Materials and methods The present study is designed as a double-blind, randomized, controlled clinical trial. A total of 70-80 adult men and women (\>18 years old) with overweight or obesity (BMI \>25 kg/m² and waist circumference \>94 cm for men and \>80 cm for women) and at least one metabolic abnormality (dyslipidemia, arterial hypertension, insulin resistance, or type 2 diabetes mellitus) will be recruited.
Individuals will be excluded if they are currently taking dietary supplements (e.g., multivitamin preparations), suffer from acute or chronic diseases (e.g., cancer, autoimmune disorders, insulin-treated diabetes), follow a strict vegetarian/vegan diet, abuse alcohol and/or illicit substances, or decline to provide written informed consent.
Participants will be instructed to consume daily, for a total duration of eight weeks either high phenolic olive oil (HPOO), containing \>1000 mg/kg total polyphenols, or low phenolic olive oil (LPOO), containing ≤100 mg/kg total polyphenols. The quantity of olive oil administered will be determined based on individual fat energy requirements, aiming to promote mild weight loss.
Participants will be allocated to one of the two intervention groups using simple randomization (1:1), conducted by an independent researcher. Both groups will receive standardized dietary guidance for mild weight loss according to the National Dietary Guidelines to ensure homogeneity of the intervention.
Measurements and data collection will be conducted at baseline and at the end of the intervention (week 0 and week 8), during morning visits following an overnight fast, at the University facilities:
* Socio-demographic characteristics and detailed medical history.
* Measurement of arterial blood pressure and heart rate.
* Measurement of resting metabolic rate (RMR) and respiratory quotient (RQ) (Cosmed Quark RMR, Rome, Italy).
* Blood sampling; a total of 20 mL of venous blood will be collected for serum and plasma isolation. Samples will be stored at -80°C until further analysis.
* Blood sample analysis; hormones, inflammatory- and oxidative stress markers assessed via enzyme-linked immunosorbent assay (ELISA) and spectrophotometric techniques, bioavailability of phenolic compounds analyzed with chromatography instrumentation available at the University of Peloponnese.
* Anthropometric measurements; height and body weight, body composition with bioelectrical impedance (TANITA Corporation, Tokyo, Japan), waist and hip circumferences, handgrip strength (Takei A5401 Hand Dynamometer, Takei Scientific Instruments Co., Ltd., Niigata, Japan).
* Questionnaires: Validated and standardized questionnaires for the Greek population will be administered, namely Food Frequency Questionnaire (FFQ), MedDiet Score, 24-hour dietary recall, 36-Item Short Form Health Survey (SF-36), Athens Insomnia Scale (AIS), Rosenberg Self-Esteem Scale (RSES), Depression Anxiety Stress Scale-21 (DASS-21), Profile of Mood States (POMS-short), Athens Physical Activity Questionnaire (APAQ).
Olive oil products (HPOO or LPOO) will be provided to participants every two weeks in identical, airtight containers suitable for olive oil storage, following microbiological quality control by the distribution company. Prior to each distribution, total phenolic content will be quantified using the Folin-Ciocalteu method.
The study follows a double-blind design: neither participants nor investigators will be aware of group allocation (HPOO or LPOO). The allocation code will be accessible only to the Principal Investigator.
Every two weeks during the intervention, telephone follow-up will be conducted to assess dietary habits, including compliance with olive oil consumption, using 24-hour dietary recall methodology. Any reported adverse effects related to olive oil intake will also be recorded.
The study will be conducted in accordance with the Code of Ethics and Research Ethics Committee of the University of the Peloponnese (4213/21-02-2025), as well as with the Declaration of Helsinki (1964) and its later amendmends. Furthermore, all applicable national and international regulations regarding the protection of personal data in research will be strictly observed. Prior to the initiation of the intervention, all participants will be thoroughly informed by the Principal Investigator about the study protocol, its potential benefits, and any possible risks. Individuals who wish to participate will be required to provide written informed consent before enrollment. All data will be stored and managed under the responsibility of the Principal Investigator. Any questions related to the study procedures or to the outcomes of the intervention will be fully addressed. Participation is strictly voluntary, and participants have the right to withdraw from the study at any time without any consequences. They may also decline to answer any questions or to undergo any measurement or blood sampling procedure without penalty.
