Inflammatory Characteristics of Oil Source

IFALD is considered by many to be a disease of inflammation. The presence of an underlying inflammatory source may be a key influence in the progression of PNALD in patients with IF. C-reactive protein (CRP), an acute inflammatory marker, has been shown to reflect disease progression in nonalcoholic steatohepatitis, a disease that in many ways resembles IFALD.

During episodes of acute inflammation, hepatocytes upregulate transcription and release of CRP, primarily in response to the cytokine interleukin-6 (IL-6). This response can be enhanced by a combination of IL-1β and tumor necrosis factor (TNF)-α. There is evidence from experimental animals that PN use, in comparison with enteral nutrition (EN), is associated with greater retention of TNF in the plasma compartment. As an inflammatory marker, higher TNF levels could lead to a mild chronic inflammatory state and further amplify the CRP response in the case of IFALD.

PUFAs of the omega-6 and omega-3 family serve as substrates for eicosanoid synthesis, which directly influences an immune response. Eicosanoids are involved in modulating the intensity of an inflammatory reaction ( Fig. 9.1 ). SOLE is rich in omega-6 PUFA and linoleic acid (LA), which is the precursor of arachidonic acid (ARA), the structural backbone of proinflammatory eicosanoids (see Fig. 9.1 ; Fig. 9.2 ). In contrast, the omega-3 PUFAs found in fish oils but not in plant oils, such as docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), are converted into anti-inflammatory derivatives (see Fig. 9.1 , Fig. 9.2 ). Furthermore, ARA is a precursor of prostaglandin E2 and leukotriene B4, which have proinflammatory proprieties and influence cytokine synthesis. EPA from the omega-3 family serves as an alternative progenitor for cyclooxygenase and lipo-oxygenase pathways for eicosanoid synthesis with reduced inflammatory potencies. The balance between omega-6 and omega-3 PUFAs can affect eicosanoid synthesis, inflammatory response, and cytokine synthesis. PUFAs can influence proinflammatory gene expressions, such as peroxisome proliferator-activated receptor and nuclear factor-κB. PUFAs are necessary to prevent a deficiency of EFAs. However, when present in excess, they can have negative effects, leading to an unbalanced fatty acid pattern in cell membranes and the synthesis of eicosanoids. In a cohort of infants placed on ILEs within 48 hours of birth, Gawecka et al. showed that SOLE promoted an excess of IL-6 production compared with olive oil-containing ILEs. Similarly, Raptis et al. demonstrated the anti-inflammatory role of omega-3 PUFAs via the GPR120 receptor using fish oil-based ILEs. DHA has also been shown to inhibit palmitate-induced Toll-like receptor-4 (TLR4)–activated inflammation. In addition to high omega-6 fatty acid content, SOLE also contains high levels of phytosterols, which impair bile secretion via antagonism of the nuclear receptor Farnesoid X receptor (FXR), which will be discussed in greater detail below.

Inflammatory properties of omega-3 and omega-6 fatty acids.

From: Lee S, Gura KM, Kim S, Arsenault DA, Bistrian BR, Puder M. Current clinical applications of omega-6 and omega-3 fatty acids. Nutr Clin Pract. 2006 Aug;21[4]:323–41.

Synthesis of fatty acids.

Other Oil Sources

After the introduction of SOLE, a second-generation ILE was introduced in Europe, and it was a blend of 50% soybean oil and 50% medium-chain triglyceride (MCT) derived from coconut oil ( Table 9.1 ). These emulsions reduce the omega-6 PUFA content by 50%. MCTs are fatty acids that are 6 to 12 carbons long and include capric and caprylic acids. Unlike long-chain triglycerides, MCTs are easily metabolized and lack proinflammatory properties, both characteristics unique to this fat source. Furthermore, MCTs are resistant to peroxidation. MCTs also do not accumulate in the liver and consequently do not impair hepatic function. Despite these benefits, however, MCT oils are devoid of EFAs and thus cannot be used as a sole source of fat.

Since the 1980s, with the heightened awareness of the different inflammatory profiles of the available oil sources, there has been an evolution of ILE formulations. Emulsions have advanced from being solely made with soybean oil in the 1960s to being combined with MCTs in the 1980s, olive oil in the 1990s, and, more recently, with fish oil (see Table 9.1 ).

