Data from: How and when do insects rely on endogenous protein and lipid resources during lethal bouts of starvation? a new application for 13C-breath testing

Most of our understanding about the physiology of fasting and starvation comes from studies of vertebrates; however, for ethical reasons, studies that monitor vertebrates through the lethal endpoint are scant. Insects are convenient models to characterize the comparative strategies used to cope with starvation because they have diverse life histories and have evolved under the omnipresent challenge of food limitation. Moreover, we can study the physiology of starvation through its natural endpoint. In this study we raised populations of five species of insects (adult grasshoppers, crickets, cockroaches, and larval beetles and moths) on diets labeled with either 13C-palmitic acid or 13C-leucine to isotopically enrich the lipids or the proteins in their bodies, respectively. The insects were allowed to become postabsorptive and then starved. We periodically measured the δ13C of the exhaled breath to characterize how each species adjusted their reliance on endogenous lipids and proteins as energy sources. We found that starving insects employ a wide range of strategies for regulating lipid and protein oxidation. All of the insects except for the beetle larvae were capable of sharply reducing reliance on protein oxidation; however, this protein sparing strategy was usually unsustainable during the entire starvation period. All insects increased their reliance on lipid oxidation, but while some species (grasshoppers, cockroaches, and beetle larvae) were still relying extensively on lipids at the time of death, other species (crickets and moth larvae) allowed rates of lipid oxidation to return to prestarvation levels. Although lipids and proteins are critical metabolic fuels for both vertebrates and insects, insects apparently exhibit a much wider range of strategies for rationing these limited resources during starvation.当前学界关于禁食与饥饿生理学的认知，大多源自脊椎动物相关研究；但出于伦理考量，以致死终点为监测节点的脊椎动物相关研究则十分匮乏。昆虫则是探究饥饿应对比较策略的理想模型：其生活史多样，且始终处于食物匮乏这一普遍存在的选择压力之下。此外，我们还可通过饥饿的自然致死终点，完整研究其生理学过程。本研究中，我们分别以标记了¹³C-棕榈酸（¹³C-palmitic acid）或¹³C-亮氨酸（¹³C-leucine）的饲料饲养5种昆虫（成虫蝗虫、蟋蟀、蟑螂，以及甲虫幼虫和蛾幼虫），使它们体内的脂质或蛋白质分别获得同位素富集。待昆虫进入餐后吸收态后，再对其实施饥饿处理。我们定期检测呼出气体的δ¹³C值，以表征各物种如何调整对内源性脂质与蛋白质作为能量来源的依赖程度。研究发现，饥饿状态下的昆虫拥有多种调控脂质与蛋白质氧化的策略。除甲虫幼虫外，其余所有昆虫均能大幅降低对蛋白质氧化的依赖；但这种蛋白质节约策略在整个饥饿周期中通常难以持续。所有昆虫均提升了对脂质氧化的依赖程度，但部分物种（蝗虫、蟑螂与甲虫幼虫）在死亡时仍大量依赖脂质供能，而另一部分物种（蟋蟀与蛾幼虫）的脂质氧化速率则回落至饥饿前水平。尽管脂质与蛋白质是脊椎动物和昆虫共有的关键代谢燃料，但昆虫在饥饿期间对这些有限资源的调配策略显然更为多样。