From Natural Triacylglycerols to Novel Structured Lipids Containing n-3 Long-Chain Polyunsaturated Fatty Acids

Chapter 17

From Natural Triacylglycerols to Novel Structured Lipids Containing n-3 Long-Chain Polyunsaturated Fatty Acids Paula A. Lopes, Jose´ M. Pestana, Diogo Coelho, Marta S. Madeira, Cristina M. Alfaia and Jose´ A.M. Prates CIISA-Centro de Investigação Interdisciplinar em Sanidade Animal, Faculdade de Medicina Veterinária (FMV), Universidade de Lisboa, Av. da Universidade Técnica, Pólo Universitário do Alto da Ajuda, Lisboa, Portugal

1. INTRODUCTION The U.S. National Academy of Sciences defines fats and oils as “complex organic molecules that are formed by combining three fatty acids (FAs) with one molecule of glycerol.” Triacylglycerols (TAGs), formerly designated triglycerides, is the term used to define this molecular structure that makes up virtually all fats and oils in both animal and plant species. The molecular structure of different TAGs modulates their metabolic fate in the organism, in particular digestion and intestinal absorption, and is enhanced for FA located in the sn-2 position. Jensen et al. (1990) were among the first to confirm that FAs in the sn-2 position are preferentially absorbed, followed by Xu (2000) and Hunter (2001) reports. Later on, MoralesMedina et al. (2016) did a full characterization on the nutritional value from different fish oil species and concluded that docosahexaenoic acid (DHA, 22:6n-3) presented high regiospecificity to position sn-2, whereas the tendency of eicosapentaenoic acid (EPA, 20:5n-3) was to occupy sn-1 and sn-3 positions. EPA and DHA are n-3 long-chain polyunsaturated fatty acids (LCPUFAs) with a wide range of beneficial effects to human health (Calder, 2012), including prevention of agerelated chronic diseases from neurological and cardiovascular spectra. In the United States and some parts of Europe, the dietary intake of marine-based n-3 LCPUFA is well below the recommended values for optimal health (Kris-Etherton et al., 2002). Many difficulties stand in the way to accomplish the nutritional guidelines for n-3 LCPUFA intake, namely food habits, the concern about methyl mercury in some fish species, and the cost and poor stability of marine oils incorporated into foods. In addition, current supplies of wild and farmed fish seem also insufficient (Deckelbaum and Torrejon, 2012). Facing this discussion, it is imperative to look for alternative sources of n-3 LCPUFA to increase the bioavailability of EPA and DHA. On the one hand, microalgae production stands out as a good alternative to auto-sustainable n-3 LCPUFA resources because they do not require freshwater or arable land for their growth and have the ability for high FA accumulation (over 20%) as well as to produce highly pure EPA and DHA oils (reviewed by Madeira et al., 2017). On the other hand, dietary supplements seem to be a good solution to improve people health. Currently, a wide variety of food supplements are commercially available to consumers with different amounts, levels of purity, types, and forms of n-3 LCPUFA mostly due to the low efficiency on the conversion of a-linolenic acid (18:3n-3, ALA) into EPA and DHA (Bandarra et al., 2016; Lopes et al., 2017a,b). In this respect, much attention has been directed to the chemical or enzymatic synthesis of structured lipids (SLs), in particular of different forms of TAG containing n-3 LCPUFA. Structuration of naturally occurring fats could have implications regarding FA attainability; however, the scientific knowledge on the bioavailability and potential health benefits of SL is far from completely understood. This chapter reviews the current status on SLs from the most recent research trends of synthesis to their role in metabolic and nutritional applications in health care of hospitalized patients and child support, as well as the public at large.

