This study will learn more about how the body uses energy. Usually, the body uses sugars as energy first and then fats are used when the sugar stores are gone. Some people have trouble using fats as energy. This can lead to feeling tired, difficulty exercising, and storing too much fat where it does not belong (like in the muscle). It is believed that some boys and men with Klinefelter Syndrome may not be able to use fats as energy normally, and that a medication called fenofibrate could help this.

Inclusion Criteria:

• Male
• Ages 15 to 40 years
• Total testosterone concentration within the normal range for age and pubertal stage
• For the KS group only: genetic testing results confirming KS

Exclusion Criteria:

• Liver disease (ALT or AST > 3x upper limit of normal)
• Renal impairment (estimated creatinine clearance <80 ml/min)
• Diabetes mellitus (A1c > 6.4%)
• Untreated hypogonadism
• Treatment with a PPAR agonist (including fish oil) or statin within the past month
• Unstable medications with any medication added or removed within the past 2 weeks or plan for medication changes during the study period
• Inability to tolerate study procedures, including any medical conditions that make exercise unsafe at the discretion of the study physician
• For the KS group only: known allergy to fibrates, inability to swallow tablets