Fatty acid sources and their fluxes as they contribute to plasma triglyceride concentrations and fatty liver in humans.

REVIEW URRENT C OPINION

Fatty acid sources and their fluxes as they contribute to plasma triglyceride concentrations and fatty liver in humans M. Miriam Jacome-Sosa and Elizabeth J. Parks

Purpose of review Different sources of fatty acids (FA) used for VLDL-triglyceride synthesis include dietary FA that clear to the liver via chylomicron uptake, FA synthesized de novo in the liver from carbohydrates, nonesterified fatty acids derived from adipose tissue, nonesterified fatty acids derived from the spillover of chylomicrontriglyceride in the fasted and fed states, and FA stored in liver lipid droplets. Recent findings Data have amassed on the contributions of each of these sources to liver-triglyceride accrual, VLDLtriglyceride synthesis, and hypertriglyceridemia. Discussed here is the timing of use of FA from each of these sources for synthesis of VLDL-triglyceride. Secondly, as all of these FA sources have been shown to contribute significantly to nonalcoholic fatty liver disease (NAFLD), data are presented demonstrating how poor handling of FA and glucose in the periphery can contribute to NAFLD. Lastly, we highlight how the stress of excess FA availability on the liver can be corrected by reduction of dietary intake of sugars and fats, weight loss, and increased physical activity. Summary A better understanding of how lifestyle factors improve FA flux will aid in the development of improved treatments for the devastating condition of NAFLD. Keywords de-novo lipogenesis, fatty acids, hypertriglyceridemia, nonalcoholic fatty liver disease, spillover

INTRODUCTION Hypertriglyceridemia (HPTG) is associated with increased cardiovascular risk [1]. Concentrations of fasting triglycerides of more than 150 mg/dl are considered elevated and are targets for treatment. HPTG can result from overproduction of triglyceride-rich lipoproteins (TRL) of both intestinal and hepatic origin, and the sources of fatty acids (FA) used for lipoprotein assembly can be derived from the diet, from de-novo lipogenesis (DNL), or from FA originating from the plasma nonesterified FA (NEFA) pool. Dietary fat can contribute to HPTG directly through the production of chylomicron remnants and indirectly through uptake of chylomicrons by the liver and recycling of this lipid into VLDL-triglyceride, and spillover of dietary FA into NEFA, which can then clear to the liver. Spillover of chylomicron-triglyceride occurs as early as 30 min after a meal [2]. Later, in the postprandial period, more dietary lipid can be detected in VLDL-triglyceride than can be accounted for through liver recycling of spillover FA, and this label has been

assumed to enter the liver through chylomicronremnant uptake [3]. The timing of incorporation of FA from different sources is shown in Fig. 1 [2,4]. As compared with the plasma NEFA pool, entry of meal lipid into VLDL-triglyceride is delayed resulting in almost 1/2 of breakfast fat entering the blood after lunch [4]. As shown in Fig. 1, in healthy men, the use of dietary fat for VLDL-triglyceride synthesis was constant after consumption of two identically labeled meals, and this source accounted for 35–40% of VLDL-triglyceride by the end of the experiment. This proportion is probably a minimum

Department of Nutrition and Exercise Physiology, University of Missouri, Columbia, Missouri, USA Correspondence to Elizabeth J. Parks, PhD, Department of Nutrition and Exercise Physiology and Division of Gastroenterology and Hepatology, School of Medicine, One Hospital Drive, NW 406, Columbia, MO 65212, USA. Tel: +1 573 882 5864; e-mail: [email protected] Curr Opin Lipidol 2014, 25:213–220 DOI:10.1097/MOL.0000000000000080

0957-9672 ß 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins

www.co-lipidology.com

Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.

Lipid metabolism

Liver-triglyceride synthesis utilizes FA derived from many sources, both peripheral and intrahepatically generated. During the transition from fasting to feeding, the suppression of adipose release of FA results in less availability of these FA for liver-triglyceride synthesis, whereas dietary FA and those synthesized de novo become available. Multiple independent lines of evidence suggest that poor clearance and usage of FA peripherally results in an increased burden of FA on the liver, which results in NAFLD and HPTG. Genetic factors, such as body fat distribution, can significantly influence the efficiency of meal fat storage. Lifestyle factors, which can alleviate liver overload of FA, include increased exercise, which improves clearance and oxidation of FA, and weight loss, which leads to greater adipose insulin sensitivity and reduced hepatic de novo lipogenesis.

value as the meals fed contained only 30% of energy from fat [4]. Another source of FA used for lipoprotein-triglyceride synthesis is hepatic DNL. As described below, elevated lipogenesis may be a distinctive characteristic of HPTG associated with nonalcoholic fatty liver disease (NAFLD), whereas other forms of HPTG may result from elevated flux of plasma NEFA [5,6]. After synthesis, lipogenic FA appear to move into a ‘delay’ pool that turns over slowly in the liver before contributing to a triglyceride pool destined for VLDL secretion [3]. In Fig. 1, note the slow rise in appearance of DNL FA in VLDL-triglyceride over the course of two meals, and a similar delay is seen in studies of alcohol-induced lipogenesis [7]. Lipogenic FA may contribute to triglyceride that are added late in the VLDL assembly process because conditions that elevate DNL may be associated with increased VLDL particle size (triglyceride/particle) but not with increased VLDL apoB [8]. Fructose feeding and ethanol administration, which stimulate lipogenesis [7,9], increase both liver-triglyceride [10] and plasma-triglyceride concentrations [7,9,10]. Conversely, treatment with n-3 FA diminishes lipogenesis associated with fructose feeding [11]. Along the same lines, fish oil supplementation ameliorates HPTG by reducing the concentrations of both TRLapoB and TRL-triglyceride, but the effect on triglyceride is stronger such that the ratio of triglyceride per particle is reduced by 30% [12]. Another source of VLDL-triglyceride FA, the plasma NEFA that clears to the liver, may be 214


