Serum cholesterol precursor sterols to cholesterol proportions, especially those of cholestenol and lathosterol, were significantly increased, up to 3-fold, among patients compared with controls. The same was true for serum concentrations of cholesterol precursor sterols, excluding desmosterol. Squalene concentration and proportion to cholesterol were significantly higher among patients than among controls (Table 5).
Among the patients, ALT, AST, GT, bilirubin, prealbumin, albumin, and thromboplastin time remained close to normal or within the normal range during follow-up (Table 3, Fig. 1).
Of the liver biopsy specimens, 63% (5/8) showed some degree of fibrosis (scale 0–3, median 2, range 0–3). Other biopsy findings included cholestasis, steatosis, reactive changes, and inflammation (Table 1).
Relation of Serum Plant Sterols to Parenteral Nutrition, Liver Function, and Cholesterol Synthesis
Of the various characteristics of PN assessed, only the percentage of parenteral energy significantly correlated with serum PS to cholesterol proportions (Table 6). Serum cholesterol precursors to cholesterol proportions were negatively and GT was positively related to serum PS proportions (Table 6). There was a positive correlation among ALT, serum bile acids, and serum total PS/PN total PS ratio (r = 0.69–0.71, P < 0.05). No significant associations were observed between serum PS to cholesterol proportions or concentrations and serum concentrations of bilirubin, ALT, or prealbumin (data not shown). Serum PS concentration or proportions to cholesterol were unrelated to absolute amount of parenteral lipids or PS or the total duration of PN (r = 0.16–0.43, P = 0.19–0.63).
Among patients with liver fibrosis (n = 5), the median sitosterol, campesterol, avenasterol, stigmasterol, total PS, and cholestanol concentrations were 2.2, 2.1, 1.4, 1.1, 1.3, and 1.5 times higher, respectively, than in patients with no liver fibrosis (n = 3). The difference, however, was statistically nonsignificant probably because of the small number of patients in both groups. The serum concentration of total and individual PS tended to correlate with the degree of liver fibrosis (r = 0.55–0.60, P = 0.16–0.12). The degree of liver fibrosis was unrelated to the biochemical markers of liver function, serum total PS/PN total PS ratio, the duration or contents of PN or the amount of lipids, PS, or energy received in PN (data not shown).
Our major findings showed that, first, during follow-up, serum PS levels significantly increased, whereas serum liver enzymes remained close to the normal range. Second, the serum concentration of cholesterol remained low, accompanied by increased cholesterol synthesis evaluated with cholesterol precursor sterols and squalene. Third, histology revealed frequent liver damage despite well-preserved biochemical liver function. In addition, serum PS related positively to GT and negatively to cholesterol precursors. Serum PS tended to reflect liver fibrosis.
It is well known that PN containing PS increases its concentration in the serum (9,11,14,36,37). In the present study, the distribution of individual PS in the serum closely paralleled that of infused lipid regimen (Fig. 2). In addition, parenteral percentage of the energy positively correlated with serum PS levels that were unrelated to the amount of PS or lipids infused. Ellegård et al (38), in their study of adult patients with short bowel syndrome receiving PN, also found high serum levels of PS but no correlation between calculated administration of PS and serum levels of PS. Because serum PS reflects the balance between input and biliary excretion, PS are expected to accumulate further after liver damage (23). High delivery of parenteral energy signifies low enteral food intake. Minute enteral feeding, however, is associated with impaired emptying of the gallbladder, decreased biliary secretion, and cholestasis (39–41). Similar to our findings, low enteral food intake has been linked to high plasma levels of PS in adults with IF (11).
