Bedrest shifts fasting and postprandial fuel selection towards carbohydrate use over lipids, potentially affecting astronauts’ performance and health. We investigated whether this change occurs in astronauts after at least 3 months onboard the International Space Station (ISS). We further explored the associations with diet, physical activity (PA), and body composition. Before and during spaceflight, respiratory quotient (RQ), carbohydrate, and fat oxidation were measured by indirect calorimetry before and following a standardized meal in 11 males (age = 45.7 [SD 7.7] years, BMI = 24.3 [2.1] kg m⁻²). Postprandial substrate use was determined by 0-to-260 min postprandial incremental area under the curve (iAUC) of nutrient oxidation and the difference between maximal postprandial and fasting RQ (ΔRQ). Food quotient (FQ) was calculated from diet logs. Fat (FM) and fat-free mass (FFM) were measured by hydrometry and PA by accelerometry and diary logs. Spaceflight increased fasting RQ (P = 0.01) and carbohydrate oxidation (P = 0.04) and decreased fasting lipid oxidation (P < 0.01). An increase in FQ (P < 0.001) indicated dietary modifications onboard the ISS. Spaceflight-induced RQ changes adjusted for ground RQ correlated with inflight FQ (P < 0.01). In postprandial conditions, nutrient oxidation and ΔRQ were unaffected on average. Lipid oxidation changes negatively correlated with FFM changes and inflight aerobic exercise and positively with FM changes. The opposite was observed for carbohydrate oxidation. ΔRQ changes were negatively and positively related to FM and FFM changes, respectively. In conclusion, fasting substrate oxidation shift observed during spaceflight may primarily result from dietary modifications. Between-astronaut variability in postprandial substrate oxidation depends on body composition changes and inflight PA.

卧床休息使禁食和餐后的燃料选择转向碳水化合物而不是脂质，这可能会影响宇航员的表现和健康。我们调查了宇航员在国际空间站 (ISS) 上至少 3 个月后是否会发生这种变化。我们进一步探讨了与饮食、体力活动 (PA) 和身体成分的关联。在太空飞行之前和期间，通过间接量热法测量 11 名男性（年龄 = 45.7 [SD 7.7] 岁，BMI = 24.3 [2.1] kg m − ²）的呼吸商 (RQ)、碳水化合物和脂肪^氧化。 ）。餐后底物的使用由营养氧化曲线下 0 至 260 分钟的餐后增量面积 (iAUC) 以及餐后最大 RQ 和空腹 RQ 之间的差异 (ΔRQ) 确定。食商（FQ）是根据饮食日志计算的。脂肪 (FM) 和去脂质量 (FFM) 通过液体测量法测量，PA 通过加速度测量法和日记记录测量。太空飞行增加了空腹 RQ（P  = 0.01）和碳水化合物氧化（P  = 0.04），并减少了空腹脂质氧化（P  < 0.01）。 FQ 的增加（P  < 0.001）表明国际空间站上的饮食发生了变化。航天引起的 RQ 变化针对与飞行中 FQ 相关的地面 RQ 进行了调整（P  < 0.01）。在餐后条件下，营养素氧化和 ΔRQ 平均不受影响。脂质氧化变化与 FFM 变化和飞行中有氧运动呈负相关，与 FM 变化呈正相关。对于碳水化合物氧化观察到相反的情况。 ΔRQ 变化分别与 FM 和 FFM 变化呈负相关和正相关。总之，在太空飞行期间观察到的空腹底物氧化变化可能主要是由于饮食改变造成的。宇航员之间餐后底物氧化的变异性取决于身体成分的变化和飞行中的 PA。