Mammalian organs are nourished by nutrients carried by the blood circulation. These nutrients originate from diet and internal stores, and can undergo various interconversions before their eventual use as tissue fuel. Here we develop isotope tracing, mass spectrometry, and mathematical analysis methods to determine the direct sources of circulating nutrients, their interconversion rates, and eventual tissue-specific contributions to TCA cycle metabolism. Experiments with fifteen nutrient tracers enabled extensive accounting for both circulatory metabolic cycles and tissue TCA inputs, across fed and fasted mice on either high-carbohydrate or ketogenic diet. We find that a majority of circulating carbon flux is carried by two major cycles: glucose-lactate and triglyceride-glycerol-fatty acid. Futile cycling through these pathways is prominent when dietary content of the associated nutrients is low, rendering internal metabolic activity robust to food choice. The presented in vivo flux quantification methods are broadly applicable to different physiological and disease states.

哺乳动物的器官是靠血液循环携带的营养物质来滋养的。这些营养素源自饮食和内部储存，在最终用作组织燃料之前可以经历各种相互转化。在这里，我们开发同位素示踪、质谱分析和数学分析方法，以确定循环营养物质的直接来源、它们的相互转化率以及最终组织特异性对 TCA 循环代谢的贡献。使用 15 种营养示踪剂进行的实验能够对高碳水化合物或生酮饮食的喂食和禁食小鼠的循环代谢周期和组织 TCA 输入进行广泛的计算。我们发现大部分循环碳通量由两个主要循环携带：葡萄糖-乳酸和甘油三酯-甘油-脂肪酸。当饮食中相关营养素含量较低时，这些途径的无效循环就会很明显，从而使内部代谢活动对食物选择产生强烈影响。所提出的体内通量定量方法广泛适用于不同的生理和疾病状态。