As previously mentioned, in the 1990s, olive oil was introduced as an oil source in ILE. Olive oil is rich in omega-9 fatty acids, primarily as oleic acid, a type of monounsaturated fatty acid (MUFA), which is not considered essential because it is not a precursor of eicosanoids. Unlike soybean oil and fish oil, olive oil is considered immune neutral. The relatively small amount of LA (approximately 5%) explains why this oil source requires blending with another oil source that contains EFA. In comparison with pure soybean oil, olive oil has a lower content of phytosterols and is naturally rich in alpha-tocopherol. Currently, the only available olive oil containing ILE is an 80% blend with 20% soybean oil (see Table 9.1 ). In this preparation, the mean concentration of LA is 35.8 mg/mL (range 27.6-44.0 mg/mL), and α-linolenic acid (ALA) is 4.7 mg/mL (range 1.0-8.4 mg/mL). This product has 30% of the PUFA content of conventional SOLE. In comparison with soybean oil, olive oil is rich in MUFAs that possess less proinflammatory properties and are more resistant to oxidative stress injuries from free radicals. In many European countries, it is the preferred lipid source. Although currently not approved in the United States for pediatric use, in a randomized controlled trial (RCT) from Australia, an olive oil-containing ILE used in preterm infants younger than 28 weeks’ gestational age was found to be safe and well tolerated.

Fish oil is the newest oil source to be used in ILE. Like olive oil, ILE containing fish oil is less proinflammatory than SOLE. In comparison with those originating from omega-6 PUFAs in SOLE, the eicosanoids produced from omega-3 PUFAs in FOLE are generally less inflammatory. Unlike SOLE, FOLE has little LA and ALA but contains their downstream metabolites, ARA, EPA, and DHA. The omega-3 PUFAs present in fish oil are also natural ligands to some receptors of the GPR family. Recent evidence demonstrates that the interaction with these receptors mediates some of the therapeutic benefits of omega-3 PUFAs in tissues, such as the liver.

Le et al. assessed lipid and fatty acid profiles of 79 pediatric patients who developed IFALD while receiving standard PN with SOLE before and after being switched to FOLE. Children who developed cholestasis while receiving PN with SOLE had their ILE administration discontinued and were treated with FOLE. FOLE was started at a dose of 1 g/kg/day infused over 12 to 24 hours. The serum fatty acid values at the end of the study were compared with baseline values that reflected the use of SOLE. The results showed a dramatic increase in omega-3 PUFAs, such as EPA and DHA. However, ALA, the precursor fatty acid to both EPA and DHA, decreased almost 60% from the baseline value. Switching from SOLE to FOLE also led to a decrease in all omega-6 PUFAs; in particular, gamma-linolenic acid, dihomo-gamma-linoleic acid, and ARA. The decrease in these fatty acids occurred concurrently with the decrease in omega-9 fatty acids (oleic acid and mead acid), which indicated that these patients did not produce extra nonessential fatty acids to compensate for the decrease in both ALA and LA concentrations. Lipid profiles of the 79 children showed decreases in low-density lipoprotein, very-low-density lipoprotein, cholesterol, and serum triglycerides. In addition to the improvement in lipid profiles, which are significant predictors of metabolic derangement, CRP also decreased significantly after treatment with FOLE.

Before being used as monotherapy, FOLE was blended with other oil sources. In a European retrospective study, the outcomes of 20 neonates with short bowel syndrome treated with an olive oil/soybean oil blend supplemented with FOLE was compared with the outcomes in a historical cohort of 18 patients with short bowel syndrome receiving SOLE alone. The rationale for the use of this blend of oils was to provide an omega-6/omega-3 fatty acid ratio of 2:1 to 1:1. This was similar to a ratio that had been reported in a previous study using a 1:1 blend of soybean and fish oils. This proportion is also within the optimal range for anti-inflammatory effects and is similar to that found in breast milk. The researchers reported that in the olive oil/soybean oil/fish oil group, the direct bilirubin levels were reversed in all 14 survivors with cholestasis (direct bilirubin >50 μmol/L) with a median time to reversal of 2.9 months; two patients died as a result of liver failure (10%). In the SOLE controls, six patients (33%) died as a result of liver failure, and only two patients had normalization of bilirubin levels.