incorporated very early in the VLDL assembly process such that these NEFAs have a stronger effect in sparing apoB from degradation. In humans, a NEFA label infused into plasma can appear in plasma VLDL-triglyceride within 10–30 min ([13] and E.J. Parks, personal observation). Such incorporation of NEFA into an endoplasmic reticulum-associated triglyceride-pool, which is then added to the primordial particle, can support greater apoB particle secretion rates. In healthy, fasting individuals, although the plasma NEFA pool contributes the majority of FA used for VLDL-triglyceride synthesis, consumption of a mixed meal containing carbohydrate results in a reduction in hepatic use of NEFA for triglyceride synthesis. As can be seen in Fig. 1 between time 0 and 1 h, liver NEFA use fell precipitously at the onset of meal one and was suppressed throughout the day when two meals were fed because of an insulin-mediated reduction in adipose NEFA release [4]. Thus, adipose insulin sensitivity plays a major role in protecting the liver from excess flux of FA and elevated triglyceride synthesis [14,15].

100 Plasma NEFA

90

VLDL-TG source (%)

KEY POINTS

Dietary fatty acids

80

De novo lipogenesis

70 60 50 40 30 20 10

0 Time (h) 0

1

Meal 1

2

3

4

5

6

7

8

9

10

Meal 2

FIGURE 1. Time-courses of three FA sources used for hepatic lipoprotein assembly and secretion in healthy individuals. Healthy individuals received isotopes of palmitate in meals and continuously over 10 h. VLDL-TG was isolated from plasma and palmitate labels analyzed by gas chromatography/mass spectrometry (adapted with permission from [2,4]). By the end of the experiment (10 h), the sum of the three sources equaled 100%, meaning that all sources of VLDL-TG FA had been labeled and the VLDLTG pool had turned over completely. The source designated ‘plasma NEFA’ represents NEFA from adipose and chylomicron spillover of FA. [Data from [2] and [4]]. FA, fatty acid; NEFA, nonesterified fatty acid; TG, triglyceride. Volume 25 Number 3 June 2014


Liver fatty acid fluxes and hypertriglyceridemia Jacome-Sosa and Parks

INTESTINAL-TRIGLYCERIDE SECRETION, DIETARY FAT, AND FACTORS THAT AFFECT SPILLOVER

remains relatively steady throughout the day in muscle, and increases in muscle with fasting. Although it has been commonly thought that the majority of dietary fat is cleared through adiposeLPL, this perception is supported by studies that compared LPL activity in adipose and muscle with the data reported in units of FA released per g of tissue per min [21,22]. In lean and overweight individuals, the total mass of muscle can be greater than, or equal to, adipose mass, which results in the potential for net lipid clearance to muscle to become substantial. We have found that spillover of dietary FA is significantly and inversely associated with muscle mass [3,4,15,23] and more weakly, yet significantly, with fat mass (Fig. 2) [3,4,15,23]. The ability of muscle to take up and store NEFA has recently being investigated by Kanaley et al. [24 ] in individuals in the fasting state and during a clamp. Insulin reduced NEFA trafficking into the intramuscular pool associated with oxidation of FA and increased it into triglyceride. Saleh et al. [25] and Sniderman [26] have described lipid clearance as the balance between LPL-mediated release and tissue uptake. In the fed state, this balance can strongly influence the amount of dietary FA remaining in plasma, which can then flow to the liver or other organs. Plasma NEFA spillover can contribute up to 40% of the total plasma NEFA flux after a meal [27], and this effect can be particularly important in predisposed individuals. For instance, an excessive myocardial partitioning with concomitant reduced uptake of dietary FA in adipose tissue is observed in prediabetic individuals [28]. This impaired capacity of adipose tissue to appropriately upregulate mealfat storage in abdominally obese men [29] is in line with the increased postprandial-triglyceride storage

One of the most intriguing findings of the past 10 years has been the identification of elevated intestinal lipoprotein secretion as a cause of fasting HPTG. It has long been recognized that the presence of chylomicron remnants contributes to fasting dyslipidemia, but the primary focus of these studies has been reduced clearance of dietary-triglyceride in the fed state. An elegant series of studies from Lewis et al. [16–18] has demonstrated that insulin resistance is associated with overproduction of intestinal lipoproteins in response to an acute elevation of plasma NEFA [17], whereas this is inhibited by infusion of insulin [18], and a glucagon-like peptide-1 receptor agonist [19], in healthy individuals. Xiao et al. [20 ] found that intraduodenal coinfusion of glucose with intralipid increased apoB48 production rate by six-fold but did not affect apoB100 production rate. By contrast, when glucose was replaced by fructose the production rate of TRLapoB48 and TRL-apoB100 was increased above fasting by 1.6-fold and 1.4-fold, respectively. The concentrations of TRL-triglyceride were equally increased by glucose and fructose infusions. Dietary fat carried in chylomicrons is transported to peripheral tissues through the blood, and the release of TRL-triglyceride FA into the plasma NEFA pool (a process termed ‘spillover’) was described as early as 1969 by Eaton et al. [13]. However, the factors that impact this release have only recently become known. Release of chylomicron-FA occurs via the action of lipoprotein lipase (LPL), which is an enzyme upregulated throughout the day in adipose tissue [21,22]. LPL activity &

