The aim of this study is to examine the interindividual variability of subjective and hormonal appetite responses to a standardised meal in healthy men and explore any moderating influence of the fat mass and obesity associated gene (FTO).

Participants homozygous for the obesity risk A allele (AA) or low risk T allele (TT) of FTO rs9939609 will complete two fasted control and two standardised meal (5025 kJ energy, 47% carbohydrate, 9% protein, 44% fat) conditions in randomised sequences. Ratings of perceived appetite and venous blood samples will be taken before and after the interventions. Interindividual differences in appetite responses and the potential moderating influence of the FTO gene will be examined using bivariate correlations and linear mixed modelling.

Meal ingestion initiates a series of co-ordinated subjective and hormonal appetite responses. However, it is not known whether interindividual variability in appetite exists in response to a standardised meal. A recent approach proposed to quantify individual differences in the intervention response involves quantifying the participant-by-response interaction from replicated intervention and comparator arms. Using this approach (a replicated crossover study), the current study will (1) investigate whether the perceived appetite and appetite-related hormone responses to a standardised meal are reproducible on repeated occasions; (2) examine whether there is true individual variability in appetite responses to a standardised meal; and (3) determine whether the fat mass and obesity associated gene (FTO) moderates the magnitude of appetite responses to a standardised meal.

A total of 18 healthy men will be recruited according to their FTO rs9939609 genotype: 9 homozygous minor allele (AA) and 9 homozygous major allele (TT). Participants will complete four main experimental conditions each separated by an interval of at least three days: two fasted control and two standardised meal conditions. Participants will arrive at the laboratory at 09:00 after a 13 h overnight fast and a cannula will be inserted into an antecubital vein for blood sampling. After 60 min rest, a fasting venous blood sample and rating of perceived appetite will be taken (0 h; 10:00). Participants will rest throughout all four conditions but will be provided with a standardised breakfast meal after the fasting measurements during the two meal conditions. Breakfast will be consumed within 15 min and consist of croissants, butter, chocolate spread, cereal biscuits and milkshake which will provide 5025 kJ energy (47% carbohydrate, 9% protein, 44% fat). Subsequent venous blood samples will be taken at 0.5 h (10:30) and 1 h (11:00), and appetite perceptions will be assessed at 1 h (11:00).

Interindividual differences will be explored by correlating the two sets of response differences between meal and control conditions. Within-participant covariate-adjusted linear mixed models will be used to quantify participant-by-condition and FTO genotype-by-condition interactions.

Inclusion Criteria:

• Homozygous minor allele (AA) or major allele (TT) FTO rs9939609 genotype;
• Non-smoker;
• Weight stable for the previous 3 months.

Exclusion Criteria:

• Heterozygous FTO rs9939609 genotype (i.e., AT);
• Any medical conditions (e.g., diabetes, coagulation or bleeding disorders);
• Taking any medication that might influence appetite, fat metabolism or blood glucose;
• Dieting or restrained eating behaviours;
• Any food allergies.