Nitric oxide and long-term outcomes after kidney transplantation: Results of the TransplantLines cohort study

Impaired endogenous nitric oxide (NO) production may contribute to graft failure and premature mortality in kidney transplant recipients (KTR). We investigated potential associations of 24-h urinary NOx (NO₃^- + NO₂^-) excretion (uNOx) with long-term outcomes. uNOx was determined by HPLC and GC-MS in 698 KTR and in 132 kidney donors before and after donation. Additionally, we measured urinary nitroso species (RXNO) by gas-phase chemiluminescence. Median uNOx was lower in KTR compared to kidney donors (688 [393-1076] vs. 1301 [868-1863] before donation and 1312 [982-1853] μmol/24h after donation, P < 0.001). During median follow-up of 5.4 [4.8-6.1] years, 150 KTR died (61 due to cardiovascular disease) and 83 experienced graft failure. uNOx was inversely associated with all-cause mortality (HR per doubling of uNOx: 0.84 [95% CI 0.75-0.93], P < 0.001) and cardiovascular mortality (HR 0.78 [95% CI 0.67-0.92], P = 0.002). The association of uNOx with graft failure was lost when adjusted for renal function (HR per doubling of uNOx: 0.89 [95% CI 0.76-1.05], P = 0.17). There were no significant associations of urinary RXNO with outcomes. Our study suggests that KTR have lower NO production than healthy subjects and that lower uNOx is associated with a higher risk of all-cause and cardiovascular mortality.

Meat intake and risk of mortality and graft failure in kidney transplant recipients

BACKGROUND

It is unknown whether meat intake is beneficial for long-term patient and graft survival in kidney transplant recipients (KTR).

OBJECTIVES

We first investigated the association of the previously described meat intake biomarkers 1-methylhistidine and 3-methylhistidine with intake of white and red meat as estimated from a validated food frequency questionnaire (FFQ). Second, we investigated the association of the meat intake biomarkers with long-term outcomes in KTR.

METHODS

We measured 24-h urinary excretion of 1-methylhistidine and 3-methylhistidine by validated assays in a cohort of 678 clinically stable KTR. Cross-sectional associations were assessed by linear regression. We used Cox regression analyses to prospectively study associations of log2-transformed biomarkers with mortality and graft failure.

RESULTS

Urinary 1-methylhistidine and 3-methylhistidine excretion values were median: 282; interquartile range (IQR): 132-598 µmol/24 h and median: 231; IQR: 175-306 µmol/24 h, respectively. Urinary 1-methylhistidine was associated with white meat intake [standardized β (st β): 0.20; 95% CI: 0.12, 0.28; P < 0.001], whereas urinary 3-methylhistidine was associated with red meat intake (st β: 0.30; 95% CI: 0.23, 0.38; P < 0.001). During median follow-up for 5.4 (IQR: 4.9-6.1) y, 145 (21%) died and 83 (12%) developed graft failure. Urinary 3-methylhistidine was inversely associated with mortality independently of potential confounders (HR per doubling: 0.55; 95% CI: 0.42, 0.72; P < 0.001). Both urinary 1-methylhistidine and urinary 3-methylhistidine were inversely associated with graft failure independent of potential confounders (HR per doubling: 0.84; 95% CI: 0.73, 0.96; P = 0.01; and 0.59; 95% CI: 0.41, 0.85; P = 0.004, respectively).

CONCLUSIONS

High urinary 3-methylhistidine, reflecting higher red meat intake, is independently associated with lower risk of mortality. High urinary concentrations of both 1- and 3-methylhistidine, of which the former reflects higher white meat intake, are independently associated with lower risk of graft failure in KTR. Future intervention studies are warranted to study the effect of high meat intake on mortality and graft failure in KTR, using these biomarkers.

Urinary 3-hydroxyisovaleryl carnitine excretion, protein energy malnutrition and risk of all-cause mortality in kidney transplant recipients: Results from the TransplantLines cohort studies

BACKGROUND

Leucine is an essential amino acid and a potent stimulator of muscle protein synthesis. Since muscle wasting is a major risk factor for mortality in kidney transplant recipients (KTR), dietary leucine intake might be linked to long-term mortality. Urinary 3-hydroxyisovaleryl carnitine (3-HIC) excretion, a functional marker of marginal biotin deficiency, may also serve as a marker for dietary leucine intake.