&

Healthy

r = –0.547 P = 0.0001

NASH patients

0.50

0.50 0.45

Peak spillover NEFA (mmol/L)

0.45

Peak spillover NEFA (mmol/L)

r = –0.438 P = 0.0003

Insulin resistant

0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05

0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05

0.00

0.00 20

30

40

50

60

70

80

90

100

Lean body mass (kg)

0

10

20

30

40

50

60

70

80

Fat mass (kg)

FIGURE 2. (a) Relationships between spillover and individual lean mass and body fat. The peak spillover quantities measured in individuals during the 6 h after eating a meal (30% energy as fat) in insulin-resistant individuals (open squares, described in ref [15]), individuals with nonalcoholic fatty liver disease (closed squares, [3]), and healthy individuals from two studies (diamonds, [4,23]). (b), Spillover and body fat content from the same individuals as in (a). Data from [3,4,15] and [23]. 0957-9672 ß 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins


215


Lipid metabolism

in the liver and muscle of type 2 diabetic individuals [30]. Accumulating evidence suggests that the magnitude of dietary spillover is not only determined by the efficiency of removal/storage and oxidation in peripheral tissues (skeletal muscle and adipose tissue), but also by conditions that exacerbate the postprandial lipid response. These include age, exercise detraining, and greater quantities of fat in a given meal. Given recent focus on intramyocellular lipid and its contribution to insulin resistance [24 ], future studies are needed to elucidate the relative role of skeletal muscle in the regulation of TRLtriglyceride clearance and its potential to mitigate NAFLD and hypertriglyceridemia [31,32]. &

EXERCISE AND AGE AFFECT SPILLOVER In a comprehensive study performed by Bergouignan et al. [33 ], spillover was measured in two groups of healthy men who were either active (n ¼ 9) and then detrained, or sedentary (n ¼ 10) and then trained at 50% VO2 peak three times a week for 2 months. Chylomicron spillover was reduced 17% with training and, surprisingly increased 25% with detraining. Reduced spillover after training was accompanied by increased oxidation of dietary FA, whereas the opposite occurred after detraining. Detailed analysis of muscle biopsies revealed that training upregulated the expression of genes involved in uptake and oxidation of FA. These data show that the presence of dietary FA in the NEFA pool occurs because of a lack of uptake and oxidation by the skeletal muscle. Increased postprandial-triglyceride responses associated with aging have also been associated with increased spillover of dietary FA into plasma NEFA and small TRL [34]. In elderly versus young individuals, an increased postprandial contribution of dietary lipid (stably-labeled oleate) into the NEFA pool was found (41 versus 26%), and this occurred along with a lower postprandial capacity for oxidation of the ingested lipid (32 versus 45%). Elderly and young individuals were matched for fasting plasma-triglyceride and fat-free mass while body weight and fat mass were higher in the elderly relative to young individuals implicating age and body fat as determining factors. Whether poor removal and oxidation of dietary lipid by peripheral tissues could contribute to an increase in TRL secretion in the postprandial period was not examined in this study. The same group of elderly individuals was further studied to determine the effect of increasing concentrations of dietary fat (0.4 and 0.7 g of fat/kg body weight) on postprandial spillover of ingested fat [35]. A meal test with higher levels of fat (54–65 g/meal) more than doubled the plasma NEFA concentration, which was directly related to a 130% increase in &&

216


spillover. Here, spillover of dietary fat was inversely correlated with the percentage of body fat. As this study was conducted only in elderly individuals, it remains unknown whether greater fat feedings also increase spillover in younger individuals who are more metabolically flexible.

EFFECT OF WEIGHT LOSS AND SUPPRESSION OF ADIPOSE TISSUE LIPOLYSIS ON SPILLOVER Spillover is determined not only by the quantity of body fat, but also by a tissue’s ability to take up and store fat. Earlier studies demonstrated significant reductions in spillover after weight loss induced by gastric bypass, presumably as a result of increased efficiency of dietary fat storage in adipose tissue [36]. However, recent evidence suggests that body fat distribution and/or the size of fat depots rather than fat mass per se may be a determining factor. Almandoz et al. [37] evaluated the effect of a 5-month weight loss intervention on dietary spillover in obese type 2 diabetic individuals. Although weight loss (13% reduction) resulted in significant improvements in insulin resistance, plasma-triglyceride concentrations, and a 38% decrease in meal suppression of NEFA concentrations, it failed to reduce the fractional dietary spillover rate. Interestingly, the greater the lower-body fat depot size, the lower dietary fat spillover at baseline. After weight loss, those individuals with lower amounts of leg fat experienced weight loss induced increases in spillover, whereas those with higher quantities of leg fat exhibited weight loss induced reductions in spillover. Therefore, these data suggest that the efficiency by which dietary fat is stored in lowerbody fat depots may determine the extent of spillover in type 2 diabetes. Santosa et al. [38] found that after weight loss in upper body obese individuals, meal fat storage/g adipose lipid was preferentially routed back to the upper body depots. This effect would increase the risk for weight regain and result in a similar body fat distribution as was present before weight loss. In another study, lower body fat depots responded to overfeeding by increasing fat-cell hyperplasia, whereas in upper-body depots, fat-cell size expanded with overfeeding [39]. Clearly, adipocyte expandability can protect the liver from excess burden of FA and individual responses to overnutrition may predispose them to, or protect them, from development of fatty liver during overfeeding. Another key concept of recent investigation is that efficient storage of dietary fat may require the effective suppression of adipose tissue lipolysis. In a test of a model in which increased intracellular Volume 25 Number 3 June 2014



