Inclusion Criteria:  Stage 1: Young Adult 16-26 year olds  Individual must be aged 16-26 years Individual who has voluntarily signed informed consent  Stage 2: Children 8-16 year olds  Child aged 8-16 years old with NAFLD attending the paediatric clinic at Kings College Hospital A control group of children with a BMI z score <1.5 and no diagnosis of liver disease Individual willing to participate in the study with informed consent. If the participant is under 16, the parent should provide informed consent.  Exclusion Criteria:  Stage 1: Young Adult 16-26 year olds  Individual who is thought to be too unwell to participate by the study team, e.g. if participant has a fever or infection requiring treatment with antibiotics. Individual who has a medical condition that requires treatment with a drug that could alter BAT function, e.g. thyroid disease (thyroxine, carbimazole, propylthiouracil, beta- blockers), liver disease (ursodeoxycholic acid), any condition requiring treatment with peroxisome proliferator-activated receptor gamma (PPAR-γ) agonists or 4 beta 3 adrenoceptor (β3-AR) agonists. Individual has smoked/vaped 5 days before participating in the study.  Stage 2: Children 8-16 year olds  Participant who is thought to be too unwell to participate by the study team, e.g. if participant has a fever or infection requiring treatment with antibiotics. Individual who has a medical condition that requires treatment with a drug that could alter BAT function, e.g. thyroid disease (thyroxine, carbimazole, propylthiouracil, beta- blockers), liver disease (ursodeoxycholic acid), any condition requiring treatment with peroxisome proliferator-activated receptor gamma (PPAR-γ) agonists or 4 beta 3 adrenoceptor (β3-AR) agonists Individual has smoked/vaped 5 days before participating in the study