Extra-hepatic postprandial metabolism of dietary fructose

高糖摄入，更具体地说是果糖摄入，会导致与2型糖尿病(T2D)、心血管疾病(CVD)和非酒精性脂肪性肝病风险增加相关的血脂谱。导致这些疾病的潜在代谢机制在很大程度上仍不清楚。这项提案的母公司研究R01DK116033专注于果糖在肝脏中的作用和代谢命运，在历史上，果糖被认为是处理果糖的主要组织。这一建议考虑了我们的初步数据，支持非肝组织作为果糖代谢部位的主要作用，以及载脂蛋白清除因素影响餐后血脂的情况。在参加父母试验的糖尿病前期和对照组受试者中，我们使用高果糖和低果糖膳食解决了三个与非肝性果糖代谢相关的问题。我们的第一个问题涉及小肠在果糖代谢中的作用；具体地说，通过从头脂肪生成(DNL)将果糖转化为脂肪，以及乳糜粒中载脂蛋白(CM)的组成，即载脂蛋白CIII和载脂蛋白B，以及它们对餐后高脂血症的相对贡献。我们开发了一种免疫亲和方法来分离运输小肠中包装的脂肪的CM。这将使我们能够首次量化肠道DNL和CM的载脂蛋白组成，而不会受到肝脏中产生并由极低密度脂蛋白(VLDL)运输的脂肪的污染。我们的第二个问题解决了逃逸肠道和肝脏代谢并出现在循环中的果糖的数量，从而使其他组织可以获得果糖。为此，我们将使用最近发表的双稳定同位素方法来测量血浆果糖水平。此外，这种方法将使我们能够首次探索人类肠道中的糖异生作用。对于我们的第三个问题，我们将通过确定完全氧化产生标记为13CO2的果糖的量来衡量有多少果糖被用作燃料。最近的研究表明，氧化是果糖代谢的另一条主要途径。在参加父母试验的18名糖尿病前期受试者和18名对照组受试者中，口服和静脉注射稳定同位素，并应用分离TRL的技术，可以量化CM和VLDL载脂蛋白以及肠道和肝脏DNL，同时测量外周循环中的果糖，估计肠道糖异生和果糖氧化。通过测量DNL、清除因子、循环果糖和果糖氧化，结合母体研究的测量(肝脏糖异生、TRL-DNL和糖原储存)，我们将对果糖代谢有一个更全面的表征，并回答关于果糖在两个不同群体中命运的长期问题。重要的是，我们将展示肝外果糖代谢如何导致T2D和CVD风险。

High sugar consumption, and more specifically fructose intake, leads to lipid profiles associated with increased risk of type 2 diabetes (T2D), cardiovascular disease (CVD), and non-alcoholic fatty liver disease. The underlying metabolic mechanisms leading to these conditions remain largely unknown. The parent study for this proposal, R01DK116033, focuses on the role and metabolic fates of fructose in the liver, which historically was considered the main tissue handling fructose. This proposal takes into account our preliminary data supporting a major role of non-hepatic tissues as sites of fructose metabolism, and of apolipoprotein clearance factors influencing postprandial lipid profiles. We address three questions related to non-hepatic fructose metabolism using high- and low-fructose meals in pre-diabetic and control subjects enrolled in the parent trial. Our first question regards the role of the small intestine in fructose metabolism; specifically the conversion of fructose to fat by de novo lipogenesis (DNL) and the composition of apolipoproteins in chylomicrons (CM), namely apoCIII, and apoB, and their relative contribution to postprandial hyperlipidemia. We developed an immunoaffinity method to isolate CM that transport the fat packaged in the small intestine. This will allow us to quantify, for the first time, intestinal DNL and the apolipoprotein composition of CM without contamination from fat made in the liver and transported by very-low density lipoproteins (VLDL). Our second question addresses the amount of fructose that escapes intestinal and hepatic metabolism and appears in the circulation, thereby becoming available to other tissues. For this aim, we will use a recently published dual stable isotope method to measure plasma fructose levels. Additionally, this method will allow us to explore, for the first time, intestinal gluconeogenesis in humans. For our third question, we will measure how much fructose is used as fuel by determining the amount of fructose that is completely oxidized to produce labeled 13CO2. Recent studies have suggested that oxidation is another major route of fructose metabolism. The administration of oral and intravenous stable isotopes in 18 pre-diabetic and 18 control subjects enrolled in the parent trial and the application of a technique to fractionate TRL allows for the quantification of CM and VLDL apolipoproteins and intestinal- and hepatic- DNL, along with measurements of fructose in the peripheral circulation, estimation of intestinal gluconeogenesis, and fructose oxidation. By measuring DNL, clearance factors, circulating fructose, and fructose oxidation in conjunction with the measurements of the parent study (hepatic gluconeogenesis, TRL-DNL and glycogen storage), we will have a more complete characterization of fructose metabolism and answer longstanding questions regarding the fates of fructose in two distinct populations. Importantly, we will demonstrate how extra-hepatic fructose metabolism contributes to T2D and CVD risk.