adipose-triglyceride turnover reduces spillover, Muthusamy et al. [40] demonstrated that insulin infusion reduced plasma NEFA concentrations (40%) and adipose tissue lipolysis (25%), but dietary spillover remained unchanged while niacin infusion reduced spillover 30% [41]. The impact of spillover occurring in tissues other than adipose was not considered in this model. In summary, it has become apparent that the efficiency by which dietary FA are taken up by adipose tissue and the muscle during conditions that exaggerate the postprandial response (e.g., aging, insulin resistance, and high-fat feeding) is a major determinant of the magnitude of dietary spillover. As a result these factors will influence liver triglyceride accrual.

FACTORS AFFECTING DE NOVO LIPOGENESIS AND ITS ROLE IN NONALCOHOLIC FATTY LIVER DISEASE

100 90 80 70 60 50 40 30 20 10

Fasting VLDL-TG (mg/dL)

Hepatic de-novo lipogenesis is the process by which endogenous FA are synthesized from carbohydrates [42]. The lipogenic pathway appears to underlie the mechanism of excess hepatic-triglyceride storage during carbohydrate overfeeding. This effect was recently examined by measuring liver fat accumulation following 3 weeks of carbohydrate overfeeding (1000 extra kcal/day) in overweight individuals with the PNPLA3 geneotype rs738409 [43], which influences the prevalence of fatty liver [44]. Although DNL was not measured directly, the ratio of palmitate to linoleate in VLDL-triglyceride (a surrogate marker of DNL) was related to a 27% increase in liver fat. In healthy individuals, lipogenesis is suppressed by high-fat diets and fasting [45], whereas in insulin-resistant individuals with NAFLD, the appropriate suppression of DNL with fasting fails to occur [3,15]. For example, in fasted, lean individuals, DNL may contribute less than 10% of FA in VLDL-triglyceride [46]; in obesity [47] and insulin resistance [3,48], lipogenesis may provide up to 50% of palmitate in VLDL-triglyceride. The pathway of DNL produces saturated FA, and these FA are elevated in NAFLD [49]. Zong et al. [50] found that FA likely to be derived from lipogenesis (C16 : 0, C16 : 1) were prospectively associated with increased risk of type 2 diabetes in people with diets habitually high in carbohydrate. This raises an important question: given that dietary sugars stimulate DNL, which produces saturated FA, should dietary sugar intake be considered as atherogenic as dietary saturated FA? It is unknown whether newly synthesized saturated FA have a similar impact on hepatic metabolism as exogenous saturated FA, but data from multiple lines of evidence suggest that de-novo production may be linked to mitochondrial toxicity

[51] and increased esterification of FA. In a recent study comparing individuals with high and lowliver fat, who were matched for body composition and insulin sensitivity, the key difference between those with and without fatty liver was the extent of DNL [15]. The stimulatory effects of DNL on hepatictriglyceride accrual may not be limited to increased synthesis of FA but also involve the concomitant inhibition of transport of FA into the mitochondria, thus suppressing FA oxidation [52]. Further, the relationship of liver-triglyceride to VLDL-triglyceride synthesis may be present only in the setting of excess energy, when lipogenesis is high. In the same study of patients with NAFLD described above, a subset of those individuals participated in a dietary program to lose 10% body weight. Liver-triglyceride was significantly reduced but as shown in Fig. 3, weight loss had a greater effect to reduce liver-triglyceride than it did to reduce VLDL-triglyceride concentration. Before weight loss, the ranges of liver-triglyceride contents and plasma-triglyceride concentrations were both governed by excess caloric intake and genetic factors. By contrast, after weight loss, when individuals were acclimated to significantly lower food intakes, the range of liver-triglyceride plummeted but VLDL-triglyceride did not fall as far. It is notable that when the individuals achieved stability at the lower body weights, they were still obese. Post-weight loss liver-triglyceride contents were all within normal ranges, whereas there remained a wide range of VLDL-triglyceride

0 30%

25%

20%

15%

10%

5%

0%

Liver-TG (%)

FIGURE 3. Changes in liver-TG and VLDL-TG with 10% weight loss. Ten men and women with NAFLD had liver-TG measured by magnetic resonance spectroscopy and VLDL isolated from plasma in the fasting state. Weight loss was induced by a 550 kcal reduction in energy intake over 6 month, after which the individuals were weight stable. Open symbols, preweight loss data, closed symbols, postweight loss data. Circles, women; squares, men (E.J. Parks, unpublished). NAFLD, nonalcoholic fatty liver disease; TG, triglyceride.



217


Lipid metabolism

concentrations, which presumably were now governed by genetic factors. In summary, livertriglyceride content is strongly dependent on recent energy balance, and, although fatty liver can cause HPTG, it contributes only to a portion of the HPTG of obesity.

