Sleeve gastrectomy in rats improves postprandial lipid clearance by reducing intestinal triglyceride secretion

Abstract

Background & aims: Postprandial hyperlipidemia is a risk factor for atherosclerotic heart disease and is associated with the consumption of high-fat diets and obesity. Bariatric surgeries result in superior and more durable weight loss than dieting. These surgeries are also associated with multiple metabolic improvements, including reduced plasma lipid levels. We investigated whether the beneficial effects of vertical sleeve gastrectomy (VSG) on plasma lipid levels are weight independent.

Methods: VSG was performed on Long-Evans rats with diet-induced obesity. Controls were sham-operated animals who were either pair-fed or ad libitum-fed. We measured fasting and postprandial levels of plasma lipid. To determine hepatic and intestinal triglyceride secretion, we injected the lipase inhibitor poloxamer 407 alone or before oral lipid gavage. (13)C-Triolein was used to estimate postprandial uptake of lipid in the intestine.

Results: Rats that received VSG and high-fat diets had markedly lower fasting levels of plasma triglyceride, cholesterol, and phospholipid than obese and lean (pair-fed) controls that were fed high-fat diets. Rats that received VSG had a marked, weight-independent reduction in secretion of intestinal triglycerides. VSG did not alter total intestinal triglyceride levels or size of the cholesterol storage pool nor did it affect the expression of genes in the intestine that control triglyceride metabolism and synthesis. VSG did not affect fasting secretion of triglyceride, liver weight, hepatic lipid storage, or transcription of genes that regulate hepatic lipid processing.

Conclusions: VSG reduced postprandial levels of plasma lipid, independently of body weight. This resulted from reduced intestinal secretion of triglycerides following ingestion of a lipid meal and indicates that VSG has important effects on metabolism.

Figure 1. VSG reduces plasma lipids in a weight-independent manner

A: Blood from 4-h fasted animals was sampled on post-operative day 50. At this time, VSG animals weighed less than SHAM (P<0.05) animals. CHOW rats were lighter than SHAM (P<0.001), VSG (P<0.05), and PF (P<0.01) rats on the day of study. B: Fat mass was reduced in VSG (P<0.001) and PF (P<0.05) animals as compared with SHAM. CHOW animals had reduced fat mass as compared with SHAM (P<0.001), VSG (P<0.05), and PF (P<0.001) animals. Lean mass was unaffected by surgery, as compared with SHAM and PF groups. CHOW animals had reduced lean mass as compared with SHAM (P<0.05) and VSG (P<0.05) animals. C: Plasma triglyceride levels were reduced in VSG and CHOW animals as compared with SHAM (P<0.01 vs. VSG, P<0.001 vs. CHOW) and PF (P<0.01 vs. VSG, P<0.001 vs. CHOW) rats. D: Plasma cholesterol levels were reduced by VSG as compared with SHAM (P<0.001) and PF (P<0.05). E: Phospholipid levels were reduced in plasma from VSG and CHOW animals as compared with either SHAM or PF rats (P<0.001 for all comparisons). F: Plasma NEFA levels were reduced in CHOW animals as compared to PF (P<0.05) but were not reduced significantly by VSG.

Figure 2. Reductions in plasma lipids are most dramatic during short periods of fasting

Triglyceride and cholesterol levels were measured in plasma sampled at 0, 4, 8, and 24 hours of fasting. A: In unfasted blood, triglycerides were reduced by VSG as compared with SHAM (P<0.001) and PF (P<0.01). Triglycerides were also reduced in VSG animals after 4 and 8 hours of fasting (P<0.001 vs. SHAM at each time point), but this reduction was not weight-independent (P>0.05 vs. PF). B: Area under the curve for plasma triglycerides across 24 hours of fasting was reduced for VSG animals as compared with either SHAM (P<0.001) or PF (P<0.05). C: Plasma cholesterol did not differ significantly between groups across the 24 hour fast. D: Area under the curve for plasma cholesterol across the 24 hour study was unchanged by surgery or weight loss. E: Cholesterol in fractionated plasma from VSG rats did not reveal a shift in the size of plasma lipoproteins. However, VSG improved fasting hypercholesterolemia observed in SHAM animals, as demonstrated by reduced cholesterol content in VLDL and HDL peaks. The reduction in HDL cholesterol content appears to be unique to VSG vs. PF or CHOW groups.

Figure 3. VSG impairs dietary triglyceride secretion without affecting fasting triglyceride secretion

A: Under fasted conditions, plasma triglycerides were increased along a similar trajectory in all animals after a 1 g/kg i.p. dose of P-407 (treatment X time, interaction P=0.6684; effect of treatment, P=0.7272; effect of time, P<0.0001). B: VSG did not affect the rate of appearance of triglycerides in the blood across the 24-hour experiment (ANOVA: P=0.6053). C: After an i.p. injection of 1 g/kg of P-407, a 0.47 g/kg lipid gavage resulted in an attenuated appearance of triglycerides in the plasma of VSG animals as compared with PF (P<0.001 at 4- and 6-hour timepoints) and SHAM (P<0.01 at 4-hour timepoint and P<0.001 at 6-hour timepoint). Interaction of treatment and time, P<0.0001. D: The rate of appearance of triglycerides in the plasma during the 6-hour experiment was reduced by VSG as compared with SHAM (P<0.01) and PF (P<0.001) animals. E: Plasma ApoB48 (P=0.1193), ApoB100 (P=0.8792), and total ApoB (P=0.6486) content were unchanged 6 hours after the oral lipid load. VSG did not significantly alter the ratio of triglyceride to total ApoB at this time point (P=0.3502).

Figure 4. VSG does not affect hepatic lipid storage

A: Plasma bile acid content is increased by weight loss (vs. SHAM: P<0.05 for VSG, P<0.05 for PF, and P<0.001 for CHOW; P=0.0006). B: Plasma bilirubin levels did not differ among groups (P=0.0776). C: Hepatic triglyceride content was unaffected by VSG (P=0.3902). D: Cholesterol content was unchanged in livers from VSG animals (P=0.3920). E: No differences in the ratio of hepatic cholesterol ester to total cholesterol content were detected (P=0.7771). F: Wet liver weight (expressed as a ratio to body weight) did not differ among groups (0.2831).

Figure 5. VSG does not affect the size of intestinal lipid storage pools

A: ¹³C enrichment in M1 and M2 intestinal samples after ¹³C-Triolein gavage did not differ between groups (M1, P=0.1274; M2, P=0.9866). B: Weight of neither M1 (P=0.5527), M2 (P=0.7453), M3 (P=0.9788), nor M4 (P=0.8089) was affected by VSG surgery or by weight loss. C: Triglyceride content did not differ among groups for any quartile of the small intestine (M1, P=0.6202; M2, P=0.9445; M3, P=0.2394; M4, P=0.4164). D: Differences in cholesterol content were not detected in any gut region (M1, P=0.5310; M2, P=0.8361; M3, P=0.3177; M4, P=0.7376). E: No obvious differences in the morphology of intestinal villi in cresyl violet-stained M2 sections were observed.