparent between the effects of MCT and LCT. It is concluded that lipolysis by the increased level and activity of HSL, which was induced by the activation of cAMP-dependent PKA in WAT, was partially responsible for the lower fat accumulation in C57BL/6J mice fed with MCT.     

Intra- and extracellular forms of lipoprotein lipase in adipose tissue.

The location of lipoprotein lipase activity in rat adipose tissue was studied using intact epididymal fat pads, isolated adipocytes, and lipoprotein lipase activity secreted from adipocytes as enzyme sources. The enzyme activities of these preparations were characterized by gel filtration. The method used for isolation of adipocytes had been modified to minimize activation of lipoprotein lipase during the procedures. Extracts of intact adipose tissue separated into two major lipoprotein lipase activity peaks, designated "a" and "b", the "a" fraction representing about 30 (fasted rats) to 50% (fed rats) of the total enzyme activity. An intermediate fraction (designated "i") was frequently observed. Extracts of isolated adipocytes from fed rats contained about 35% and those from fasted rats about 65% of the lipoprotein lipase activity present in intact tissue. The "b" fraction constituted 80--97% of the adipocyte lipoprotein lipase activity. In contrast, the enzyme activity secreted from the adipocytes contained only the "a" and "i" fractions. These data implicate the existance of one intracellular form of lipoprotein lipase (corresponding to the "b" fraction), different from extracellular forms of the enzyme (corresponding to fractions "a" and "i"). A transformation of the intracellular to the extracellular forms appears to occur in conjunction with secretion of enzyme from the fat cell.     

The Adipose Tissue Phenotype of Hormone‐Sensitive Lipase Deficiency in Mice

Objective: To directly ascertain the physiological roles in adipocytes of hormone‐sensitive lipase (HSL; E.C. 3.1.1.3), a multifunctional hydrolase that can mediate triacylglycerol cleavage in adipocytes. Research Methods and Procedures: We performed constitutive gene targeting of the mouse HSL gene (Lipe), subsequently studied the adipose tissue phenotype clinically and histologically, and measured lipolysis in isolated adipocytes. Results: Homozygous HSL^?/? mice have no detectable HSL peptide or cholesteryl esterase activity in adipose tissue, and heterozygous mice have intermediate levels with respect to wild‐type and deficient littermates. HSL‐deficient mice have normal body weight but reduced abdominal fat mass compared with normal littermates. Histologically, both white and brown adipose tissues in HSL^?/? mice show marked heterogeneity in cell size, with markedly enlarged adipocytes juxtaposed to cells of normal morphology. In isolated HSL^?/? adipocytes, lipolysis is not significantly increased by β₃‐adrenergic stimulation, but under basal conditions in the absence of added catecholamines, the lipolytic rate of isolated HSL^?/? adipocytes is at least as high as that of cells from normal controls. Cold tolerance during a 48‐hour period at 4 °C was similar in HSL^?/? mice and controls. Overnight fasting was well‐tolerated clinically by HSL^?/? mice, but after fasting, liver triglyceride content was significantly lower in HSL^?/? mice compared with wild‐type controls. Conclusions: In isolated fat cells, the lipolytic rate after β‐adrenergic stimulation is mainly dependent on HSL. However, the observation of a normal rate of lipolysis in unstimulated HSL^?/? adipocytes suggests that HSL‐independent lipolytic pathway(s) exist in fat. Physiologically, HSL deficiency in mice has a modest effect under normal fed conditions and is compatible with normal maintenance of core body temperature during cold stress. However, the lipolytic response to overnight fasting is subnormal.     

High‐fat diet feeding induces sex‐dependent changes in inflammatory and insulin sensitivity profiles of rat adipose tissue

The aim of the study was to determine, in rats of both sexes, the effect of HF diet feeding on the expression of adipokines involved in inflammatory status and insulin sensitivity and on the levels of proteins involved in lipid handling of retroperitoneal adipose tissue. Eight‐week‐old Wistar rats of both sexes were fed a control diet (2.9% w/w fat) or an HF diet (30% w/w fat) for 14 weeks. Adiponectin, peroxisome proliferator–activated receptor γ and inflammatory marker mRNA levels were analyzed by real‐time polymerase chain reaction. Levels of insulin receptor, glucose transporter 4, carnitine palmitoyltransferase 1, fatty acid synthase, hormone‐sensitive lipase and lipoprotein lipase were determined by Western blot. HF diet feeding did not induce hyperphagia or body weight gain but did promote an increase in adiposity although only in male rats. HF diet impaired glucose tolerance and the expression of inflammatory and insulin sensitivity markers in adipose tissue of male rats, but not in female rats. Male rats seem to be more prone to disorders associated with an unbalanced composition of the diet, even in the absence of hyperphagia. In contrast, female rats counteract excessive fat intake by improving their ability to use lipid fuels, which limits adiposity and maintains insulin sensitivity. Copyright © 2012 John Wiley & Sons, Ltd.     