SUBSTRATE PARTITIONING, OBESITY, AND INSULIN RESISTANCE As increased carbohydrate supply stimulates the hepatic DNL process, reduced glucose uptake by extrahepatic tissues (primarily the skeletal muscle) could, therefore, result in increased availability of lipogenic substrates for new synthesis of FA in the liver. This hypothesis was investigated in a group of insulin resistant, elderly individuals (n ¼ 12) by measuring changes in muscle and liver glycogen and lipid content after consumption of high-carbohydrate meals, relative to pair-matched young individuals (n ¼ 12) [53]. The reduced postprandial muscle glycogen synthesis (45%) in elderly individuals paralleled a two-fold increase in hepatic DNL and was associated with a three-fold increase in hepatic-triglyceride content and 1.5-fold higher plasma-triglyceride concentration. The opposite was observed when young, insulin-resistant individuals (n ¼ 12) performed an acute exercise bout before consuming a carbohydrate-rich meal. Relative to resting conditions, training increased muscle glycogen synthesis by three-fold, whereas hepatic DNL and triglyceride synthesis were reduced by 30 and 40%, respectively [32]. Recent evidence suggests that DNL can influence the metabolic consequences of obesity and insulin resistance in a tissue-specific manner. Eissing et al. [54] assessed the expression of enzymes necessary for DNL in adipose and liver samples from nonobese as well as obese individuals with and without type 2 diabetes. The expression of liver genes involved in DNL was found to be upregulated in obese individuals, the expression of adipose lipogenic genes and that of the glucose transporter-4 were found to be substantially decreased. Interestingly, the dysregulation in lipogenic markers in adipose tissue was reversed by weight loss through bariatric surgery. Consistent with these findings, Tuvdendorj et al. [55 ] reported a reduced DNL and triglyceride synthesis in subcutaneous abdominal adipose tissue in obese, insulin-resistant individuals. Whether reduced adipose-triglyceride turnover in obesity is related to reduced adipose DNL or reduced intracellular lipolysis or re-esterification of plasma NEFA, cannot be determined from these studies. Nonetheless, these data suggest that the impaired lipogenic capacity in adipose tissue &&

218


limits adipose tissue storage capacity characteristic of these conditions and may contribute to HPTG through greater flux of NEFA to the liver.

ADIPOSE TISSUE FLUX AND VLDLTRIGLYCERIDE PRODUCTION Plasma NEFA are major substrates for hepatic-triglyceride esterification and secretion – accounting for 62–100% of the total liver-triglyceride secreted in the form of VLDL [3,46]. Numerous early studies demonstrated that elevations in plasma NEFA flux directly result in increased VLDL-triglyceride secretion rate and for many years, the plasma NEFA pool was thought to be the sole source of FA used for VLDL-triglyceride synthesis [5,6,56]. However, recent studies have highlighted some disassociation between the rate of plasma NEFA flux and VLDLtriglyceride secretion rates. A greater availability of plasma NEFA does not necessarily contribute to significantly greater VLDL-triglyceride secretion [57]. Koutsari et al. [58] measured the proportion of the total circulating NEFA pool that was incorporated into VLDL-triglyceride in lean and obese men and women under different physiological conditions (postabsorptive, postprandial, and walking). They found that despite major differences in systemic NEFA availability and turnover, only 7% of circulating NEFA was directed to VLDL-triglyceride secretion in overweight/obese normotriglyceridemic individuals. The only exception was found in the postprandial state when 15% of systemic NEFA concentrations were directed toward VLDL-triglyceride, even though adipose tissue lipolysis was the lowest during these conditions. Visceral fat mass was the only and the strongest predictor of VLDLtriglyceride secretion. Interestingly, an opposing finding was observed in a study of African-American and Caucasian women wherein the proportion of VLDL-triglyceride derived from nonsystemic FA was positively correlated with intra-hepatic triglyceride but not with visceral adipose tissue mass [59].

CONCLUSION Historically, the study of the causes of HPTG focused on quantifying the magnitude of liver-triglyceride secretion and identifying genetic and lifestyle factors that reduced triglyceride clearance from plasma. As the prevalence of obesity and diabetes has grown over the last decade, so too has concern for these conditions to be associated with HPTG, which is caused by increased intestinal secretion of lipoproteins and, more importantly, excess liver-triglyceride levels. Much new data have become available to aid in the understanding of the causes of NAFLD, Volume 25 Number 3 June 2014



and strong evidence has pointed to poor peripheral FA and glucose handling as causes. Current views in this field rest on the concept of the liver as an innocent bystander, which after taking up excess FA and glucose, increases production of triglyceride and secretion of VLDL-triglyceride. Clearly, both adipose and muscle insulin sensitivity play major roles. Acute energy excess causes fatty liver, and this in turn can lead to greater liver-triglyceride secretion and HPTG. However, once the energy excess is alleviated by exercise or dietary restriction, liver fat is reduced, and HPTG is also reduced, although less so. Given the significant advances made in the understanding of whole body partitioning of FA, lifestyle strategies to improve peripheral lipid clearance should be more strongly advocated to treat fatty liver and dyslipidemia, thereby reducing cardiovascular risk. Acknowledgements The authors are appreciative to the many scientists in this field who have shared their enthusiasm and insights during lively discussions of data. The author’s research is supported by grants from the NIH and the American Diabetes Association. Conflicts of interest There are no conflicts of interest.

