 Study design: To establish the Upper Level of Safe intake USLI of methionine intake, it is essential to create a dose-response approach of possible negative effects of methionine. To define the Lowest-Observed-Adverse-Effect Level LOAEL, the investigators suggest a 4 weeks intervention and 2 to 4 weeks washout in healthy volunteers with free methionine at 10 mgkg body weightday requirement, 25 mgkg body weightday, 50 mgkg body weightday and 100 mgkg body weightday. Pure methionine will be provided by ICAAS. The 4-week intervention period is based on a study in pigs 6. Methionine will be consumed in 3 daily doses. Participants will be advised about their daily dietary intake during the study. Besides the screening visit, each subjects will come back for up to 13 study visits up to 5 Short study Days SD, approx. 30 min and 8 Long study Days LD, approx. 5 h. On all study days both SD and LD the investigators will check parameters of toxicity and measure metabolites of methionine catabolism on the day before each dose intervention, during intervention 2 weeks after the start of each dose intervention; range: - 3 days, on the last day of each dose intervention, and 2 weeks after the last dose intervention. Because methionine as a dietary supplement clearly rises a safety issue, we will conduct this experiment in steps. First, subjects will receive the 10 mgkg body weightday. The investigators then will analyse all parameters of toxicity, interpret the data and when there are no safety concerns, continue with the next intake level. The study will be stopped at the level when there are apparent safety concerns. 