Recently, an article with the title “Hemolysis contributes to anemia during long-duration space flight” by Trudel et al. has been published in Nature Medicine.¹ The authors propose that accelerated hemolysis of circulating red blood cells (RBCs) significantly contributes to “space anemia.” After monitoring Hb-concentration [Hb] and exhaled carbon monoxide (CO) in 14 astronauts during 5 months-long space missions on the International Space Station (ISS), and up to one year after return, they base this conclusion on an ≈11% decrease in [Hb] between pre- and post-mission values, together with a ≈50% increase in the rate of CO elimination, which was entirely ascribed to extravascular hemolysis of RBCs. In our opinion, their interpretation is not consistent with the data, and we believe that the term “anemia” is not justified, because [Hb] was at most decreased to low-normal.² We propose here alternative explanations both for the low [Hb] and for the elevation of exhaled CO, which, of course, require future experimental proof.

The steady-state RBC number is regulated by the rate of RBC production, under the control of hypoxia-inducible factor (HIF)-2α and erythropoietin (EPO), whereas their clearance rate appears to be constant, reflecting the removal of senescent RBCs mainly.³ It is still debated whether the reduction in total RBCs observed during space missions (or upon return to sea level from high altitude adaptation, or in chronic kidney patients after terminating an EPO-therapy), might occur faster because of accelerated clearance of pre-senescent RBCs, mainly “neocytes.” According to this “neocytolysis” hypothesis, young RBCs are destroyed preferentially when EPO levels drop.⁴ In sum, erythrocatheresis would be increased by a combination of decreased RBC production, removal of neocytes, and continued clearance of senescent RBCs at its normal rate. Neocytolysis gained credit as a possible explanation of “space anemia” and as one of the many health risks associated with space missions.⁴ However, this hypothesis has recently been falsified in the case of return from high altitude,⁵ when EPO levels drop and the elevated total number of circulating RBCs decreases to normal, pre-altitude values, which can easily be explained by the decreased rate of RBC production.

Is the elevated amount of exhaled CO alone an adequate measure of RBC destruction for subjects exposed to microgravity? We think it's not. Under basal conditions, a healthy adult expires ≈0.43 ml/h of endogenously produced CO, corresponding to ≈1.92 × 10⁻⁵ mol/h of catabolized heme, resulting in equal amounts of CO, Fe, and biliverdin (bilirubin).^{6, 7} Approximately 75% to 85% of eliminated CO originates from the breakdown of Hb heme, the remainder part comes from the breakdown of liver hemoproteins.⁸ Thus, ≈1.54 × 10⁻⁵ mol/h of Hb-dependent CO elimination corresponds to ≈5.9 g Hb metabolized per day, that is, ≈0.8% of the total Hb of an adult with blood [Hb] of 14.5 g/dL and a total blood volume of 5 liters. This value is consistent with the normal clearance rate of human RBCs, which survive ≈120 days and are eliminated on a time-dependent basis with minimal random destruction.³ During such a steady state, the same number of reticulocytes enters the circulation every day. Deviations from this regimen are only known, until now, for hemolytic anemia, where RBCs turn over faster due to an increase in random, pre-senescent clearance, which is only partially compensated by accelerated erythropoiesis.³ In these conditions, the rate of Hb catabolism, as reflected by the levels of exhaled CO, can become up to one order of magnitude higher than normal.⁶

We would like to raise an issue regarding the possible correlation between CO elimination and the extent of hemolysis in the astronauts under study.¹ Based on previous data,⁹ the rate of CO elimination in space in the study in question¹ was, on average, ≈2.59 × 10⁻⁵ mol/h, persisting for 5 months on the ISS. This corresponds to a >50% increase with respect to pre-mission values, and, according to the authors,¹ to ≈9.7 g of Hb removed per day of the 157 days at the ISS. Even assuming that erythropoiesis continued at a physiological rate, producing ≈6 g Hb daily (which is unlikely because EPO levels dropped below basal and stayed low for a few weeks after arrival at the ISS), at the end of the 5-month mission each crew member would have presented with an [Hb] of <2 g/dL.