REFERENCES AND RECOMMENDED READING Papers of particular interest, published within the annual period of review, have been highlighted as: & of special interest && of outstanding interest 1. Miller M, Stone NJ, Ballantyne C, et al. Triglycerides and cardiovascular disease: a scientific statement from the American Heart Association. Circulation 2011; 123:2292–2333. 2. Barrows BR, Timlin MT, Parks EJ. Spillover of dietary fatty acids and use of serum nonesterified fatty acids for the synthesis of VLDL-triacylglycerol under two different feeding regimens. Diabetes 2005; 54:2668–2673. 3. Donnelly KL, Smith CI, Schwarzenberg SJ, et al. Sources of fatty acids stored in liver and secreted via lipoproteins in patients with nonalcoholic fatty liver disease. J Clin Invest 2005; 115:1343–1351. 4. Timlin MT, Barrows B, Parks EJ. Increased dietary substrate delivery alters hepatic fatty acid recycling in healthy men. Diabetes 2005; 54:2694– 2701. 5. Kissebah AH, Adams PW, Wynn V. Plasma free fatty acid and triglyceride transport kinetics in man. Clin Sci Mol Med 1974; 47:259–278. 6. Kissebah AH, Alfarsi S, Adams PW, et al. Transport kinetics of plasma free fatty acid, very low density lipoprotein triglycerides and apoprotein in patients with endogenous hypertriglyceridaemia: effects of 2,2-dimethyl, 5(2, 5-xylyoxy) valeric acid therapy. Atherosclerosis 1976; 24:199–218. 7. Siler SQ, Neese RA, Parks EJ, Hellerstein MK. VLDL-triglyceride production after alcohol ingestion, studied using [2-13C1] glycerol. J Lipid Res 1998; 39:2319–2328. 8. Brouwers MC, van Greevenbroek MM, Bilderbeek-Beckers MA, et al. Fatty liver-based identification of two distinct hypertriglyceridemic subgroups in familial combined hyperlipidemia. Metabolism 2007; 56:1311–1317. 9. Stanhope KL, Schwarz JM, Keim NL, et al. Consuming fructose-sweetened, not glucose-sweetened, beverages increases visceral adiposity and lipids and decreases insulin sensitivity in overweight/obese humans. J Clin Invest 2009; 119:1322–1334. 10. Le KA, Ith M, Kreis R, et al. Fructose overconsumption causes dyslipidemia and ectopic lipid deposition in healthy subjects with and without a family history of type 2 diabetes. Am J Clin Nutr 2009; 89:1760–1765.

11. Faeh D, Minehira K, Schwarz JM, et al. Effect of fructose overfeeding and fish oil administration on hepatic de novo lipogenesis and insulin sensitivity in healthy men. Diabetes 2005; 54:1907–1913. 12. Tinker LF, Parks EJ, Behr SR, et al. (n-3) fatty acid supplementation in moderately hypertriglyceridemic adults changes postprandial lipid and apolipoprotein B responses to a standardized test meal. J Nutr 1999; 129:1126– 1134. 13. Eaton RP, Berman M, Steinberg D. Kinetic studies of plasma free fatty acid and triglyceride metabolism in man. J Clin Invest 1969; 48:1560–1579. 14. Sanyal AJ, Campbell-Sargent C, Mirshahi F, et al. Nonalcoholic steatohepatitis: association of insulin resistance and mitochondrial abnormalities. Gastroenterology 2001; 120:1183–1192. 15. Lambert JE, Ramos-Roman MA, Browning JD, Parks EJ. Increased de novo lipogenesis is a distinct characteristic of individuals with nonalcoholic fatty liver disease. Gastroenterology 2014; 146:726–735. 16. Duez H, Lamarche B, Uffelman KD, et al. Hyperinsulinemia is associated with increased production rate of intestinal apolipoprotein B-48-containing lipoproteins in humans. Arterioscler Thromb Vasc Biol 2006; 26:1357– 1363. 17. Duez H, Lamarche B, Vale´ro R, et al. Both intestinal and hepatic lipoprotein production are stimulated by an acute elevation of plasma free fatty acids in humans. Circulation 2008; 117:2369–2376. 18. Pavlic M, Xiao C, Szeto L, et al. Insulin acutely inhibits intestinal lipoprotein secretion in humans in part by suppressing plasma free fatty acids. Diabetes 2010; 59:580–587. 19. Xiao C, Bandsma RH, Dash S, et al. Exenatide, a glucagon-like peptide-1 receptor agonist, acutely inhibits intestinal lipoprotein production in healthy humans. Arterioscler Thromb Vasc Biol 2012; 32:1513–1519. 20. Xiao C, Dash S, Morgantini C, Lewis GF. Novel role of enteral monosacchar& ides in intestinal lipoprotein production in healthy humans. Arterioscler Thromb Vasc Biol 2013; 33:1056–1062. This publication presents the latest data from a series of elegant studies investigating the role of intraluminal and plasma substrates in the control of apoB48 secretion in health and insulin resistance. 21. Ruge T, Svensson M, Eriksson JW, Olivecrona G. Tissue-specific regulation of lipoprotein lipase in humans: effects of fasting. Eur J Clin Invest 2005; 35:194–200. 22. Berk ES, Johnson JA, Lee M, et al. Higher postabsorptive skeletal muscle LPL activity in African American vs. non-Hispanic White premenopausal women. Obesity (Silver Spring, Md ) 2008; 16:199–201. 23. Chavez-Jauregui RN, Mattes RD, Parks EJ. Dynamics of fat absorption and effect of sham feeding on postprandial lipema. Gastroenterology 2010; 139:1538–1548. 24. Kanaley JA, Shadid S, Sheehan MT, et al. Hyperinsulinemia and skeletal & muscle fatty acid trafficking. Am J Physiol Endocrinol Metab 2013; 305:E540–548. This is an excellent demonstration of the impact of insulin (clamp) on FA partitioning in skeletal muscle. The data support the hypothesis that in vivo, during hyperinsulinemia, plasma oleate is trafficked to triglyceride droplets and away from the acylcarnintine pool and oxidation. 25. Saleh J, Sniderman AD, Cianflone K. Regulation of plasma fatty acid metabolism. Clinica Chimica Acta 1999; 286:163–180. 26. Sniderman AD. Postprandial hypertriglyceridemia(s): time to enlarge our pathophysiologic perspective. Eur J Clin Invest 2000; 30:935–937. 27. Lambert JE, Parks EJ. Postprandial metabolism of meal triglyceride in humans. Biochimica Et Biophysica Acta 2012; 1821:721–726. 28. Labbe SM, Grenier-Larouche T, Noll C, et al. Increased myocardial uptake of dietary fatty acids linked to cardiac dysfunction in glucose-intolerant humans. Diabetes 2012; 61:2701–2710. 29. McQuaid SE, Hodson L, Neville MJ, et al. Downregulation of adipose tissue fatty acid trafficking in obesity: a driver for ectopic fat deposition? Diabetes 2011; 60:47–55. 30. Ravikumar B, Carey PE, Snaar JE, et al. Real-time assessment of postprandial fat storage in liver and skeletal muscle in health and type 2 diabetes. Am J Physiol Endocrinol Metab 2005; 288:E789–E797. 31. Trovato GM, Catalano D, Martines GF, et al. Western dietary pattern and sedentary life: independent effects of diet and physical exercise intensity on NAFLD. Am J Gastroenterol 2013; 108:1932–1933. 32. Rabol R, Petersen KF, Dufour S, et al. Reversal of muscle insulin resistance with exercise reduces postprandial hepatic de novo lipogenesis in insulin resistant individuals. Proc Natl Acad Sci U S A 2011; 108:13705–13709. 33. Bergouignan A, Momken I, Lefai E, et al. Activity energy expenditure is a major && determinant of dietary fat oxidation and trafficking, but the deleterious effect of detraining is more marked than the beneficial effect of training at current recommendations. Am J Clin Nutr 2013; 98:648–658. This well controlled research study used a combination of isotopic labeling, meal feeding, and tissue biopsies before and after separate groups of individuals exercise trained, or detraining. The methods used here represent the state-ofthe art with respect to determining the impact of physical activity on peripheral FA clearance, storage, and oxidation. 34. Puga GM, Meyer C, Everman S, et al. Postprandial lipemia in the elderly involves increased incorporation of ingested fat in plasma free fatty acids and small (Sf 20–400) triglyceride-rich lipoproteins. Am J Physiol Endocrinol Metab 2011; 301:E356–361.



