Social determinants such as education, income, marital status, and social networks play a crucial role in cancer outcomes, including breast cancer. One potential influencing factor is diet, and the Mediterranean diet has been shown to benefit both the health and the microbiome. This raises the question: Does adherence to the Mediterranean diet impact the microbiome, quality of life, overall well-being, and outcomes in breast cancer patients undergoing (neo)adjuvant treatment, particularly among those with unfavorable socioeconomic determinants? The Mediterranean diet consists of plant foods such as vegetables, fruits, nuts, seeds, berries, legumes, herbs, and spices. It also features animal proteins like fish, meat, and cheese, along with fat from olive oil. Thus, adherence to the Mediterranean diet is adherence to the official Danish diet recommendations. This phase II clinical trial is an interdisciplinary study combining nutrition, sociology, and health research. Breast cancer patients candidates for (neo)adjuvant treatment at four oncology departments in Region South Denmark will be randomized 2:1 to the Mediterranean diet (with individual dietary guidance from a nutrition therapist aiming at daily consumption of a minimum of 30 grams of dietary fiber and weekly consumption of a minimum of 30 different plant foods; minimizing the amount of ultra-processed food) versus the patient's regular diet, in conjuction with the (neo)adjuvant chemotherapy. The trial's primary endpoint will be changes in gut microbiota composition in feces. The study evaluates how the Mediterranean diet affects the microbiota (evaluated using Oxford Nanopore Technology 16S sequencing on fecal samples); short-chain fatty acids in stool and plasma (assessed by mass spectrometry); immune system (using flow cytometry for immune cell determination in blood and immunohistochemical determination of immune cells in the tumor tissue); and patients' social quality of life, overall well-being, sense of empowerment, and personal feeling of agency (using the WHOQOL-100 questionnaire); taking into account the impact of the distance from the patient residence to the hospital (tracked from GEOTEAM at Statistics Denmark), and socioeconomic factors (using the DREAM database). The evaluation of biological parameters is based on the suggested mechanism of action. A diet rich in various plant foods and dietary fiber will alter the gut microbiota, promoting bacteria producing high amounts of short-chain fatty acids. These fatty acids will activate immune cells, aiding in the destruction of cancer cells. The study will also assess whether the Mediterranean diet will positively impact the patients' social quality of life, overall well-being, sense of empowerment, and personal feeling of agency. The Mediterranean diet may not only lead to improved dietary habits but also serve as a crucial tool for breast cancer patients, particularly among those with unfavorable socioeconomic determinants. Enhanced empowerment and improved coping tools could be an important step in combating cancer-related inequalities.