Because EPO levels were 16% higher than pre-space in the second half of the mission, it may be argued that an increase in erythropoietic rate could have compensated for the accelerated hemolysis. However, all the surrogate parameters of erythrokinetics (elevated plasma iron, ferritin, saturated transferrin), and previously published results (reviewed in⁴), seem to exclude this possibility.

Any imbalance in RBC turnover can only be appreciated by measuring total Hb mass, not [Hb] because the latter is subjected to fluctuations in plasma volume that occur upon entering microgravity and on return. Therefore, [Hb] does not reflect possible variations in the total amount of circulating Hb.⁴ However, Trudel et al. only measured [Hb] in the crewmembers, and did this, unfortunately, on two occasions only, far apart from each other: 3 months before launch (median [Hb] = 14.3 g/dL), and 4 days after landing (12.7 g/dL), which is 5 months after arriving at the ISS, and 8 months after the initial measurement.¹ The difference of 1.6 g/dL, corresponding to an ≈11% decrease from baseline, is a small change that does not indicate increased hemolysis. Neither is an [Hb] of 12.7 g/dL a sign of anemia. A finer resolution of [Hb] kinetics would have been possible, if [Hb] was quantified also during the 5-month stay at the ISS, but, surprisingly, it was not. There is clearly a discrepancy between the relatively modest decrease in [Hb] and the tremendous and sustained increase in CO elimination,¹ which cannot possibly be accounted for by an increased turnover of Hb. However, the results are compatible with alternative explanations as proposed below.

Early studies showed that the amounts of eliminated CO were always larger than expected from plasma bilirubin levels⁸ because bilirubin derived from the catabolism of liver hemoproteins is directly excreted into the gallbladder, and is not secreted in plasma. Conversely, bilirubin derived from extravascular hemolysis of RBCs appears in plasma, to be delivered and processed further in the liver. In both cases, CO is mainly eliminated with the expired air. Increased extravascular hemolysis should, therefore, correspond to increased levels of plasma bilirubin, which has not been observed in the astronauts (Figure 1). Therefore, the extra CO must be derived from a different source. A possible contributor could be ineffective erythropoiesis¹⁰ because of low EPO levels. In this case, CO would be produced by the breakdown of heme of erythroid precursors in the bone marrow, not of circulating RBCs. The lack of decrease in haptoglobin in the astronauts supports this notion.¹ Actually, there was an increase in haptoglobin, that was left uncommented in the report by Trudel et al.¹ (Figure 1), which is unexpected and whose source needs to be identified.

As an alternative to accelerated and intramedullary hemolysis, the excess of CO elimination is more plausibly explained by stimulated turnover of liver hemoproteins, among which the short-lived cytochrome P-450 accounts for 70% of heme synthesized in the liver.⁸ The gene encoding the inducible isoform of heme oxygenase (HO-1), the enzyme responsible for the CO-producing first reaction of heme catabolism, and most abundantly expressed in spleen and liver,¹¹ appears to be activated by a far greater variety of factors than any other gene.¹² It is not known if its expression is stimulated in microgravity. Another contributor to CO elimination might be the breakdown of myoglobin heme, which has never been measured in space. Finally, an additional contribution to exhaled CO may result from the stimulated expression of HO-1 because of increased oxidative stress conditions in space.¹³

The supplementary tables¹ contain additional information that has not been discussed. The results show that most of the increase in expired CO resulted from males, whereas there was no increase from female astronauts, indicating a strong sex difference. Haptoglobin increased more in females than in males, and plasma bilirubin changed both in males and females, but in opposite directions, resulting in no change when values were averaged¹ (Figure 1). This dimorphism certainly requires further investigation.

Data from Trudel et al. are of great interest as they outline a scenario of profound deviation from normality in the kinetics of CO elimination in space versus earth conditions. The conclusions drawn are however unrealistic because many of the conflicting results reported, especially as supplementary material, were left uncommented or explained away as minor inconsistencies. The reinterpretation of those data may be of broader interest. For instance, recently proposed methods for the determination of RBC lifespan based on measurements of CO in the exhaled air may turn out to be unreliable when applied to subjects exposed to microgravity.¹⁴ A paradigm shift should be introduced to properly interpret an otherwise valuable dataset and foster new research in space medicine.