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2. STRUCTURED LIPIDS AND BIOFUNCTIONALITY Lipids that have been chemically and/or enzymatically modified to produce novel TAGs with desired health, functional, or nutritional values are often referred to as structured lipids (SL) (Akoh and Kim, 2008). The design of an SL with a particular chemical structure enables to modulate TAG behavior. Based on this perspective, the structuration of TAGs has received much attention as they are TAG modified through chemical or enzymatic methods, to alter their FA composition and/or to rearrange FA positions relative to the glycerol backbone (Hunter, 2001). Medium-long-medium (MLM), medium-medium-long (MML), long-medium-long (LML), and long-long-medium (LLM) are four different types of FA arrangement on the glycerol backbone (Kim and Akoh, 2015). Among these several types of structured lipids, MLM type comprising medium-chain FAs esterified in the sn-1,3 positions and long-chain FA in the sn-2 position of the glycerol backbone are considered the desired structure of SL (Nunes et al., 2012; Kim and Akoh, 2015) being well absorbed through the intestinal mucosa (Jandacek et al., 1987). The reaction schemes of these structured TAGs are shown in Fig. 17.1. The combinations to produce MLM-type structured lipids from various plant oils are diverse and described in detail elsewhere (Jennings et al., 2010; Kim et al., 2010; Ozturk et al., 2010; Sengupta and Ghosh, 2011b; Silroy and Ghosh, 2011; Chnadhapuram and Sunkireddy, 2012; Choi et al., 2012; Nunes et al., 2012; Savaghebi et al., 2012; Wang et al., 2012; Gokce et al., 2013; Caballero et al., 2014; Qin et al., 2014; Silroy et al., 2014). MLM-type SLs are expected to have different physiological attributes from LCT, the typical TAG form of many edible fats and oils because of the presence of both medium- and long-chain FA in the same TAG molecules (Kim et al., 2008b).

2.1 Scale-Up Synthesis and Positional Distribution of Fatty Acids in Structured Lipids SL can be prepared by two kinds of catalysts: chemical interesterification and enzymatic interesterification. Chemical interesterification is cheaper and takes less reaction time than enzymatic interesterification (Klinkesorn et al., 2004), but some problems might occur with stereospecificity, unless lower temperatures are strictly controlled to avoid the production of FA random rearrangements (Farfán et al., 2013).

2.1.1 SL May Be Produced via Different Reaction Types Interesterification reactions involve exchange of fatty acyl groups between two or more TAG molecules (Idris and Mat Dian, 2005). The reaction starts with TAG hydrolysis into FFA followed by reesterification of the FFA into the glycerol backbone. Examples of commercial fats that use this reaction are Betapol (Lipid Nutrition) and Salatrim (short and acyltriacylglycerol molecules) (Farfán et al., 2013). While Betapol is an example of human milk fat (HMF) analogs (Zou et al., 2016), Salatrim contains low-energy value (w5 kcal/g) synthesized by short-chain FAs that supply fewer calories than long-chain FAs. Acidolysis reactions involve the transfer of acyl group between an acid and an ester (Lee and Akoh, 1998). Several studies (Iwasaki et al., 1999; Hamam and Shahidi, 2004; Wang et al., 2015) used enzymatic acidolysis for the synthesis of novel SL with high content of LCPUFA in the sn-2 position of TAG molecules. Alcoholysis reactions involve the exchange of alkoxy group between an alcohol and an ester, such as glycerol (glycerolysis) or ethanol (ethanolysis) (Lee and Akoh, 1998). The reaction scheme is illustrated in Fig. 17.2. The enzymatic synthesis of SL uses lipases and phospholipases as biocatalysts to modify fats and oils (Sproston et al., 2017) mainly due to their selectivity and regiospecificity (Chu et al., 2001; Wang et al., 2006; Farfán et al., 2013). For

FIGURE 17.1 Schematic representation of structured lipids. The result is a triacylglycerol with combinations of various chain lengths of fatty acids on each glycerol backbone. LCT, long-chain triacylglycerols; MCT, medium-chain triacylglycerols; MLM, medium, long, medium-chain triacylglycerols; LML, long, medium, long-chain triacylglycerols; LLM, long, long, medium-chain triacylglycerols; MML, medium, medium, long-chain triacylglycerols; SL, structured lipids.

