Department of Nutrition and Dietetics, Level 6, Gate 10, Brunel Building, Southmead Hospital, Bristol, BS105 NB, UK. Electronic address: stephen.taylor@nbt.nhs.uk. Department of Nutrition and Dietetics, Level 6, Gate 10, Brunel Building, Southmead Hospital, Bristol, BS105 NB, UK. Electronic address: kaylee.allan@nbt.nhs.uk. Department of Nutrition and Dietetics, Level 6, Gate 10, Brunel Building, Southmead Hospital, Bristol, BS105 NB, UK. Electronic address: Helen.McWilliam@nuth.nhs.uk. Department of Anaesthetics, Level 3 Gate 38, Brunel Building, Southmead Hospital, Bristol, BS10 5NB, UK. Electronic address: alex.manara@nbt.nhs.uk. Department of Anaesthetics, Level 3 Gate 38, Brunel Building, Southmead Hospital, Bristol, BS10 5NB, UK. Electronic address: jules.brown@nbt.nhs.uk. Research Design Service - South West, University Hospitals Bristol NHS Foundation Trust, Level 3 Education Centre, Upper Maudlin Street, Bristol, BS2 8AE, UK. Electronic address: rosemary.greenwood@UHBristol.nhs.uk. Department of Engineering Design and Mathematics, University of the West of England, Frenchay Campus, Bristol, BS16 1QY, UK. Electronic address: Deirdre.toher@uwe.ac.uk.