最近，Trudel 等人发表了一篇题为“溶血导致贫血在长时间太空飞行中”的文章。已发表在《自然医学》上。¹作者提出循环红细胞 (RBC) 的加速溶血会显着导致“空间性贫血”。在国际空间站 (ISS) 为期 5 个月的太空任务期间以及返回后长达一年的时间里，他们监测了 14 名宇航员的 Hb 浓度 [Hb] 和呼出的一氧化碳 (CO)，他们将这一结论建立在 ≈11任务前和任务后值之间 [Hb] 降低 %，同时 CO 消除率增加约 50%，这完全归因于红细胞的血管外溶血。在我们看来，他们的解释与数据不一致，我们认为“贫血”一词是不合理的，因为 [Hb] 最多降至正常低值。²我们在此提出了对低 [Hb] 和呼出 CO 升高的替代解释，这当然需要未来的实验证明。

在缺氧诱导因子 (HIF)-2α 和促红细胞生成素 (EPO) 的控制下，稳态红细胞数量受红细胞生成速率的调节，而它们的清除率似乎是恒定的，主要反映了衰老红细胞的去除. ³在太空任务期间（或从高海拔适应返回海平面时，或在终止 EPO 治疗后的慢性肾病患者中）观察到的总红细胞减少是否会因为加速清除前衰老的红细胞，主要是“新细胞”。根据这种“新细胞溶解”假说，当 EPO 水平下降时，年轻的红细胞会优先被破坏。⁴总而言之，红细胞生成减少、新细胞清除和衰老红细胞以正常速率持续清除会增加红细胞灌注。新细胞溶解症被认为是“太空贫血”的一种可能解释，也是与太空任务相关的众多健康风险之一。⁴然而，这一假设最近在从高海拔返回的情况下被证伪，⁵当 EPO 水平下降且循环红细胞总数升高降至正常的海拔前值时，这可以很容易地用下降率来解释。红细胞生成。

对于暴露于微重力的受试者来说，仅呼出的 CO 量升高是否足以衡量 RBC 的破坏？我们认为不是。在基础条件下，健康成年人呼出约 0.43 ml/h 的内源性 CO，对应于 ≈1.92 × 10 ^-5 mol/h 的分解代谢血红素，导致等量的 CO、Fe 和胆绿素（胆红素）。^{6, 7}大约 75% 到 85% 的 CO 消除来自 Hb 血红素的分解，其余部分来自肝脏血红素的分解。⁸因此，≈1.54 × 10 ^-5mol/h 的 Hb 依赖性 CO 消除对应于 ≈5.9 g Hb 每天代谢，即 ≈0.8% 的成年人血液 [Hb] 为 14.5 g/dL 和总血容量为 5 升. 该值与人类红细胞的正常清除率一致，红细胞存活约 120 天，并在时间依赖性基础上以最小的随机破坏被消除。³在这种稳定状态下，每天有相同数量的网织红细胞进入循环系统。到目前为止，这种方案的偏差仅在溶血性贫血中才为人所知，由于随机的衰老前清除率增加，RBCs 转得更快，而加速红细胞生成只能部分补偿。³在这些情况下，呼出的 CO 水平反映的 Hb 分解代谢率可能比正常水平高出一个数量级。⁶

我们想提出一个问题，即 CO 消除与正在研究的宇航员溶血程度之间可能存在的相关性。¹根据之前的数据，^9在问题1的研究中，空间中的 CO 消除率平均约为 2.59 × 10 ^-5 mol/h，在国际空间站上持续了 5 个月。这相当于任务前值增加了 > 50%，并且根据作者的说法，¹在国际空间站的 157 天中，每天去除约 9.7 克 Hb。即使假设红细胞生成以生理速度继续，每天产生约 6 g Hb（这不太可能，因为 EPO 水平下降到基础水平以下并在到达国际空间站后的几周内保持低水平），在每个为期 5 个月的任务结束时机组成员的 [Hb] 会小于 2 g/dL。

