The aim of the study is to determine the optimal plant-based dietary model in terms of weight loss and health that would be both acceptable and environmentally friendly dietary model based on limiting the consumption of meat and animal products. Such findings will lead to new knowledge and allow us to formulate new principles for the nutritional treatment of people suffering from obesity.

Obesity is not only one of the most serious health problems in the world, it is also associated with an increased risk of many other diseases. Despite many preventive measures, the number of people suffering from obesity is constantly increasing, also in Poland. One of the integral elements of treatment is a change in lifestyle, including a change in eating habits. In this context, new dietary models are constantly being considered and eagerly adopted by patients. These are not always rational and evidence-based choices. What's more, what we eat is not only important for our health, but also for the planet. Recently, high-fat diets with a significant carbon footprint have gained popularity.

It is therefore worth considering more plant-based dietary models in the treatment of obesity and its associated disorders. Such a change can simultaneously help reduce greenhouse gas emissions and improve environmental quality, as well as benefit our health - both directly and indirectly.

There is a growing body of scientific evidence suggesting that limiting meat consumption may be beneficial to health, leading to weight loss, improvements in selected cardiovascular risk parameters and inflammation. However, many questions remain.

For this reason, more plant-based dietary patterns are worth considering in the treatment of obesity and its accompanying disorders. There is growing scientific evidence that limiting meat consumption promotes good health. It has been shown that the implementation of a vegetarian diet can result in weight loss, as well as help in maintaining optimal weight levels more effectively in the long term. This is extremely important for people suffering from obesity. Adherence to meat-restricted diets is also associated with other health benefits, which include an improved lipid profile and other cardiovascular risk parameters, as well as beneficial effects on inflammation and oxidative stress parameters.

Currently, there are only a few intervention studies available that have investigated the effectiveness of plant-based diets in a population of obese adults who previously have not restricted their meat consumption. Most of the studies concern vegetarians, whose awareness of taking care of their diet and lifestyle is, after all, much higher. To some extent, this may explain such optimistic conclusions from observational studies. It is not the only issue associated with research focused on plant-based diets. In experimental studies, the results of dietary interventions are very often compared with no intervention whatsoever. It would therefore be interesting to verify the effectiveness of a vegan or vegetarian diet against current dietary recommendations or another health-promoting dietary pattern - the Mediterranean diet. After all, vegan or vegetarian diets are elimination diets. Should they be recommended for the treatment of obesity and disorders associated with it? Do body weight, cholesterol and glucose levels decrease faster? What risks do they entail and are they difficult to implement? The investigators only know the answers to some of these questions.

In light of the above facts, the study will aim to evaluate the efficacy of the following plant-based diets: (1) the Mediterranean diet, (2) the 'planetary diet' as defined in the EAT-Lancet Commission report (3), the lactovegetarian diet, and (4) the vegan diet, in terms of weight reduction and effects on selected metabolic factors among obese patients. The last group (5) will receive the current national dietary recommendations for the Polish population.

To the investigators best knowledge, this is the first such study in Poland comparing several different plant-based diets. The investigators plan to recruit 100 obese adults under 60 years of age. The cohort will be randomly divided into five groups, each receiving a different intervention for 12 weeks. After 12 to 14 weeks from the end of the intervention, a follow-up meeting will take place again (i.e. after 24 to 26 weeks from the start of the nutritional intervention).

The study collect socio-demographic data and information on lifestyle (including dietary habits, physical activity, substance use and diet supplementation), psychological questionnaires (Perception of their own ability to change their diet (personality resources), which will be determined by the intensity of self-efficacy (GSES scale); Positive attitude towards the future, which will be determined by assessing the intensity of optimism (LOT-R); Coping dealing with emotions, which will be determined by assessing emotional control (CECS emotional control scale); Human ability to respond flexibly to changing situation requirements (stress, conflict, uncertainty), which will be determined by use The Brief Resilience Coping Scale (BRCS) created by V. G. and K. Sinclair A. Wallston) and to carry out biochemical tests and anthropometric measurements. Patients will be given dietary recommendations appropriate to the diet and specially designed dietary schemes, the main element to be intervened upon being the frequency of consumption of meat and other animal products or their elimination. In addition, an average carbon footprint will be estimated for each diet. Meetings with the dietitian will take place in a hybrid format (face-to-face and on-line). Venous blood for biochemical testing will be collected from patients before and after the study and at week 6 of the intervention. At the same time, anthropometric measurements will also be taken and information on wellbeing or difficulties in meeting dietary recommendations will be recorded. The investigators plan to examine the effects of the intervention on changes in body weight, body composition, blood pressure and, among others, changes in total cholesterol, LDL fraction, HDL fraction, changes in triacylglycerols, blood glucose, insulin, reduction in blood uric acid or changes in ALT, AST, GGTP activity.

Also, biochemical tests will be performed, including tests for inflammatory parameters (C-reactive protein (CRP), interleukin-6 (IL-6), L-selectin, intercellular adhesion molecule-1 (ICAM-1), and adiponectin), as well as anthropometric measurements and body composition assessments based on electrical bioimpedance. TMAO concentrations will be examined in relation to selected appetite traits. In addition, metabolites and cofactors of the methionine cycle (S-(5'-Adenosyl)-L-homocysteine (SAH), homocysteine, methionine, S-(5'-Adenosyl)-L-methionine (SAM), choline, betaine) will be monitored. In addition, selected polymorphisms of genes related to the homocysteine-methionine cycle and lipid metabolism will be evaluated. Patients will be given dietary recommendations according to their dietary habits, with the frequency of consumption or elimination of meat and other zoonotic products and their products as the main intervention element. Meetings with the dietitian will be held at baseline, week 6 and week 12. Between meetings, weekly online visits with a psychodietitian will be scheduled.

The aim of the study is to determine the optimal dietary model in terms of weight loss and health that would be both acceptable and environmentally friendly dietary model based on limiting the consumption of meat and animal products. The goal of this study is also to evaluate the efficacy of selected plant-based diets on weight loss, effects on selected metabolic and inflammatory parameters in obese adults. Such findings will lead to new knowledge and allow us to formulate new principles for the nutritional treatment of people suffering from obesity.

Inclusion Criteria:

• Age >18 years (adults able to give informed consent to participate in the study) ≤ 60 years of age
• BMI >30 kg/m2 or BMI 25-29.9 kg/m2 with waist circumference in women 88 cm, in men 94cm.

Exclusion Criteria:

• age <18 years and >60 years,
• severe medical conditions, including alcoholism, major and extensive surgery, chronic kidney and liver disease, history of myocardial infarction, unstable angina pectoris within the last 6 months, history of stroke within the last 6 months, cancer within the last 5 years,
• psychiatric condition preventing participation in the study and cooperation with the investigators,
• use of enteral or parenteral nutrition,
• use of medications that cause significant weight loss (e.g., GLP-1 analogues)
• pregnancy or breast feeding,
• attempts to change dietary habits, including reduction of caloric intake and elimination of meat and animal products in the past 6 months,
• lack of consent to participate in the study.