The effect of caffeine on appetite and body weight is controversial. Mostly epidemiological studies exist that show either a negative effect (reduction of appetite and body weight) or no effect. In this trial we are going to study the results of the consumption of 5mgr/kgr body weight of caffeine on appetite, food consumption and hormones ghrelin, asprosin, leptin and pancreatic polypeptide in relation to gene polymorphisms that affect caffeine metabolism and body weight. Seventy subjects will participate in a cross sectional study consisting of two trials (with or without the consumption of caffeine) in order to study the aforementioned parameters. Differences of total calories consumption between fast metabolizers of caffeine and the rest of the participants is the primary outcome.

In more detail, participants will be recruited through local advertisements in the university campus so that to be of normal weight (BMI<25) and apparently healthy. Smokers, special population groups, i.e. athletes, pregnant women etc., those having a chronic or acute disease and those on medication will be excluded from the study.

Experimental protocol. Each volunteer will take part in 2 trials (with 7days interval between them) in a random order (using a random-number table). Female participants will be on the follicular phase of their menstrual cycle during the experiments to avoid variability in appetite [doi.org/10.1016/j.appet.2018.01.029.]. The week preceding each experimental day, participants will be instructed to abstain from any caffeine. Furthermore, the day preceding each experimental day participants will be instructed to abstain from alcohol source and physical exercise, to get enough sleep (~7 h) and to come to the lab after an overnight fast of 10 h. Furthermore, the days before the experiment, volunteers will be asked to consume similar foods and quantities, keeping more or less constant eating patterns. Participants will arrive at the lab in the morning 9 am after an overnight fast of 10 h. They will consume a breakfast snack along with one of the three experimental drinks within 5 min. The snack will consist of 1 slice of white bread, 5 g of butter and 10 g of white sugar, providing 142 kcal (6.5% of energy from proteins, 62.5% from carbohydrates and 31.0% from lipids). The experimental drinks will be either (a) 200 mL of instant coffee providing 5 mg caffeine/kg body weight or (b) 200 mL of water (Control). After a 3-h period, participants will be offered an ad libitum lunch meal from a buffet, consisting of common Greek diet foods (pasta, tomato sauce, beef, salad, cheese, yogurt, fruit and juice). They will be instructed to consume as much food as they desire until they feel satiated, within 30 min. Researchers will record the exact amount of food consumed by weighting the foods that will be chosen and their remnants. In addition, a detailed recording of the foods participants are going to consume later in the evening will be performed from a registered dietician through recall questionnaires.

Molecular and biochemical parameters that are going to be recorded with the utilization of standardized generally accepted protocols are blood pressure, body weight and other anthropometric parameters, blood levels of ghrelin, asprosin, leptin, pancreatic polypeptide, insulin and glucose, and genetic markers that are related to caffeine metabolism (CYP1A2 rs2069514 και CYP1A2 rs762251), as well as to an obesity genetic risk score (32SNPs).

Primary and secondary outcomes Based on genotype data, participants will be categorized as fast, medium and slow metabolizers of caffeine. For sample size calculation differences of total calories consumptions between groups will be utilized as primary outcome. More specifically, since a small participation of slow metabolizers are expected, these subjects will be grouped together with medium metabolizers. A difference of total calories consumption that exceeds 20% between the two groups is considered clinically significant. Taking into account an attrition rate of 10% between the two trials (caffeine and control) a total of about 70 participants are required for the study.

Macronutrients (protein, carbohydrates and lipids) consumption, responses on visual analog scales for satiety and appetite, hormones concentrations will be studied as secondary outcomes

Inclusion Criteria:

• normal weight (BMI<25)
• apparently healthy participants

Exclusion Criteria:

• smokers,
• special population groups, i.e. athletes, pregnant women etc.,
• people with a chronic or acute disease and
• those on medication