因为在任务的后半段，EPO 水平比前空间高 16%，所以可以说红细胞生成率的增加可以补偿加速的溶血。然而，红细胞动力学的所有替代参数（血浆铁、铁蛋白、饱和转铁蛋白升高）和先前发表的结果（见⁴综述）似乎排除了这种可能性。

RBC 转换的任何不平衡只能通过测量总 Hb 质量而不是 [Hb] 来了解，因为后者会受到进入微重力和返回时发生的血浆体积波动的影响。因此，[Hb] 不能反映循环 Hb 总量的可能变化。⁴然而，Trudel 等人。只测量了机组人员的 [Hb]，不幸的是，只有两次，彼此相距甚远：发射前 3 个月（中值 [Hb] = 14.3 g/dL）和着陆后 4 天（12.7 g /dL)，即到达国际空间站后 5 个月，以及初次测量后 8 个月。¹1.6 g/dL 的差异对应于从基线下降约 11%，是一个微小的变化，并不表明溶血增加。12.7 g/dL 的 [Hb] 也不是贫血的迹象。如果 [Hb] 在国际空间站停留 5 个月期间也被量化，那么 [Hb] 动力学的更精细分辨率是可能的，但令人惊讶的是，事实并非如此。[Hb] 的相对适度下降与 CO 消除的巨大而持续的增加之间显然存在差异，¹这不可能通过 Hb 的营业额增加来解释。然而，结果与下面提出的替代解释兼容。

早期研究表明，消除的 CO 量总是大于血浆胆红素水平⁸的预期值，因为来源于肝血蛋白分解代谢的胆红素直接排泄到胆囊中，而不是分泌到血浆中。相反，来自红细胞血管外溶血的胆红素出现在血浆中，在肝脏中进一步输送和加工。在这两种情况下，CO 主要通过呼出的空气消除。因此，增加的血管外溶血应该对应于血浆胆红素水平的增加，这在宇航员中没有观察到（图 1）。因此，额外的 CO 必须来自不同的来源。一个可能的贡献者可能是无效的红细胞生成¹⁰因为EPO水平低。在这种情况下，一氧化碳将由骨髓中红系前体血红素的分解产生，而不是循环红细胞的分解。宇航员的触珠蛋白没有减少支持了这一观点。¹实际上，触珠蛋白有所增加，这在 Trudel 等人的报告中未作评论。¹（图 1），这是出乎意料的，需要确定其来源。

作为加速溶血和髓内溶血的替代方法，过量的 CO 消除更合理地解释为刺激肝脏血红素的更新，其中短寿命的细胞色素 P-450 占肝脏合成血红素的 70%。⁸编码血红素加氧酶 (HO-1) 的可诱导异构体的基因，该酶负责血红素分解代谢产生 CO 的第一反应，在脾脏和肝脏中表达最多，¹¹似乎被更多种类的比任何其他基因的因素。¹²不知道它的表达是否在微重力下受到刺激。消除二氧化碳的另一个因素可能是肌红蛋白血红素的分解，这在太空中从未被测量过。最后，由于空间中氧化应激条件的增加，HO-1 的刺激表达可能会导致呼出的 CO 的额外贡献。¹³

补充表¹包含尚未讨论的附加信息。结果表明，呼出二氧化碳的增加大部分来自男性，而女性宇航员没有增加，这表明性别差异很大。女性的触珠蛋白比男性增加更多，男性和女性的血浆胆红素都发生了变化，但方向相反，因此在平均值¹时没有变化（图 1）。这种二态性当然需要进一步研究。

来自 Trudel 等人的数据。引起了极大的兴趣，因为它们概述了在空间与地球条件下 CO 消除动力学严重偏离常态的情景。然而，得出的结论是不切实际的，因为报告的许多相互矛盾的结果，特别是作为补充材料，没有评论或解释为轻微的不一致。对这些数据的重新解释可能会引起更广泛的兴趣。例如，最近提出的基于呼出空气中 CO 的测量来确定 RBC 寿命的方法在应用于暴露于微重力的受试者时可能会变得不可靠。¹⁴应引入范式转变以正确解释原本有价值的数据集并促进空间医学的新研究。