The goal of this case-control study is to investigate energy metabolism and brown adipose tissue (BAT) activity in individuals with Gilbert's syndrome (GS) and controls. The main focus of the study is to analyze:

1. the link between bilirubin metabolism and metabolic health.
2. energy metabolism and body composition in individuals with Gilbert's syndrome and control subjects
3. brown adipose tissue activity in Gilbert's syndrome and healthy controls.

Participants will undergo the following investigations:

1. cold exposure
2. PET-CT imaging with 18-F-FDG
3. MRI imaging of liver, abdominal fat and muscle
4. blood sampling
5. indirect calorimetry
6. bioelectrical impedance analysis
7. infrared thermography

Researchers will compare individuals with GS and control subjects in terms of metabolic health, body composition and BAT activity.

Bilirubin has long been associated with liver pathology and haemolytic conditions and was until recently assumed to possess little or no biological function. However, unconjugated bilirubin (UCB) rarely approaches these toxic levels and approximates 10 μM in the general population, which is 30-60 times below the reported toxic threshold.

The beneficial associations of mildly elevated blood UCB concentrations with diseases of civilization (in particular those affecting the cardiovascular system and all-cause mortality) have been reported in numerous studies. In addition, a mildly elevated UCB is also associated with reduced adiposity, decreased risk of metabolic syndrome, non-alcoholic fatty liver disease and diabetes as well as overall cancer risk. Bilirubin downregulates pro-inflammatory responses often observed in disease, which complements epidemiological evidence of reduced inflammatory, autoimmune, and degenerative diseases in individuals with GS. Together, these observations may account for the reduced all-cause mortality rates reported in those with GS compared to the normobilirubinaemic population. Finally, bilirubin has been recently reported to modulate cell signalling and act as an endocrine molecule. We were amongst the first to show that - bile pigments possess substantial anti-mutagenic in vitro and ex vivo antioxidant capacity; - subjects with mild hyperbilirubinaemia - also called Gilbert ́s Syndrome (GS) have a significantly reduced BMI compared to age and gender matched controls, a lower hip circumference (HC) as well as lower total/LDL cholesterol, triglycerides and low- and pro-atherogenic sub fractions as well as IL-6. Interestingly effects of GS in older participants (≥35yrs) is most profound, with substantially reduced anthropometric data and improved lipid profile.

Very recently we and others showed that enhanced lipid catabolism in GS seems to be the key mechanism involved.

Based on these very preliminary observations, and the strong link between bilirubin metabolism and metabolic health we aim to investigate in this application more deeply mechanistic questions to better understand why GS subjects show the very specific phenotype and its metabolic potential.

Here we propose a human case control study. Therefore we will investigate 40 GS individuals vs. 40 age and gender matched controls. Participants will be recruited as performed in previous studies.

Cases: (Gilbert-Syndrom):

• Total bilirubin in the blood > 1.2 mg/dL (17.1 μM)
• Unconjugated (indirect bilirubin) > 1 mg/dL

Controls (non-Gilbert-Syndrom):

• Total bilirubin in the blood ≤ 1.2 mg/dL (17.1 μM)
• Unconjugated (indirect bilirubin) ≤ 1 mg/dL

The duration of the study lasts for every participant 3 independent days. On the preinvestigation-day a screening will be performed including a blood sampling to prove in- and exclusion criteria.

On study day 1 participants must be fastened and will undergo the measurement of the brown adipose tissue.

On study day 2 participants must be fastened and will undergo the measurement of fat distribution in liver and muscle.

One 24-hr recall, a FFQ and a physical activity questionnaire will be assessed at the beginning of study day 1.

Inclusion Criteria:

• written consent, age between 18 and 65 years of age, liver marker (AST, ALT, GGT) < 2x of the upper norm range, non-smoker, moderate physical activity, ability to understand the requirements of the course of study

Cases: (Gilbert-Syndrom):

• Total bilirubin in the blood > 1.2 mg/dL (17.1 μM)
• Unconjugated (indirect bilirubin) > 1 mg/dL

Controls (non-Gilbert-Syndrom):

• Total bilirubin in the blood ≤ 1.2 mg/dL (17.1 μM)
• Unconjugated (indirect bilirubin) ≤ 1 mg/dL

Exclusion Criteria:

• age < 18 or > 65 years; having current or any history of cardio-pulmonary, metabolic or musculoskeletal disease; breastfeeding or was/potentially pregnant; liver diseases incl. hep A and B; kidney diseases; active tumor disease; persons with an organ transplant; not willing to meet the demands of the study; not being weight stable (± 5% body mass; self-reported) for at least the 3 months prior to their involvement; or any conditions or concurrent behaviour (including medication) that may have posed undue personal risk to the participant, introduce bias to the study or were influencing liver marker in the last 5 weeks.

Cases: (Gilbert-Syndrom):

• Total bilirubin in the blood ≤ 1.2 mg/dL (17.1 μM)
• Unconjugated (indirect bilirubin) ≤ 1 mg/dL

Controls (non-Gilbert-Syndrom):

• Total bilirubin in the blood > 1.2 mg/dL (17.1 μM)
• Unconjugated (indirect bilirubin) > 1 mg/dL

cohort from previous study