title: Protein intake and kidney function in the middle-age population: contrast between cross-sectional and longitudinal data.
abstract: BACKGROUND
  Protein intake is considered a determinant of glomerular filtration rate (GFR). Urinary urea is an objective marker of protein intake. The population-based study investigated, cross-sectionally and longitudinally, the association of protein intake with GFR, indexed by estimated GFR (eGFR).
  
  
  METHODS
  Data were collected on overnight urinary urea, serum creatinine (S-cr), eGFR and other variables in 1522 men and women aged 45-64 years who participated in the Gubbio study (baseline). S-Cr, eGFR and other variables were re-assessed in 1144 of the 1425 survivors after 12-year follow-up.
  
  
  RESULTS
  At baseline, mean ± SD was 84.0 ± 11.4 mL/min × 1.73 m(2) for eGFR calculated by CKD-Epi equation and 1.34 ± 0.57 g/day per kg of ideal weight for protein intake assessed by measurements of overnight urine excretion of urea nitrogen. Cross-sectional analyses of baseline data indicated a positive correlation of protein intake with eGFR (R = 0.180, P < 0.001). In multi-variable regression, 1 g/day higher protein intake related to 4.7 mL/min × 1.73 m(2) higher eGFR [95% confidence interval (CI) = 3.7/5.7]. At follow-up, mean ± SD of 12-year eGFR change was -11.6 ± 9.0 mL/min × 1.73 m(2). Baseline protein intake correlated with more negative eGFR change (R = -0.251, P < 0.001). In multi-variable regression, 1 g/day higher protein intake related to -4.1 mL/min × 1.73 m(2) more negative eGFR change (95% CI = -5.1/-3.1) and to 1.78 risk for incidence of eGFR < 60 mL/min × 1.73 m(2) (95% CI = 1.15/2.78).
  
  
  CONCLUSIONS
  In middle-aged adults, high protein intake is associated cross-sectionally with higher GFR but longitudinally with greater GFR decline over time.
authors.affiliation: Department of Medicine and Surgery, University of Salerno, Salerno, Italy.
authors.forename: Massimo
authors.initials: M
authors.lastname: Cirillo
authors.affiliation: Department of Medicine and Surgery, University of Salerno, Salerno, Italy.
authors.forename: Cinzia
authors.initials: C
authors.lastname: Lombardi
authors.affiliation: Atella Hemodialysis Center, Orta di Atella, Italy.
authors.forename: Daniela
authors.initials: D
authors.lastname: Chiricone
authors.affiliation: Department of Medicine, Second University of Naples, Naples, Italy.
authors.forename: Natale G
authors.initials: NG
authors.lastname: De Santo
authors.affiliation: Istituto Auxologico Italiano, University of Milan, Milan, Italy.
authors.forename: Alberto
authors.initials: A
authors.lastname: Zanchetti
authors.affiliation: Department of Medicine and Surgery, University of Salerno, Salerno, Italy.
authors.forename: Giancarlo
authors.initials: G
authors.lastname: Bilancio
mesh_terms: D015992:Body Mass Index; D003430:Cross-Sectional Studies; D004044:Dietary Proteins / Q000008:administration & dosage*; D005260:Female; D005919:Glomerular Filtration Rate / Q000502:physiology*; D006801:Humans; D007668:Kidney / Q000502:physiology*; D007677:Kidney Function Tests; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D014508:Urea / Q000652:urine*
chemical_list: D004044:Dietary Proteins; D014508:Urea
keywords: Gubbio study; creatinine; eGFR; protein intake; urea