The primary objective of this study is to observe the kinetics of pre-stored and de-novo synthesized insulin that is secreted into the circulation using an in-vivo heavy water (D2O) labelling experiment in patients with postprandial hyperinsulinaemic hypoglycaemia (PHH) and non-surgical non-PHH controls.

Despite an increased prevalence, the underlying pathophysiology of PHH remains incompletely understood. It is generally assumed that PHH is caused by excess insulin secretion, either due to an intrinsic beta-cell abnormality (histology showing increased beta-cell mass or signs of hyperfunction) and/or increased postprandial insulinotropic signals (also known as the incretin-effect) as a consequence of the re-arranged gastrointestinal tract and accelerated nutrient transit and absorption. Either of the two explanations would imply an altered insulin turnover in these patients with higher amounts of pre-stored insulin and/or accelerated de-novo insulin synthesis in response to stimulus-depletion of the available insulin pool. A non-invasive in vivo technique to study insulin turnover has not been established yet and data related to PHH are consequently lacking.

The primary objective of this study is to observe the kinetics of pre-stored and de-novo synthesized insulin that is secreted into the circulation using an in-vivo heavy water (D2O) labelling experiment in patients with postprandial hyperinsulinaemic hypoglycaemia (PHH) and non-surgical non-PHH controls. The investigators hypothesize that the suggested methodological approach is feasible to assess insulin turnover and provides the foundation for further studies in different target groups.

Inclusion criteria for PHH patients:

• Aged ≥18 years
• Roux-en-Y gastric bypass ≥1 year ago
• PHH defined as postprandial plasma or sensor glucose<3.0mmol/l according to the International Hypoglycaemia Study Group and exclusion of other causes of hypoglycaemia

Inclusion criteria for non-PHH non-surgical controls:

• Aged ≥18 years
• Absence of any condition or previous surgery known to affect gastro-intestinal integrity and food absorption

Exclusion Criteria:

• Clinically relevant weight changes (≥5%) within the past 3 months
• Incapacity to give informant consent
• Historical or current diabetes based on HbA1c ≥6.5% without glucose-lowering treatment
• Haemoglobin level below 13.5 g/l
• Ongoing treatment with glucose-lowering drugs, anorectic drugs, steroids or any medications known to affect gastric motility
• Active heart, lung, liver, gastrointestinal, renal or neurological disease
• Inability to follow study procedures
• Pregnancy or breast-feeding

Participants will be recruited by referral from our outpatient endocrine clinic and collaborating Centres of Excellence for Metabolic Surgery. Non-surgical controls will be recruited through advertisement according to guidelines from swissethics.