Serum phenylalanine concentration as a marker of liver function in obese patients before and after bariatric surgery

Human obesity is associated with increased serum phenylalanine concentration, which is probably caused by liver dysfunction related to liver steatosis. This study examines whether improvements of liver function after bariatric surgery is associated with a decrease of serum phenylalanine concentration caused by an increase of phenylalanine metabolism.

Serum phenylalanine and alanine aminotransferase (an independent predictor of liver steatosis) concentrations as well as several parameters related to obesity were measured in 16 obese patients (seven men and nine women) before and 6 months after vertical banded gastroplasty. Ten (six men and four women) lean, healthy subjects served as controls.

Obese patients before surgery had approximately twofold higher serum phenylalanine concentration than control subjects. The serum phenylalanine concentration decreased 6 months after bariatric surgery. Serum alanine aminotransferase (ALT) concentration was higher in obese patients before surgery, and decreased 6 months after bariatric surgery. Changes in serum phenylalanine concentration correlated positively with changes of ALT concentration (r = 0.75; p < 0.001). The body weight, BMI, HOMA-IR, serum triacylglycerol, LDL-cholesterol/HLD-cholesterol ratio, leptin, insulin, and glucose concentrations were higher in obese patients, and decreased 6 months after bariatric surgery. Serum HDL-cholesterol concentration was lower in obese patients before surgery than in control subjects and increased 6 months after surgery.

Results obtained indicate that bariatric surgery-induced weight loss had beneficial effects on several laboratory parameters including serum phenylalanine, ALT, lipid concentrations and insulin resistance. A strong positive correlation between serum phenylalanine and serum ALT concentrations suggests that deterioration of liver function in obese patients is contributing to a decrease in phenylalanine metabolism and consequently to the increase of serum phenylalanine concentration. One can suppose that serum phenylalanine concentration could be noninvasive marker of liver dysfunction associated with liver steatosis in obese patients.