Zinc deficiency and the activities of lipoprotein lipase in plasma and tissues of rats force-fed diets with coconut oil or fish oil

The present study was performed to investigate the effect of zinc deficiency on the activities of lipoprotein lipase in postheparin serum and tissues of rats fed diets containing either coconut oil or fish oil as dietary fat, using a bifactorial experimental design. To ensure an adequate food intake, all the rats were force-fed by gastric tube. Experimental diets contained either 0.8 mg zinc/kg (zinc-deficient diets) or 40 mg zinc/kg (zinc-adequate diets). The effects of zinc deficiency on the activities of lipoprotein lipase in postheparin serum and postprandial triglyceride concentrations and distribution of apolipoproteins in serum lipoproteins depended on the type of dietary fat. Zinc-deficient rats fed the coconut oil diet exhibited a reduced activity of lipoprotein lipase in postheparin serum and adipose tissue, markedly increased concentrations of triglycerides in serum, and a markedly reduced content of apolipoprotein C in triglyceride-rich lipoproteins and high density lipoproteins compared with zinc-adequate rats fed coconut oil. By contrast, zinc-deficient rats fed the fish oil diet did not exhibit reduced activities of lipoprotein lipase in postheparin serum and adipose tissue and increased concentrations of serum lipids compared with zinc-adequate rats fed the fish oil diet. This study suggests that a reduced activity of lipoprotein lipase might contribute to increased postprandial concentrations of serum triglycerides observed in zinc-deficient animals. However, it also demonstrates that the effects of zinc deficiency on lipoprotein metabolism are influenced by dietary fatty acids.     

High Local Concentrations and Effects on Differentiation Implicate Interleukin‐6 as a Paracrine Regulator

Objective: To examine the possibility that interleukin‐6 (IL‐6) can act as a paracrine regulator in adipose tissue by examining effects on adipogenic genes and measuring interstitial IL‐6 concentrations in situ. Research Methods and Procedures: Circulating and interstitial IL‐6 concentrations in abdominal and femoral adipose tissue were measured using the calibrated microdialysis technique in 20 healthy male subjects. The effects of adipose cell enlargement on gene expression and IL‐6 secretion were examined, as well as the effect of IL‐6 in vitro on gene expression of adiponectin and other markers of adipocyte differentiation. Results: The IL‐6 concentration in the interstitial fluid was ～100‐fold higher than that in plasma, suggesting that IL‐6 may be a paracrine regulator of adipose tissue. This was further supported by the finding that adding IL‐6 in vitro at similar concentrations down‐regulated the expression of adiponectin, aP2, and PPARγ‐2 in cultured human adipose tissue. In addition, gene expression and release of IL‐6, both in vivo and in vitro, correlated with adipose cell size. Discussion: These data suggest that IL‐6 may be a paracrine regulator of adipose tissue. Furthermore, increased adipose tissue production of IL‐6 after hypertrophic enlargement of the adipose cells may detrimentally affect systemic insulin action by inducing adipose tissue dysfunction with impaired differentiation of the pre‐adipocytes and/or adipocytes and lower adiponectin.     