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FIGURE 17.2 Schematic overview of structured lipids synthesis representing interesterification, acidolysis, and alcoholysis (glycerolysis) reactions. TAG, triacylglycerols; SL, structured lipids; FFA, free fatty acids; MAG, monoacylglycerols; DAG, diacylglycerols.

example, the sn-1,3-specific lipases show marked preference for the acyl ester bonds in the first and third positions of the acylglycerols incorporating FAs at these sites without changing FA in the sn-2 position (Wang et al., 2006; Kim and Akoh, 2015). Nonspecific lipases do not show distinct specificity with respect to the position of the acyl ester group on the glycerol backbone. Moreover, enzymatic reactions are driven by mild temperatures (Rodrigues and Gioielli, 2003; Criado et al., 2007) with minor loss of original attributes of temperature-sensitive substrates and products (namely, SL) (Kim and Akoh, 2015). Finally, enzymes stand out as an environmentally friendly alternative because they reduce energy as well as the use of dangerous reagents (Kim and Akoh, 2015). The use of immobilized enzymes allows easy recovery and reuse of enzymes over time, reducing cost (Kim and Akoh, 2015). Examples of commercial products are: Lipozyme RM IM (Rhizomucor miehei lipase immobilized on a macroporus anion exchange resin, sn-1,3 specific enzyme), Lipozyme TL IM (Thermomyces lanuginosus lipase immobilized on silica gel, sn-1,3 specific enzyme), and Novozym 435 (Candida antarctica lipase B immobilized on a macroporus acrylic resin and Candida rugosa, nonspecific enzymes) (Kim and Akoh, 2015). Thin-layer chromatography method might detect sn-2 positional FA, as described by Álvarez and Akoh (2016a) modified from Luddy et al. (1964) for the pancreatic lipase-catalyzed sn-2 positional analysis of blending oils. 2-Oleyglycerol was spotted as standard and run in parallel with the sample for identification of 2-MAG band. Then, 2-MAG band was scraped off and converted to FAME. The determination of FA profile in the sn-2 position was quantified by gas chromatography (Álvarez and Akoh, 2016a). Alternatively, Guil-Guerrero et al. (2015) described the positional distribution of DHA in the sn-2 position within the TAG structure by 13C-nuclear magnetic resonance. Table 17.1 summarizes the information on the preparation methods of structured lipids.

2.2 Clinical Studies and Protective Outcomes of Structured Lipids The compilation of health benefits of structured lipids in humans and animal models is presented in Table 17.2. The first report in which structured TAG was administered to confirm safety, tolerance, and efficacy was carried out by Bellantone

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TABLE 17.1 Brief Summary of the Preparation Methods of Structured Lipids Type of Method

Reaction

Nutritional Applications

References

Chemical method

Interesterification

Rousseau and Marangoni, 2008

Enzymatic method

Interesterification using sn-1,3 specific lipases

Trans-free margarines, snack foods, shortenings Infant formula (e.g., Betapol) Infant formula enriched with ARA, DHA, and MCFA

´ lvarez and Akoh, 2016a; A ´ lvarez A and Akoh, 2016b

Reduced calorie fat (e.g., Salatrim)

