A Methodological Study to Learn More About Kidney Function in Healthy People and in People With Reduced Kidney Function

The main purpose of this study is to investigate how reliable a test assessing the renal functional reserve (RFR) and the maximum glomerular filtration rate (maxGFR) are. MaxGFR and RFR assessments are repeated twice at 1-month intervals. If precise, this test may improve the measure of kidney function and be useful in future studies for people with chronic kidney problems. To do this, the researchers will collect and analyze information about kidney function from up to 50 adults, including healthy people and people with stable kidney problems. Each person will come to the study center for a screening visit and then three study days, each about a month apart. On each study day, participants will have blood and urine samples taken before and after drinking a protein shake. The tests will measure things like creatinine, cystatin C, and other markers in the blood and urine to see how the kidneys are working. The study will also record basic health information, weight, blood pressure, and any medicines being taken. The study will last about 2 to 2.5 months for each participant.

Inclusion Criteria: * Participant must be 18 to 75 years of age inclusive, at the time of signing the ICF. * Stable medical condition defined as no new drug or other treatment prescribed for a chronic condition within 60 days prior to Study Day 1. * Body weight between 50 and 100 kg. * Female Participants: A participant is eligible to participate if not pregnant or breastfeeding. If a urine test cannot be confirmed as negative at screening (e.g., an ambiguous result), the participant must be excluded from participation. Exclusion Criteria: * Recent surgical intervention within 60 days prior to Study Day 1. * New clinical diagnosis established within 60 days prior to Study Day 1. * New drug prescription within 60 days prior to Study Day 1. * Relevant change in severity of any medical condition within 60 days prior to Study Day 1, e.g. disease flare. * Known congenital amino acid metabolism disorders. * Severe hypersensitivity to milk products. * Known liver cirrhosis. * Regular use of non-steroidal anti-inflammatory drugs (NSAIDs). * Impaired kidney function with eGFR \<30 mL/min/1.73m2