Role of Phytosterols in the Pathogenesis of IFALD

Phytosterols are naturally occurring compounds found in plant cell membranes and are structurally similar cholesterol. In animal studies, phytosterols have been shown to interrupt hepatocyte FXR signaling and the expression of downstream bile acid transporters, thus decreasing bile flow. In animal models, phytosterols have been shown to increase the risk of sepsis by altering the migratory and phagocytic functions of neutrophils, which can also contribute to IFALD. Iyer et al. investigated the effect of intravenous plant sterols in piglets at doses equivalent to those used with commercial ILE preparations. Although they did not demonstrate any clinical or histologic changes of cholestasis within the 14-day study period, serum bile acid levels were elevated in the sterol-treated piglets, suggesting early onset of cholestasis. As part of the same investigations, using a rat hepatocyte model, Iyer et al. showed that a commercial ILE enriched with plant sterols led to significant inhibition of hepatocyte secretory function. Similarly, El Kasmi et al. used mouse models to show that in comparison with FOLE, SOLE led to accumulation of stigmasterol in serum and in the liver and that it was associated with hepatic injury and cholestasis, along with reduced expression and function of the Abcb11 and Abcc2 genes, which play a critical role in bile transport. On a molecular basis, they found that the expression of the canalicular exporter for stigmasterol, Abcg5/g8, was downregulated, resulting in hepatic accumulation of stigmasterol. This was associated with an inhibition in the expression of nuclear receptor FXR within the hepatocyte, which, in turn, reduced the expression of FXR-dependent genes, including bile salt export pumps that are responsible for driving bile flow.

In neonates, phytosterols appear to accumulate rapidly. Nghiem-Rao et al. conducted a prospective cohort study in which 45 neonates (36 SOLE recipients versus 9 controls) underwent serial blood sample measurements of sitosterol, campesterol, and stigmasterol. They reported that very preterm infants receiving SOLE had higher sitosterol exposure and concluded that the poorly developed mechanisms of eliminating phytosterols might be a contributing factor for preterm infants being at greater risk for developing IFALD.

The U.S. Food and Drug Administration (FDA), however, is concerned about the phytosterols content in ILEs. As part of the postmarketing process, manufacturers of several recently approved ILEs are now required to conduct studies in this area as a condition of the product’s approval. The requirements include developing and validating analytical methods for determining individual component phytosterol content and testing for individual component phytosterol content. On the basis of the results, data would be used to establish limits for individual phytosterol components. The FDA also requires manufacturers to develop and validate analytical methods for measuring phytosterol levels in plasma. Furthermore, FDA recommends that RCTs in pediatric patients (including neonates) be performed to compare an ILE product with a phytosterol-depleted formulation and a standard-of-care ILE to evaluate the incidence of liver injury, including IFALD. Table 9.2 compares the phytosterol content of commonly used ILEs.

Not all animal models support the role of phytosterols as a causative factor in IFALD. In one study, premature piglets fed on an exclusively PN diet with different ILEs found that it was not the phytosterol content but, rather, the vitamin E or omega-3 PUFA content that was more hepatoprotective. In fact, in subsequent studies using the same piglet model, the addition of phytosterols to 100% FOLE did not produce IFALD. There is one major difference between the piglet model of IFALD and the murine model that may be responsible for some of the observed differences in results. It is customary to treat the piglets with broad-spectrum antibiotics, which may alter the intestinal microbiota, resulting in no intestinal injury or inflammation and thus no subsequent increase in intestinal permeability leading to IFALD.

On the basis of this limited information, it would make sense to use an ILE with as little phytosterol content as possible. One means of reducing the phytosterol content in plant-based ILEs is to winterize the oil. This is a labor-intensive and somewhat expensive process and involves crystallization (or partial solidification) of the oil, followed by a separation of solids and fats.

Impact of Lipid Dose on Predisposing Patients to IFALD

Regardless of type of substrate, macronutrient excess in PN has been shown to be harmful. Excessive lipid provision may be especially detrimental. To date, data on the benefits of reducing lipid intake have been conflicting. SOLE dose reduction from 2 to 3 g/kg/day to 1 g/kg/day has not been proven to decrease the incidence of IFALD. Low-dose SOLE may, however, slow the progression of hepatic disease, but when done to the extreme, it also carries the risk of predisposing patients to developing essential fatty acid deficiency (EFAD).

As part of an analysis of factors contribution to IFALD in adults receiving home parenteral nutrition, Cavicchi et al. reported that SOLE doses of >1 g/kg/day were significantly associated with chronic cholestasis and liver disease. Similarly, Cober et al. compared limiting doses of SOLE to 1 g/kg/day twice weekly in infants and compared total bilirubin levels with historical controls who received 3 g/kg/day of SOLE. A marked reduction in SOLE intake was associated with a progressive decline in bilirubin levels in infants with IFALD, without a significant effect on growth, although there was a trend toward development of EFAD.