OBJECTIVE

In this study we aimed to investigate the cross-sectional determinants of urinary 3-HIC excretion and to prospectively investigate the association of urinary 3-HIC excretion with all-cause mortality in KTR.

DESIGN

Urinary 3-HIC excretion and plasma biotin were measured in a longitudinal cohort of 694 stable KTR. Cross-sectional and prospective analyses were performed using ordinary least squares linear regression analyses and Cox regression analyses, respectively.

RESULTS

In KTR (57% male, 53 ± 13 years, estimated glomerular filtration rate 45 ± 19 mL/min/1.73 m²), urinary 3-HIC excretion (0.80 [0.57-1.16] μmol/24 h) was significantly associated with plasma biotin (std. β = -0.17; P < 0.001). Subsequent adjustment for potential covariates revealed urinary creatinine excretion (std. β = 0.24; P < 0.001) and urinary urea excretion (std. β = 0.53; P < 0.001) as the primary determinant of urinary 3-HIC excretion. Whereas plasma biotin explained only 1% of the variance in urinary 3-HIC excretion, urinary urea excretion explained >45%. During median follow-up for 5.4 [4.8-6.1] years, 150 (22%) patients died. Log₂-transformed urinary 3-HIC excretion was inversely associated with all-cause mortality (HR: 0.52 [0.43-0.63]; P < 0.001). This association was independent of potential confounders.

CONCLUSIONS

Urinary 3-HIC excretion more strongly serves as a marker of leucine intake than of biotin status. A higher urinary 3-HIC excretion is associated with a lower risk of all-cause mortality. Future studies are warranted to explore the underlying mechanism.

TRIAL REGISTRATION ID

NCT02811835. TRIAL REGISTRATION URL: https://clinicaltrials.gov/ct2/show/NCT02811835.

Effect of High versus Low Dairy Consumption on the Gut Microbiome: Results of a Randomized, Cross-Over Study

The influence of dairy on the gut microbiome has not been studied extensively. We performed a randomized cross-over study to analyze the effect of high dairy intake on the gut microbiome. Subjects were randomly assigned to a high-dairy diet (HDD) (5-6 dairy portions per day) and a low-dairy diet (LDD) (≤1 dairy portion per day) for 6 weeks with a washout period of 4 weeks in between both diets. The gut microbiome was assessed using 16S rRNA gene sequencing. Compositionality and functionality of the gut microbiome was assessed using Quantitative Insights Into Microbial Ecology (QIIME) and Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt). Stool consistency was evaluated using the Bristol stool chart. In total, 46 healthy overweight subjects (BMI range 25-30 kg/m²) completed both intervention periods. During the HDD, there was a significantly higher abundance of the genera Streptococcus, Leuconostoc, and Lactococcus, and the species Streptococcus thermophilus, Erysipelatoclostridium ramosum and Leuconostoc mesenteroides (p_FDR < 0.10). Furthermore, during the HDD, there was a significantly lower abundance of the genera Faecalibacterium and Bilophila, and the species Faecalibacterium prausnitzii, Clostridium aldenense, Acetivibrio ethanolgignens, Bilophila wadsworthia and Lactococcus lactis (p_FDR < 0.10). There were eight subjects who became constipated during the HDD and these subjects all had a lower abundance of F. prausnitzii. This is the first cross-over study in which the effect of an HDD compared to an LDD on the gut microbiome has been studied. An HDD led to a significantly different composition of the gut microbiome, with a particularly lower abundance of F. prausnitzii and a higher abundance of S. thermophilus. Constipation was observed in several subjects during the HDD. Predicted metabolic pathways were not significantly altered due to an HDD.