In the present study, serum PSs were inversely related to cholesterol precursors, suggesting that cholesterol homeostasis was not interfered with in the study population. In addition, the association also suggested that the higher the cholesterol synthesis, the lower the serum proportions of PS. High cholesterol synthesis is associated with increased biliary cholesterol secretion, which parallels biliary secretion of sitosterol and campesterol (22). Thus, increased cholesterol synthesis and biliary cholesterol secretion among the patients may have reduced a further increase in serum PS content by increasing their secretion in bile. The difference between serum campesterol percentages in patients compared with parenteral emulsions used was statistically significant (Fig. 2). First, this suggests that in patients with IFALD during olive oil–based PN, the ABCG5/8 transporters in hepatocytes operate relatively normally, and second, that they transport sitosterol more efficiently than campesterol, as previously reported in healthy adults (22). In accordance, we previously showed that PSs, especially sitosterol, enrich in gallstones during PN (42). In a recent study by Nikkilä et al (43), high serum levels of campesterol, sitosterol, and cholestanol in end-stage PBC were normalized after liver transplantation, suggesting that damaged liver cannot efficiently excrete PS or synthesize cholestanol. In our patients, serum PS levels reflected the content of parenteral lipid emulsions, and no relative accumulation of PS was found suggesting that liver PS excretion to bile was efficient despite frequent fibrosis in liver biopsies.
In phytosterolemia, a rare familial lipid storage disease leading to accelerated atherosclerosis, serum concentrations of sitosterol, and campesterol are extremely high, reaching values up to 1500 and 640 μmol/L in homozygotes, respectively, owing to enhanced absorption of the PS (13). In patients with heterozygous phytosterolemia who are clinically symptomless, the serum sitosterol levels are <30 μmol/L (13). In the present study, patients had a higher serum sitosterol concentration compared with controls or patients with heterozygous phytosterolemia, but clearly lower than in patients with homozygous phytosterolemia.
The patients had consistently abnormally low serum levels of total cholesterol, LDL-cholesterol, and HDL-cholesterol. At the same time, serum cholesterol precursor to cholesterol proportions, especially those of cholestenol and lathosterol, were significantly increased, up to 3-fold, among patients, reflecting increased endogenous cholesterol synthesis (14,16). This suggests that the amount of parenterally provided cholesterol alone was insufficient to compensate for the intestinal losses of cholesterol, and, therefore, cholesterol incorporated to expanding tissues in these growing children. Of note, the cholesterol content of the olive oil–based lipid regimen was several times lower than that of PS (Table 2). The magnitude of the increase in serum cholesterol precursor proportions was comparable with children with short bowel syndrome after weaning off PN (44).
During follow-up, patients received a median of 48% of total daily energy and 0.9 g · kg−1 · day−1 of lipids from olive oil–based PN. Previous studies have linked high serum PS levels to IFALD both in adults and in children (9–11,14,38), whereas excessive PN fat dose overcoming the ability of the liver to clear phospholipids and fatty acids can lead to steatosis, cholestasis, and eventually fibrosis (7). A PN lipid dose <1.0 g · kg−1 · day−1 is found to be effective in preventing biochemical signs of IFALD (1,5). During follow-up, the biochemical markers of liver function remained close to the normal range, excluding 1 patient in whom a >20-fold increase in serum PS level decreased together with ALT and bilirubin after transition from soy-based to olive oil–based PN shortly after inclusion in the present study (Fig. 1). Hallikainen et al (14) reported a similar finding in an adult patient with long-term PN caused by short bowel syndrome. Despite the fact that biochemical markers of liver function remained close to the normal range, histological liver damage was common and liver fibrosis tended to reflect serum PS. In addition, there was a positive relation between serum PS and GT and between serum total PS/PN total PS ratio and ALT and serum bile acids. A further refinement of diagnostic strategies and mechanisms behind PN-associated liver damage is needed to differentiate the significance between the total lipid dose and PS. Meanwhile, lipid preparations with low PS content, for example, by combining olive oil– and fish oil-based lipid emulsions, may be advisable. Serum PS concentration can be used to monitor and guide PN lipid dosage.
In conclusion, children with IF receiving olive oil–based PN have higher serum PS levels compared with healthy controls, but the levels are lower than those observed in phytosterolemia. The PS distribution in serum reflects their contents in the parenteral lipid emulsion. Despite the fact that liver enzyme levels remained close to the normal range and the biochemical liver function was well preserved, the biopsies showed liver damage in most of the patients. Serum PS appeared to mirror liver fibrosis.
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Keywords:Copyright 2011 by ESPGHAN and NASPGHAN
cholesterol metabolism; intestinal failure; intestinal failure–associated liver disease; parenteral nutrition; plant sterols