219


Lipid metabolism 35. Puga GM, Meyer C, Mandarino LJ, Katsanos CS. Postprandial spillover of dietary lipid into plasma is increased with moderate amounts of ingested fat and is inversely related to adiposity in healthy older men. J Nutr 2012; 142:1806–1811. 36. Faraj M, Jones P, Sniderman AD, Cianflone K. Enhanced dietary fat clearance in postobese women. J Lipid Res 2001; 42:571–580. 37. Almandoz JP, Singh E, Howell LA, et al. Spillover of fatty acids during dietary fat storage in type 2 diabetes: relationship to body fat depots and effects of weight loss. Diabetes 2013; 62:1897–1903. 38. Santosa S, Hensrud DD, Votruba SB, Jensen MD. The influence of sex and obesity phenotype on meal fatty acid metabolism before and after weight loss. Am J Clin Nutr 2008; 88:1134–1141. 39. Tchoukalova YD, Votruba SB, Tchkonia T, et al. Regional differences in cellular mechanisms of adipose tissue gain with overfeeding. Proc Natl Acad Sci U S A 2010; 107:18226–18231. 40. Muthusamy K, Nelson RH, Singh E, et al. Effect of insulin infusion on spillover of meal-derived fatty acids. J Clin Endocrinol Metab 2012; 97:4201– 4205. 41. Nelson RH, Vlazny D, Smailovic A, Miles JM. Intravenous niacin acutely improves the efficiency of dietary fat storage in lean and obese humans. Diabetes 2012; 61:3172–3175. 42. Hudgins LC, Hellerstein M, Seidman C, et al. Human fatty acid synthesis is stimulated by a eucaloric low fat, high carbohydrate diet. J Clin Invest 1996; 97:2081–2091. 43. Sevastianova K, Santos A, Kotronen A, et al. Effect of short-term carbohydrate overfeeding and long-term weight loss on liver fat in overweight humans. Am J Clin Nutr 2012; 96:727–734. 44. Koutsari C, Lazaridis KN. Emerging genes associated with the progression of nonalcoholic fatty liver disease. Hepatology (Baltimore, Md ) 2010; 52:807– 810. 45. Schwarz JM, Neese RA, Turner S, et al. Short-term alterations in carbohydrate energy intake in humans. Striking effects on hepatic glucose production, de novo lipogenesis, lipolysis, and whole-body fuel selection. J Clin Invest 1995; 96:2735–2743. 46. Parks EJ, Krauss RM, Christiansen MP, et al. Effects of a low-fat, highcarbohydrate diet on VLDL-triglyceride assembly, production, and clearance. J Clin Invest 1999; 104:1087–1096.