Release of 12 Adipokines by Adipose Tissue,Nonfat Cells,and Fat Cells From Obese Women

The relative release in vitro of endothelin‐1, zinc‐α2‐glycoprotein (ZAG), lipocalin‐2, CD14, RANTES (regulated on activation, normal T cell expressed and secreted protein), lipoprotein lipase (LPL), osteoprotegerin (OPG), fatty acid–binding protein 4 (FABP‐4), visfatin/PBEF/Nampt, glutathione peroxidase‐3 (GPX‐3), intracellular cell adhesion molecule 1 (ICAM‐1), and amyloid A was examined using explants of human adipose tissue as well as the nonfat cell fractions and adipocytes from obese women. Over a 48‐h incubation the majority of the release of LPL was by fat cells whereas that of lipocalin‐2, RANTES, and ICAM‐1 was by the nonfat cells present in human adipose tissue. In contrast appreciable amounts of OPG, amyloid A, ZAG, FABP‐4, GPX‐3, CD14, and visfatin/PBEF/Nampt were released by both fat cells and nonfat cells. There was an excellent correlation (r = 0.75) between the ratios of adipokine release by fat cells to nonfat cells over 48 h and the ratio of their mRNAs in fat cells to nonfat cells at the start of the incubation. The total release of ZAG, OPG, RANTES, and amyloid A by incubated adipose tissue explants from women with a fat mass of 65 kg was not different from that by women with a fat mass of 29 kg. In contrast that of ICAM‐1, FABP‐4, GPX‐3, visfatin/PBEF/Nampt, CD14, lipocalin‐2, LP, and endothelin‐1 was significantly greater in tissue from women with a total fat mass of 65 kg.     

Increased norepinephrine by medium-chain triglyceride attributable to lipolysis in white and brown adipose tissue of C57BL/6J mice

A further investigation of the lipolysis induced by medium-chain triglyceride (MCT) was conducted on C57BL/6J mice fed with a diet containing 2% MCT or 2% long-chain triglyceride (LCT). Blood norepinephrine, body fat and blood lipid variables, and the protein or mRNA expression of the genes relevant to lipolysis were measured and analyzed in the white and brown adipose tissue (WAT, BAT). Decreased body fat and improved blood lipid profiles attributable to MCT were confirmed. A higher level of blood norepinephrine was observed with the MCT diet. The adipose triglyceride lipase (ATGL) activity and its mRNA expression, the expression of protein and mRNA of the beta 3 adrenergic receptor (β3-AR) in both WAT and BAT, and the hormone-sensitive lipase (HSL) activity and its mRNA expression in BAT were significantly increased in the mice with MCT feeding. The lipolysis induced by MCT might be partially mediated by increasing norepinephrine, thereafter signaling the up-regulation of β3-AR, ATGL, and HSL in WAT and BAT.     

Distribution and source of lipoprotein lipase in mouse mammary gland

During lactation lipoprotein lipase (LPL) is elevated in mammary tissue and depressed in adipose tissue to redirect lipids for incorporation into milk fat. The cellular origin of lipoprotein lipase in mammary tissue is thought to be the mammary epithelial cell which is the predominant cell type noticeable in the lactating gland; however, mammary adipocytes are also present. If lipoprotein lipase is produced by adipocytes in other sites of the body, then the question remains as to why mammary adipocytes have not been shown to produce lipoprotein lipase. In this study we present several lines of evidence that indicate that the mammary adipocyte is a source of LPL in the lactating mammary gland of mice. This evidence includes the absence of extracellular and intracellular lipoprotein lipase activity in two types of primary mammary epithelial cell cultures and a similarity in the changes of lipoprotein lipase activity in genital adipose tissue from nonpregnant mice and lactating mammary tissue to the nutritional state of the animal. Other evidence presented here includes strong localization of lipoprotein lipase protein and messenger RNA by fluorescence immunohistochemistry and in situ hybridization, respectively, to interstitial cells located between epithelial structures. We postulate that these interstitial cells are regressed, lipid-deleted mammary adipocytes.     

Regulation of lipoprotein lipase. Induction by insulin.

Lipoprotein lipase activity in intact epididymal adipose tissue of fasted rats increased rapidly after treatment with insulin in vivo. In contrast, lipoprotein lipase activity in adipocytes isolated from the contralateral fat pads remained essentially unchanged. When adipocytes were incubated for 30 min at ambient temperature in vitro, about 2 times more lipoprotein lipase activity was found in the medium of cells from insulin-treated rats than in medium from cells of control animals. Following insulin treatment, extracts of tissue acetone powders separated by gel chromatography showed increases in both enzyme activity fractions obtained (designated lipoprotein lipase a and b). However, no consistent differences were observed between fractions derived from adipocyte acetone powders of insulin-treated and control animals. All the observed effects of insulin on lipoprotein lipase activity were abolished by cycloheximide treatment in vivo. These data indicate that following insulin treatment, increased lipoprotein lipase activity in adipose tissue results from enhanced enzyme secretion by the fat cell and subsequent accumulation in the tissue, thus implicating the adipocyte secretory mechanism as a major site of regulation of lipoprotein lipase activity in adipose tissue.     