Farfa´n et al., 2013

Farfa´n et al., 2013

Chemical method

Acidolysis

Bakery products

Rousseau and Marangoni, 2008

Enzymatic method

Acidolysis using sn-1,3 specific lipases

Synthesis of MLM type

Kim et al., 2007; Kim et al., 2008b

Infant formula

Sørensen et al., 2010; Li et al., 2014

Infant formula enriched with ARA, DHA, and MCFA

Pande et al., 2013b

Chemical method

Alcoholysis

Emulsifiers, surfactants

Feltes et al., 2013

Enzymatic method

Ethanolysis using sn-1,3 specific lipases

Emulsifiers MAG

Wang et al., 2014

Glycerolysis using sn-1,3 specific lipases

Emulsifiers DAG oil

Flickinger and Matsuo, 2003

et al. (1999) in postoperative patients. TAG levels and other clinical variables were found similar between structured TAG and long-chain TAG (Bellantone et al., 1999). A few years before, Sandström et al. (1995) had already demonstrated that provision of an SL emulsion containing medium- and long-chain FA, esterified randomly to glycerol in a TAG structure, was associated with increased whole-body fat oxidation in postoperative patients. In line with these findings, patients undergoing major abdominal surgery receiving an enteral diet containing a fish oil/medium-chain TAG SL experienced an improvement of hepatic, renal, and immune function, and a significant reduction in eicosanoids from peripheral blood mononuclear cells (Swails et al., 1997). Apart from the aforementioned studies with humans, literature on this topic relies, to a large extent, on the use of laboratory animal models allowing for extrapolation to humans. In experiments with mice, SL used as functional oil for suppressing high-fat-induced obesity reduced plasma TAG (Cao et al., 2013). Also, DHA-enriched structured lipiddiacylglycerol (SL-DAG) was found beneficial for lowering body fat and ameliorating hepatic steatosis in obese models by improving hepatic fatty acid and cholesterol metabolic enzyme activity as well as gene-related expression (Kim et al., 2008a). Moreover, rice bran oil enriched with ALA from linseed oil and n-3 EPA and DHA from cod liver oil in the sn-1 and sn-3 positions of TAG revealed hypocholesterolemic and hypolipidemic effects (Chopra and Sambaiah, 2009; Kanjilal et al., 2013). Also, SL using groundnut oil and n-3 FA from linseed oil exhibited hypocholesterolemic and hypotriglyceridemic action (Sharma and Lokesh, 2013). All in all, these findings support the evidence that SL beneficially reduced serum lipids (Lee et al., 1999) and lipid deposition in animals fed atherogenic diets (Kanjilal et al., 2013). A couple of studies by Nagata et al. (2003, 2004) hypothesized that: (1) lipid emulsions of highly purified SL containing medium-chain FA in the sn-1 and 3 positions and linoleic acid (18:2n-6, LA) in the sn-2 position are rapidly hydrolyzed; (2) feeding highly purified LML type could improve serum and liver lipid profile; and (3) MLM type might be a preferable substrate for pancreas contributing to energy supply. In cholesterol-rich blood, erythrocytes appeared fragile and deformed. These features were partially reversed by SL, such as EPA- and DHA-rich mustard oil by improving hematological and histological parameters and lowering hypercholesterolemia in rats (Sengupta and Ghosh, 2010, 2011a). In contrast, Kim et al. (2007) reported that the dietary effects of sesame oilebased MLM-type SL on plasma lipid profile and cardiovascular variables were not different from those of original sesame oil (LCT), but induced tachycardia in hypertensive rats (Kim et al., 2008b).

TABLE 17.2 Compilation of Health Benefits of Structured Lipids Methods of SL Synthesis

Beneficial Effects

Experimental Model

References

MCT and LCT

SL Emulsion

Increased body fat oxidation in postoperative patients

Humans

Sandstro¨m et al., 1995

Fish oil/MCT

Fish oil SL

Improved hepatic, renal, and immune function Reduced eicosanoids from PBMC

Humans

Swails et al., 1997

SL containing EPA, DHA, and caprylic acid

Transesterification

Reduced cholesterol and TAG

Mice

Lee et al., 1999

Rapeseed oil-based MLM type

Interesterification

Improved FA hydrolysis and absorption

Rats

Straarup and Høy, 2000

LML type MLM type

Transesterification

MLM type was rapidly hydrolyzed and contributed to energy supply LML improved serum and liver lipids

Rats

Nagata et al., 2003

MLM type LMM type

Transesterification

Both SL reduced serum lipids and cholesterol

Rats

Nagata et al., 2004

Rapeseed oil-based MLM type

Interesterification

Improved fecal fat and nitrogen digestibility Better deposition of long-chain FA

Post-weaning piglets

Straarup et al., 2006

Sesame oil-based MLM type

Acidolysis

No effect on cardiovascular function

Spontaneously hypertensive rats

Kim et al., 2007

DHA-enriched SL-DAG

Glycerolysis

Improved FA and cholesterol metabolism

Mice

Kim et al., 2008a

Sesame oil-based MLM type

Acidolysis

Caused tachycardia

Spontaneously hypertensive rats

Kim et al., 2008b

SL with cod liver oil, SL with linseed oil

Interesterification

Hypolipidemic and hypocholesterolemic effects

Rats

Chopra and Sambaiah, 2009

MCT-containing mustard oil PUFA-containing mustard oil

Transesterification

Both SL reduced platelet aggregation and hypercholesterolemia

Hypercholesterolemic rats

Sengupta and Ghosh, 2010

MCT-rich mustard oil PUFA-rich mustard oil

Transesterification

Minimized the adverse effect of excess cholesterol in erythrocyte’s membrane

Rats

Sengupta and Ghosh, 2011a

SL containing SCFA

Interesterification Triacetin þ camellia oil FAME

Reduced TAG

Mice

Cao et al., 2013

SL with sunflower and SL with soybean oil with ethyl behenate

Interesterification

Reduced lipemia and lipid deposition

Rats and rabbits

Kanjilal et al., 2013

SL with groundnut oil, SL with linseed oil

Interesterification (for step 1) Acidolysis (for step 2)