220


47. Marques-Lopes I, Ansorena D, Astiasaran I, et al. Postprandial de novo lipogenesis and metabolic changes induced by a high-carbohydrate, lowfat meal in lean and overweight men. Am J Clin Nutr 2001; 73:253–261. 48. Vedala A, Wang W, Neese RA, et al. Delayed secretory pathway contributions to VLDL-triglycerides from plasma NEFA, diet, and de novo lipogenesis in humans. J Lipid Res 2006; 47:2562–2574. 49. Oresic M, Hyotylainen T, Kotronen A, et al. Prediction of nonalcoholic fattyliver disease and liver fat content by serum molecular lipids. Diabetologia 2013; 56:2266–2274. 50. Zong G, Zhu J, Sun L, et al. Associations of erythrocyte fatty acids in the de novo lipogenesis pathway with risk of metabolic syndrome in a cohort study of middle-aged and older Chinese. Am J Clin Nutr 2013; 98:319–326. 51. Listenberger LL, Han X, Lewis SE, et al. Triglyceride accumulation protects against fatty acid-induced lipotoxicity. Proc Natl Acad Sci U S A 2003; 100:3077–3082. 52. McGarry JD. Banting lecture 2001: dysregulation of fatty acid metabolism in the etiology of type 2 diabetes. Diabetes 2002; 51:7–18. 53. Flannery C, Dufour S, Rabol R, et al. Skeletal muscle insulin resistance promotes increased hepatic de novo lipogenesis, hyperlipidemia, and hepatic steatosis in the elderly. Diabetes 2012; 61:2711–2717. 54. Eissing L, Scherer T, Todter K, et al. De novo lipogenesis in human fat and liver is linked to ChREBP-beta and metabolic health. Nat Commun 2013; 4:1528. 55. Tuvdendorj D, Chandalia M, Batbayar T, et al. Altered subcutaneous abdom&& inal adipose tissue lipid synthesis in obese, insulin-resistant humans. Am J Physiol Endocrinol Metab 2013; 305:E999–E1006. Adipose triglyceride turnover was labeled in vivo using d2O in lean and obese individuals to demonstrate limited FA storage in obesity that was associated with muscle and adipose insulin resistance. 56. Julius U. Influence of plasma free fatty acids on lipoprotein synthesis and diabetic dyslipidemia. Exp Clin Endocrinol Diabetes 2003; 111:246–250. 57. Nielsen S, Karpe F. Determinants of VLDL-triglycerides production. Curr Opin Lipidol 2012; 23:321–326. 58. Koutsari C, Mundi MS, Ali AH, et al. Systemic free fatty acid disposal into very low-density lipoprotein triglycerides. Diabetes 2013; 62:2386–2395. 59. Miller BV 3rd, Patterson BW, Okunade A, Klein S. Fatty acid and very low density lipoprotein metabolism in obese African American and Caucasian women with type 2 diabetes. J Lipid Res 2012; 53:2767–2772.

Volume 25 Number 3 June 2014


Role of triglyceride-fatty acid cycle in controlling fat metabolism in humans during and after exercise.

Plasma phospholipids and fatty acid composition differ between liver biopsy-proven nonalcoholic fatty liver disease and healthy subjects.

Changes in plasma triglyceride fatty acids during lactation.

Fatty Liver and Fatty Heart-Where do They Stand in the AMIS Syndrome?

Liver fatty acid-binding protein as a diagnostic marker for non-alcoholic fatty liver disease.

Plasma free Fatty Acid concentrations as a marker for acute myocardial infarction.

Characterization of Retail Conventional, Organic, and Grass Full-Fat Butters by Their Fat Contents, Free Fatty Acid Contents, and Triglyceride and Fatty Acid Profiling.

Rapid effects of environmental disturbance on rat plasma unesterified fatty acid and tryptophan concentrations and their prevention by antilopolytic drugs.

Fatty acid metabolism via triglyceride in the salmon heart.

Long-chain polyunsaturated fatty acid sources and evaluation of their nutritional and functional properties.

Serum fatty acid and lipoprotein subclass concentrations and their associations in prepubertal healthy Norwegian children.

"Non alcoholic fatty liver disease and eNOS dysfunction in humans".

Dose-dependent effect of insulin on plasma free fatty acid turnover and oxidation in humans.

Modulation of plasma N-acylethanolamine levels and physiological parameters by dietary fatty acid composition in humans.

Cheddar Cheese Ripening Affects Plasma Nonesterified Fatty Acid and Serum Insulin Concentrations in Growing Pigs.

Measuring oral fatty acid thresholds, fat perception, fatty food liking, and papillae density in humans.

Effects of antihypertensive and triglyceride-lowering agents on hepatic copper concentrations in rats with fatty liver disease.

Liver Mitochondrial DNA Copy Number and Deletion Levels May Contribute to Nonalcoholic Fatty Liver Disease Susceptibility.

Effect of nutritional status on rat adipose tissue, muscle and post-heparin plasma clearing factor lipase activities: their relationship to triglyceride fatty acid uptake by fat-cells and to plasma insulin concentrations.

fatty acid cycling is increased after exercise.

The triglyceride and glucose index (TyG) is an effective biomarker to identify nonalcoholic fatty liver disease.

ER stress and autophagy dysfunction contribute to fatty liver in diabetic mice.

Plasma non-esterified fatty acid levels in children and their relationship with sex steroids.

PCB126-Induced Disruption in Gluconeogenesis and Fatty Acid Oxidation Precedes Fatty Liver in Male Rats.