Anthropometric variables and metabolism in polycystic ovarian disease

Anthropometric, endocrine and metabolic variables, were examined in women with polycystic ovarian syndrome (PCO), and in normal control women. Obese women with PCO had higher plasma insulin values than non obese women with PCO, but lean body mass, glucose tolerance, plasma triglycerides and blood pressure were not different in spite of almost twice the body fat mass in the obese PCO women. However, in comparisons between non-obese PCO and control women, with equal body fat mass, the PCO women had higher blood pressure, plasma triglycerides and insulin, as well as a tendency to increased lean body mass. Both PCO groups had a high waist/hip ratio and larger abdominal fat cells than controls, indicating a preferential abdominal accumulation of adipose tissue. In comparison with abdominal adipocytes, femoral adipocytes were larger and had higher lipoprotein lipase activity in the control women, while in the PCO women these regional differences were not found. Basal and norepinephrine stimulated lipolysis were higher in the abdominal than femoral adipocytes in all groups. Substitution of the PCO women with ethinyl estradiol plus desogestrel during 6 months resulted in a regression of clinical androgenic symptoms as well as a normalization of plasma concentrations of free testosterone and sex hormone binding globulin. However, neither body composition nor metabolism were normalized. It was concluded that body fat distribution is more closely related to hypertension and metabolic derangements than total fat mass in the PCO syndrome. It is suggested that the relative paucity of femoral adipose tissue is due to a lack of specific effects of progesterone on adipocytes in this region.     

Skeletal muscle adiposity is associated with serum lipid and lipoprotein levels in Afro‐Caribbean men

Objective: When compared with other ethnic groups, African ancestry individuals have lower triglycerides and higher High‐density lipoprotein cholesterol (HDL‐C) levels, although the mechanisms for these differences remain unclear. A comprehensive array of factors potentially related to fasting serum lipid and lipoprotein levels in African ancestry men was evaluated. Design and Methods: Men (1,821) underwent dual‐energy X‐ray absorptiometry measures of total body fat and quantitative computed tomography assessments of calf skeletal muscle adiposity [subcutaneous and intermuscular adipose tissue (AT), and muscle density as a measure of intra‐muscular AT]. Results: Multivariable linear regression analysis identified age (?), total body fat (+), subcutaneous AT (?), fasting glucose (+), fasting insulin (+), diastolic blood pressure (+), and non‐African ancestry (+) as independent correlates of triglycerides (all P < 0.05). Total body fat (+), intra‐muscular AT (?), and diastolic blood pressure (+) were independent correlates of Low‐density lipoprotein cholesterol (LDL‐C) (all P < 0.001). Age (+), waist circumference (?), fasting insulin (?), physical activity (+), and alcohol intake (+) were independent correlates of HDL‐C (all P < 0.05). Conclusions: A novel relationship between skeletal muscle adiposity and serum lipid and lipoprotein levels in African ancestry men, independent of total and central adiposity was illuminated. In African ancestry populations, genetic factors are likely a significant determinant of triglycerides levels.     

Effects of dietary cholesterol on adipose tissue lipoprotein lipase in the baboon

The effects of infant diet (breast milk or formula containing 2, 30 or 60 mg/dl cholesterol) and subsequent dietary cholesterol (0.02, 1.0 or 1.7 mg/kcal) and fat (saturated or unsaturated) on heparin-releasable lipolytic activity from omental adipose tissue was estimated from 99 baboons of 5-8 years of age. This lipase activity was characterized as lipoprotein lipase based on salt inhibition and apolipoprotein C-II activation. Lipoprotein lipase activity released from adipose tissue by heparin was significantly (P less than 0.002) lower in high cholesterol-fed baboons than in those fed low cholesterol. Most of this difference was due to impaired long-term heparin release of lipoprotein lipase. Adipose tissue lipoprotein lipase increased with increasing fat cell size regardless of diet, but there was no effect of diet on adipocyte size. There were no significant effects of infant cholesterol intake nor adult saturated or unsaturated fat on lipoprotein lipase activity. Adult baboons breast fed as infants had lower adipose tissue lipoprotein lipase activity (P less than 0.07) than adults fed formula as infants.