Reduced LDL cholesterol and TAG

Rats

Sharma and Lokesh, 2013

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Type of SL

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Using rat models of normal and malabsorption, Straarup and Høy (2000) concluded that the optimal absorption of a structured fat depends on the regiospecific structure. Dietary SL containing medium- and long-chain FAs improved fecal fat and nitrogen digestibility and deposition of long-chain FAs in postweaning piglets (Straarup et al., 2006). Another aspect worth identifying is whether increasing the consumption of SLs could compromise (or improve) phagocytosis, which is paramount for host defense against pathogens. The effect of structured TAG enriched in EPA or DHA on splenocyte FA composition and leucocyte phagocytosis was studied in vitro by Kew et al. (2003). Contrarily to DHA, Kew et al. (2003) observed an influence of EPA in the sn-2 position of TAG but not in the sn-1 or sn-3 positions, on its incorporation into cell phospholipids and on the activity of phagocytic cells in a dose-dependent manner.

2.3 Human Milk Fat Analogs for Infant Formula Formulation HMF is the second largest component of breast milk by concentration (3%e5% in mature milk) and contributes nearly half of the energy provided to infants through dietary source (Wells, 1996). HMF is a rich source of essential fatty acids (EFAs), such as ALA and LA, and their derivatives, such as LCPUFA, DHA, and arachidonic acid (20:4n-6, ARA), which are present in human milk in trace amounts (<1%, each one) (Picciano, 1998; Lopez-Lopez et al., 2002). Bioavailability of EFA and LCPUFA are critical during infancy for appropriate brain growth, motor and cognitive skills, neurological reflexes, and sensory functions (Yehuda et al., 2005). An increased interest in DHA has arisen, associated in particular with memory and visual skills in infant development (Li et al., 2015). Therefore, most of the infant formulas available today are supplemented with DHA and ARA, especially in the case of preterm infants (Zou et al., 2016). Carlson et al. (1986) and Pita et al. (1990) reported that even when the conventional formulas include substantial amounts of LA and ALA (which are precursors of endogenous synthesis of ARA and DHA, respectively) they were incapable to retain postnatal acceptable levels of LCPUFA levels in plasma and erythrocytes relative to infants fed human breast milk. Elongation-desaturation enzymes are not sufficiently active during the early stages of life to fully desaturate and elongate LA and ALA (Uauy et al., 2003). Moreover, commercial infant formulations differ at positional distribution of some of the most important FA on TAG molecules compared to HMF (Bracco, 1994; Álvarez and Akoh, 2016b). For example, DHA (60%) and ARA (45%) are mostly located in the sn-2 position in HMF, whereas in commercial infant formulas their distribution is almost equivalent across all three positions of TAG (Álvarez and Akoh, 2016b). DHA- and ARA-rich single cell oils (DHASCO and ARASCO, respectively) employed as blends for preparation of the formula do not exhibit clear positional specificity, with FA allocated almost indistinctly in all three positions (Myher et al., 1996). Also in infants, the FA position of TAG plays a fundamental role at the level of absorption, distribution, and metabolism of fat (Lucas et al., 1997; Ramírez et al., 2001). Newborn rats fed oils containing DHA and ARA in the sn-2 position presented higher levels of DHA and ARA in brain compared to those fed with oils containing these FAs randomly distributed (Christensen and Høy, 1997). DHA and ARA in the sn-1 and sn-3 positions of TAG induce resistance to pancreatic lipase and, thus, relatively low absorption of these FAs might be expected (Bottino et al., 1967). As such, HMF analogs can be defined as SL similar to human milk fat in terms of FA composition and distribution produced for use in infant formulas (Kim and Ahoh, 2015). Betapol was the first SL commercially produced as HMF analog although it is still LCPUFA insufficient (Zock et al., 1996; Pande et al., 2013a; Kim and Akoh, 2015). That is why novel SLs enriched with LCPUFAs have been developed for appropriate infant growth and development using a series of different oils combinations (Sørensen et al., 2010; Teichert and Akoh, 2011a, 2011b; Pande et al., 2013a,b) and with reasonable amounts of DHA (Pande et al., 2013a,b; Li et al., 2014) in the sn-2 position (Li et al., 2014). Nagachinta and Akoh (2012) and Turan et al. (2012) produced HMF analogs suitable for infant formulas that could efficiently deliver DHA and ARA as well as other LCPUFAs for pregnant women and vegans. More recently, Álvarez and Akoh (2016a,b) successfully tested an infant formula fat analog with larger amounts of DHA and ARA in the sn-2 position. All the information concerning the methods of synthesis and effects of human milk fat analogs are summarized in Table 17.3.

