Serum apolipoprotein A-IV and lipoprotein cholesterol in patients undergoing total parenteral nutrition.
The distinctive biological properties of apolipoprotein (apo) A-IV suggest that serum levels of this apoprotein should be profoundly and selectively depressed by nutritional modalities that totally bypass the intestine. To test this hypothesis we have measured serum lipid and apoprotein levels in 18 fasting patients receiving total parenteral nutrition and in a group of 31 normal controls. Measurement of total serum lipids revealed that the patients had significantly higher serum triglyceride levels and lower total serum cholesterol levels than the controls. The lower total serum cholesterol levels were a consequence of decreases in both low-density lipoprotein and high-density lipoprotein cholesterol. The mean serum level of apo A-IV in the patients was 15 +/- 13 AU/dl, compared with 93 +/- 29 AU/dl in the controls. This difference was almost threefold greater than the differences between levels of apo A-I, apo B, or lipoprotein subfractions. Moreover, apo A-IV was undetectable in 4 patients who had undergone significant small intestinal resection. A trend toward lower apo A-IV levels was seen in the resected patients as compared with those with intact small intestine. Linear regression analysis showed that levels of apo A-IV were not correlated with any apoprotein or lipoprotein parameter in either the patient or control group, with the exception of a positive correlation between serum apo A-IV levels and total serum cholesterol in the controls only, suggesting that apo A-IV synthesis is regulated independently of the synthesis of the major classes of plasma lipoproteins. We conclude that serum apo A-IV levels are especially sensitive to the interruption of enteral feeding and rapidly fall to very low values during prolonged fasting.