Urinary sulfate excretion and risk of late graft failure in renal transplant recipients - a prospective cohort study

Hydrogen sulfide (H₂S), produced from metabolism of dietary sulfur‐containing amino acids, is allegedly a renoprotective compound. Twenty‐four‐hour urinary sulfate excretion (USE) may reflect H₂S bioavailability. We aimed to investigate the association of USE with graft failure in a large prospective cohort of renal transplant recipients (RTR). We included 704 stable RTR, recruited at least 1 year after transplantation. We applied log‐rank testing and Cox regression analyses to study association of USE, measured from baseline 24 h urine samples, with graft failure. Median age was 55 [45–63] years (57% male, eGFR was 45 ± 19 ml/min/1.73 m²). Median USE was 17.1 [13.1–21.1] mmol/24 h. Over median follow‐up of 5.3 [4.5–6.0] years, 84 RTR experienced graft failure. RTR in the lowest sex‐specific tertile of USE experienced a higher rate of graft failure during follow‐up than RTR in the middle and highest sex‐specific tertiles (18%, 13%, and 5%, respectively, log‐rank P < 0.001). In Cox regression analyses, USE was inversely associated with graft failure [HR per 10 mmol/24 h: 0.37 (0.24–0.55), P < 0.001]. The association remained independent of adjustment for potential confounders, including age, sex, eGFR, proteinuria, time between transplantation and baseline, BMI, smoking, and high sensitivity C‐reactive protein [HR per 10 mmol/24 h: 0.51 (0.31–0.82), P = 0.01]. In conclusion, this study demonstrates a significant inverse association of USE with graft failure in RTR, suggesting high H₂S bioavailability as a novel, potentially modifiable factor for prevention of graft failure in RTR.

Urinary Oxalate Excretion and Long-Term Outcomes in Kidney Transplant Recipients

Epidemiologic studies have linked urinary oxalate excretion to risk of chronic kidney disease (CKD) progression and end-stage renal disease. We aimed to investigate whether urinary oxalate, in stable kidney transplant recipients (KTR), is prospectively associated with risk of graft failure. In secondary analyses we evaluated the association with post-transplantation diabetes mellitus, all-cause mortality and specific causes of death. Oxalate excretion was measured in 24-h urine collection samples in a cohort of 683 KTR with a functioning allograft ≥1 year. Mean eGFR was 52 ± 20 mL/min/1.73 m2. Median (interquartile range) urinary oxalate excretion was 505 (347-732) µmol/24-h in women and 519 (396-736) µmol/24-h in men (p = 0.08), with 302 patients (44% of the study population) above normal limits (hyperoxaluria). A consistent and independent inverse association was found with all-cause mortality (HR 0.77, 95% CI 0.63-0.94, p = 0.01). Cause-specific survival analyses showed that this association was mainly driven by an inverse association with mortality due to infection (HR 0.56, 95% CI 0.38-0.83, p = 0.004), which remained materially unchanged after performing sensitivity analyses. Twenty-four-hour urinary oxalate excretion did not associate with risk of graft failure, post-transplant diabetes mellitus, cardiovascular mortality, mortality due to malignancies or mortality due to miscellaneous causes. In conclusion, in KTR, 24-h urinary oxalate excretion is elevated in 44% of KTR and inversely associated with mortality due to infectious causes.

Causal path analyses of the association of protein intake with risk of mortality and graft failure in renal transplant recipients

The effect of a low protein intake on survival in renal transplant recipients (RTR) is unknown. A low protein intake may increase risks of malnutrition, low muscle mass, and death. We aimed to study associations of protein intake with mortality and graft failure and to identify potential intermediate factors. Protein intake was estimated from 24-h urinary urea excretion (24-h UUE). Graft failure was defined as return to dialysis or retransplantation. We used Cox regression analyses to analyze associations with outcome and potential intermediate factors in the causal path. In 604 RTR, mean ± SD 24-h UUE was 380 ± 114 mmol/24-h. During median follow-up for 7.0 yr (interquartile range: 6.2-7.5 yr), 133 RTR died and 53 developed graft failure. In univariate analyses, 24-h UUE was associated with lower risk of mortality (HR [95% CI] = 0.80 [0.69-0.94]) and graft failure (HR [95% CI] = 0.72 [0.56-0.92]). These associations were independent of potential confounders. In causal path analyses, the association of 24-h UUE with mortality disappeared after adjustment for muscle mass. Low protein intake is associated with increased risk of mortality and graft failure in RTR. Causal path analyses reveal that the association with mortality is explained by low muscle mass. These findings suggest that protein intake restriction should not be advised to RTR.