Impact of Lipid Dispensing/Infusion Practices

Even the method used to dispense or infuse ILEs may contribute to the development of IFALD. It has been well known for many years that photodegradation of amino acids may lead to IFALD. Neuzil et al. hypothesized that ILE can be negatively affected by ambient light. This is based on the premise that alpha-tocopherol could serve as a pro-oxidant in isolated lipoprotein suspensions, such as ILEs. Neuzil et al. exposed SOLE to a single spotlight, commonly used in the treatment of neonatal jaundice, and measured the formation of triglyceride hydroperoxides by using high-performance liquid chromatography. They observed that the concentrations of these hydroperoxides in different batches of SOLE increased by 60-fold after 24 hours of exposure to phototherapy lights. Triglyceride hydroperoxides were formed during phototherapy light exposure whether the SOLE was given via plastic intravenous administration sets used routinely for infusion or via glass containers. Although to a much lesser extent than observed with phototherapy, ambient light was also shown to cause significant peroxidation of ILEs. The authors concluded that phototherapy light-induced formation of triglyceride hydroperoxides could be prevented by covering SOLE with aluminum foil or supplementing it with sodium ascorbate before light exposure.

To further complicate matters, the type of oil in the ILE may also be important factor in determining the impact of photo-oxidation. Assuming that photo-oxidation of PN may result in production of 4-hydroxynonenal, which is suspected to be involved in the pathogenesis of IFALD, Miloudi et al. attempted to find a practical means to reduce 4-hydroxynonenal in PN and assessed the in vivo impact of PN containing low concentrations of 4-hydroxynonenal. Using a newborn guinea pig model, hepatic markers of oxidative stress (glutathione, F[2α]-isoprostanes [GS-HNE]) and inflammation (messenger RNA [mRNA] of TNF-α and IL-1) were measured after the animals received infusions of PN compounded with either SOLE or FOLE over a 4-day period. Compared with SOLE, FOLE was found to reduce oxidative stress associated with PN and prevent hepatic inflammation.

Given the conflicting data and the lack of risk associated with protecting PN solutions from light, the choice remains one of risk to benefit, with many neonatal intensive care units (NICUs) opting to cover PN solutions and ILEs because this may confer some benefits and avoid the risk of undue harm.

A separate factor that has been suggested to play a role in the development of IFALD is use of tubing containing phthalates (di[2-ethylhexyl]phthalate [DEHP]) for administration of PN and ILEs. DEHP is a substance that can lead to an increase in oxidative stress and toxicity, especially in preterm infants and neonates who receive intensive care. DEHP is a phthalic acid ester and is used as a plasticizer in polyvinylchloride (PVC) and other plastics. Most PVC infusion systems are plasticized with up to 60% of DEHP. Because DEHP is not covalently bound to the plastic matrix, DEHP is easily extracted from the tubing by PN solutions and has been shown to have toxic effects on various organ systems, including the livers of animals and humans. DEHP has the potential to reduce the canalicular excretion of bilirubin and, thus, to contribute to the development of cholestasis. The extent of DEHP leaching, however, is dependent on the type of fluid being infused. Crystalloids tend not to be problematic, but blood products and ILE are. Moreover, the type of ILE can also influence the amount of DEHP that can be extracted from the container and tubing. In a study assessing the potential role of the type of ILE on the quantity of DEHP leached, it was shown after 24 hours of exposure to PVC-based tubing that DEHP migration varied significantly ( P = 0 .0000152) according to lipid type. The olive oil-based ILE leached the most DEHP (65.8 μg/mL), followed by FOLE (37.8 μg/mL). SOLE showed comparable degrees of leaching (19.6-27.8 μg/mL). Von Rettberg et al. assessed the effect of using DEHP-based tubing versus DEHP-free tubing in patients who had received PN containing ILEs for at least 14 days in pediatric intensive care units. In the preintervention period, all intravenous infusion sets, including those for administering blood products, contained PVC. After 2001, all intravenous infusion sets were switched to non-PVC sets. Of the 30 patients in the PVC group, 15 developed signs of cholestasis, whereas of the 46 patients who were treated without using PVC infusion sets, only 6 did. This was equivalent to a reduction in the incidence of cholestasis from 50% to 13%. The authors concluded that the use of infusion systems that contained PVC lead to a 5.6-fold increase in risk for the development of hepatobiliary dysfunction.