3. CONCLUDING REMARKS The potential health effect of a specific FA depends both on its structure and administration form. The glycerol esters of fatty acids are the most common fat occurring in human diet, devoid of toxicological effects and more chemically stable. The stereospecific position of FA in TAG is of great importance, as FAs located in the sn-2 position are preferentially absorbed, thus modulating their bioavailability, physiological properties, and metabolic fate in the organism. Conversely, TAGs that have been chemically and enzymatically modified by incorporation of new molecules or by changing the positions of their original FAs are classified as SLs. The nature of the physiological response seems to depend on the structure of the lipid, namely the FA chain length rather than the structural specificity. In line with the original

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TABLE 17.3 Overall Findings on the Methods of Structured Lipids Synthesis and Effects of Human Milk Fat Analogs Methods of SL Synthesis

Test Formulation

Beneficial Effects

References

Acidolysis

Butterfat þ soybean oil FA and rapeseed oil FA

Low oxidative stability Potential use as infant formula

Sørensen et al., 2010

Interesterification

18:4n-3 soybean oil þ tripalmitin

Health benefits of omega-3 FA Potential use as infant formula

Teichert and Akoh, 2011a

Interesterification (step 1) Acidolysis (step 2)

Step 1: 18:4n-3 soybean oil þ tripalmitin ¼ SL Step 2: SL þ18:3n-6 or DHA

Reasonable amounts of 18:3n-6 and DHA Potential use as infant formula

Teichert and Akoh, 2011b

Acidolysis

Palm olein þ DHASCO-FFA and ARASCO-FFA

High amounts of ARA and DHA in the sn-2 position Potential use as infant formula and for pregnant women

Nagachinta and Akoh, 2012

Interesterification (for both steps)

Step 1: hazelnut oil þ 16:0 ethyl ester ¼ 16:0-rich SL Step 2: 16:0-rich SL þ ARASCO and DHASCO

Provide health benefits associated with ARA and DHA Potential use as infant formula

Turan et al., 2012

Interesterification þ acidolysis

Extra virgin olive oil þ tripalmitin þ ARASCO-FFA þ DHASCO-FFA

Reasonable amount of DHA in the sn-2 position Potential use as infant formula

Pande et al., 2013a

Acidolysis

Tripalmitin þ extra virgin olive oil-FFA þ DHASCO-FFA

Reasonable amount of DHA in the sn-2 position Potential use as infant formula

Pande et al., 2013b

Acidolysis

Refined olive oil þ 16:0 þ DHA

Fair amount of DHA in the sn-2 position Potential use in infant formula

Li et al., 2014

Interesterification

Synthesis of high sn-2 DHA and ARA oils through DHASCO and ARASCO

High amount of DHA and ARA in the sn-2 position Potential use as infant formula

´ lvarez and Akoh, A 2016a

Interesterification (for step 1)

Step 1: sn-2 16:0 SL þ capric acid ¼ SLCA Step 2: Blending SLCA with canola oil, corn oil, high sn-2 DHA, and high sn-2 ARA

DHA and ARA predominantly in the sn-2 position Suitable infant formula enriched with medium-chain FA, ARA, and DHA

´ lvarez and Akoh, A 2016b

metabolic and biochemical concept of structured TAG, the addition of a proper antioxidant is required for improving its oxidative stability (Lee et al., 2006) since the consumption of oxidized lipids has a significant negative effect on a range of biomarkers, including lipid metabolism, oxidative stress, and vascular function (Turner et al., 2006). The ability of a-tocopherol, b-carotene, and soy isoflavones to exhibit prooxidant activity in fat emulsions, under certain conditions, obliges manufacturers to experiment with these compounds before adding them to SL-based products as functional ingredients (Osborn-Barnes and Akoh, 2003). One type of lipid structuration largely used in oils and fats is the process of enzymatic interesterification. In particular, MLM SL type containing medium-chain FA in the sn-1 and sn-3 positions of glycerol and a long-chain functional FA in the sn-2 position are recognized as the best formula to potentiate FA protective outcomes. In addition, SL may be useful as human milk fat analogs for infant formula development with inclusion of preformed DHA to deliver extra health benefits.

