Enrichment from the easily accessible blood amino acid pool is commonly used as precursor enrichment to calculate rates of muscle protein fractional synthesis in relevant human studies in lieu of the less accessible muscle fluid amino acid pool. However, the accuracy of this approach depends largely on the extent to which there is low discrepancy in free amino acid enrichment between blood and muscle. Steady-state gradient (i.e., ratio) of amino acid enrichment between blood and muscle fluid in the basal state and in response to amino acid infusion were determined in five healthy subjects, and in association with two separate tracers: d9-leucine, introduced endogenously by the metabolism of d10-leucine (i.e., l-[2,3,3,4,5,5,5,6,6,6-2H10]leucine) infused in blood, and 13C6-phenylalanine introduced/infused in blood. The blood-to-muscle fluid amino acid enrichment ratio was lower (P < 0.05) for d9-leucine compared to 13C6-phenylalanine both before (1.5 ± 0.1 vs. 2.5 ± 0.1) and during (1.1 ± 0.1 vs. 1.2 ± 0.1) amino acid infusion. Importantly, the decrease in this ratio in association with the amino acid infusion was considerably less for the d9-leucine than the 13C6-phenylalanine (−0.38 ± 0.03 vs. −1.29 ± 0.07; P < 0.05). In conclusion, blood d9-leucine enrichment introduced endogenously by intravenous infusion of d10-leucine provides a closer estimate of the muscle fluid amino acid enrichment, and its associated changes, than blood phenylalanine enrichment to calculate rates of muscle protein synthesis in humans.

Background Prevalence of falciparum malaria on Bioko Island remains high despite sustained, intensive control. Progress may be hindered by high proportions of subpatent infections that are not detected by rapid diagnostic tests (RDT) but contribute to onward transmission, and by imported infections. Better understanding of the relationship between subpatent infections and RDT-detected infections, and whether this relationship is different from imported versus locally acquired infections, is imperative to better understand the sources of infection and mechanisms of transmission to tailor more effective interventions. Methods Quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) was performed on a sub-set of samples from the 2015 Malaria Indicator Survey to identify subpatent infections. Households with RDT(+) individuals were matched 1:4 with households with no RDT(+) individuals. The association between living in a household with an RDT(+) individual and having a subpatent infection was evaluated using multivariate hierarchical logistic regression models with inverse probability weights for selection. To evaluate possible modification of the association by potential importation of the RDT(+) case, the analysis was repeated among strata of matched sets based on the reported eight-week travel history of the RDT(+) individual(s). Results There were 142 subpatent infections detected in 1,400 individuals (10.0%). The prevalence of subpatent infections was higher in households with versus without an RDT(+) individual (15.0 vs 9.1%). The adjusted prevalence odds of subpatent infection were 2.59-fold greater (95% CI: 1.31, 5.09) for those in a household with an RDT(+) individual compared to individuals in a household without RDT(+) individuals. When stratifying by travel history of the RDT(+) individual, the association between subpatent infections and RDT(+) infections was stronger in the strata in which the RDT(+) individual(s) had not recently travelled (adjusted prevalence odds ratio (aPOR) 2.95; 95% CI:1.17, 7.41), and attenuated in the strata in which recent travel was reported (aPOR 1.76; 95% CI: 0.54, 5.67). Conclusions There is clustering of subpatent infections around RDT(+) individual(s) when both imported and local infection are suspected. Future control strategies that aim to treat whole households in which an RDT(+) individual is found may target a substantial portion of infections that would otherwise not be detected.

The amino acid enrichment in blood (Eb) is often used in clinically relevant stable isotope studies to calculate muscle protein fractional synthesis rates (FSR), but it underestimates the FSR compared to methods using intracellular enrichment in muscle (Ei). We compared the FSR calculated with intravenous infusion of the novel tracer L‐[2,3,3,4,5,5,5,6,6,6‐2H10]leucine ([2H10]leu) to infusion of the traditional tracer L‐[13C6]phenylalanine ([13C6]phe) using Eb and Ei. Because [2H10]leu is intracellularly metabolized to [2H9]leu prior to reentry into the blood, we hypothesize that using [2H10]leu will minimize the discrepancy between FSR values calculated using Eb or Ei compared to [13C6]phe‐calculated FSR values using the corresponding Eb or Ei. To test our hypothesis, six healthy subjects received simultaneous infusions of the two stable‐isotope tracers following standard protocols. The [2H9]leu‐calculated FSR (%h‐1) was 0.08±0.01 based on Eb and 0.18±0.01 based on Ei. The corresponding [13C6]phe‐calculated FSR values were 0.06±0.01 and 0.25±0.02, respectively. Moreover, the difference in the [2H9]leu‐calculated FSR values using Ei vs Eb was significantly less (P<0.01; 0.10±0.01) compared to the difference in the corresponding [13C6]phe‐calculated FSR values (0.19±0.02). We conclude that, compared to [13C6]phe, the [2H10]leu tracer offers a more accurate approach to calculating FSR when Eb is sought as an alternative to the Ei. Grant Funding Source: Supported by NIH/NIDDK Grant# R01 DK094062; NCATS Grant# UL1 TR000135