mutation p.V254E, with a normal polygenic triglyceride risk score. Conclusions: This case illustrates the interplay of obesity, diabetes, and a rare APOE mutation (p.V245E) in causing severe combined hyperlipidemia, pancreatitis, and extensive plantar xanthomas. This patient responded well to lifestyle and medication therapy. 338 Familial Chylomicronemia Syndrome is associated with increased dietary fatty acid spillover in the circulation despite lower total postprandial triglycerides Andre C. Carpentier, MD, Frederique Frisch, MSc, Diane Brisson, PhD, Daniel Gaudet, MD, PhD, (Sherbrooke, QC) Lead Author’s Financial Disclosures: None. Study Funding: IRSC grant (AC MOP 341582). Background/Synopsis: Familial Chylomicronemia Syndrome (FCS) is a rare monogenic autosomal recessive disorder characterized by severe reduction of lipoprotein lipase (LPL)-mediated triglyceride (TG)-rich lipoprotein lipolysis, leading to severe hypertriglyceridemia and high- risk of pancreatitis. Data on postprandial dietary fatty acid (FA) metabolism are however limited in this population. Objective/Purpose: To compare postprandial meta- bolism among FCS patients, healthy controls and subjects with multifactorial chylomicronemia (MCM). Methods: Twenty-one patients with FCS (11 women/10 men) aged 55 [43-60] (median [IQR]) with a body mass index (BMI) of 23.4 [21.6-25.5] kg/m2) underwent a low- fat (13g) liquid meal test labeled with 3H-palmitate and were compared to 32 healthy controls (18 women/14 men) aged 47 [33-55] with a BMI of 25.9 [24.9-27.6] kg/m2 and to 12 MCM subjects (4 women/8 men) aged 58 [50-62] with a BMI of 32.5 [29.2-34.2] who underwent a high-fat (100g) liquid meal test with either 3H- or 13C-palmitate labeling. A subset of participants simultaneously underwent intravenous infusion of d5-glycerol (9 FCS, 21 controls, 11 MCM) or d2-palmitate (7 FCS, 19 controls, 10 MCM) to measure plasma glycerol and nonesterified FA appearance rates and dietary FA spillover. Results: Fasting plasma glucose levels and postprandial glucose area under the curve (AUC) were higher in FCS than in controls (P,0.05), but similar to MCM whereas plasma fasting insulin levels and postprandial insulin AUC in FCS were similar to levels in controls, but lower than in MCM (P,0.05). Fasting plasma TG levels were higher in FCS (23.50 [12.71-29.95]) than in controls (0.67 [0.50- 1.11] ) and MCM (2.42 [1.69-4.62] (P,0.05). Plasma TG AUC was 22 and 7-fold higher in FCS than in controls and MCM, respectively (P,0.05). Chylomicron tracer activity AUC was also much higher in FCS (% ingested dose.360- min 5 62 [40-115]) than in controls (17 [11-32], P,0.05) and MCM (22 [5-76], P 5 NS). Plasma glycerol appear- ance rate AUC, a marker of total TG lipolysis, was almost 3 and 4-fold lower in FCS than in controls and MCM, respectively (P,0.05) while plasma palmitate appearance rate was similar in all groups. Dietary FA spillover rate was almost 4-fold higher in FCS than in controls (4.3 [1.5-7.5] vs. 1.0 [0.8-1.3] % ingested dose.360min) (P,0.05) but similar to MCM (5.0 [1.5-7.4]). Conclusions: FCS is characterized by increased dietary FA spillover despite profound impairment in LPL-mediated chylomicron lipolysis, suggestive of impaired adipose tissue dietary FA storage. More studies are needed to determine the mechanisms of this pathophysiological feature of FCS. 355 Correlation between chylomicronemia diagnosis scores and post-heparin lipoprotein lipase activity Etienne Khoury, PhD, Daniel Gaudet, MD, PhD, Diane Brisson, PhD, (Saguenay, QC) Lead Author’s Financial Disclosures: None. Plantar Xanthomas on Right Foot: Achilles Tendon Xanthomas on Right Foot: Clinical Applications of Biomarkers, Lipoprotein Testing e25