SUMMARY POINTS l l

Lipids are essential components of body composition and necessary in daily food intake. Triacylglycerols (TAGs) is the term used to define three fatty acids esterified (bonded) to a glycerol backbone.

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l

l

l

TAG is the natural molecular form that makes up virtually all fats and oils in both animal and plants species, is safe to consume and chemically stable. The design of a structured lipid (SL) with a particular chemical structure enables to control TAG behavior and, therefore, may improve their nutritional effects. SLs are custom-made lipids with built-in essential FA components that promise greater flexibility in patient care, prevention of age-related chronic diseases, and child nutritional support.

KEY FACTS l

l

l

l

l l

l

The glycerol esters of fatty acids (TAGs) are the most common fats occurring in the human diet, devoid of toxicological effects and chemically stable. The efficiency of human digestibility and metabolism of TAGs are sensitive to the position of FAs within the glycerol backbone, as those located in the sn-2 position are easily absorbed. TAGs that have been chemically and enzymatically modified by incorporation of new molecules or by changing the positions of their original FAs are classified as SLs. MLM SL type comprising medium-chain FAs in the sn-1,3 positions and long-chain FA in the sn-2 position of the glycerol backbone are considered the desired structure of SL. SLs have many applications in food and nutraceutical industries. SLs can be developed as functional oils to enhance TAG metabolism and retard fatty liver development, to combat obesity, and to prevent risk of atherosclerosis and age-related chronic diseases. SLs are suitable as infant formula fat analogs.

DICTIONARY OF WORDS AND TERMS ARASCO Arachidonic acid single cell oil DHASCO Docosahexaenoic acid single cell oil LCT Long-chain triacylglycerols LLM Long, long, medium-chain triacylglycerols LML Long, medium, long-chain triacylglycerols MCFA Medium chain fatty acids MCT Medium-chain triacylglycerols MLM Medium, long, medium-chain triacylglycerols MML Medium, medium, long-chain triacylglycerols SCFA Short-chain fatty acids SL Structured lipids SL-DAG Structured lipid-diacylglycerol sn Stereospecific numbering TAG Triacylglycerols

ACKNOWLEDGMENTS This book chapter was supported by Fundação para a Ciência e a Tecnologia (FCT, Lisbon, Portugal) through grant PTDC/CVT-NUT/5931/ 2014, CIISA project UID/CVT/00276/2013 and individual fellowships to JMP (SFRH/BPD/116816/2016), DC (SFRH/BD/126198/2016) and MSM (SFRH/BPD/97432/2013). We also acknowledge P2020 grant 08/SI/2015/3399 and postdoctoral fellowship to PAL funded by the Ministry of Economy and the Ministry of Science and Higher Education (Portugal).

REFERENCES Akoh, C.C., Kim, B.H., 2008. Structured lipids. In: Akoh, C.C., Min, D.B. (Eds.), Food Lipids: Chemistry, Nutrition, and Biotechnology, third ed. CRC Press, Boca Raton, pp. 841e864. Álvarez, C.A., Akoh, C., 2016a. Enzymatic synthesis of high sn-2 DHA and ARA modified oils for the formulation of infant formula fat analogues. J. Am. Oil Chem. Soc. 93, 383e395. Álvarez, C.A., Akoh, C., 2016b. Preparation of infant formula fat analog containing capric acid and enriched with DHA and ARA at the sn-2 position. J. Am. Oil Chem. Soc. 93, 531e542. Bandarra, N.M., Lopes, P.A., Martins, S.V., Ferreira, J., Alfaia, C.M., Rolo, E.A., Correia, J.J., Pinto, R.M.A., Ramos-Bueno, R.P., Batista, I., Prates, J.A.M., Guil-Guerrero, J.L., 2016. DHA at the sn-2 position of structured triacylglycerols improved n-3 polyunsaturated fatty acid assimilation in tissues of hamsters. Nutr. Res. 36, 452e463.

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