Background: Preclinical research has shown that fasting and/or a fasting mimicking diet (FMD) in addition to chemotherapy can enhance cancer treatment effectivity and reduce side-effects. The previous DIRECT1 phase 2 study (de Groot S, Nat. Commun. 2020) indicated that FMD could increase effectivity of neoadjuvant chemotherapy in patients with HER2-negative breast cancer. Methods: DIRECT2 is a multicenter, randomized, open label phase 3 study that investigates whether FMD compared to a regular diet during neoadjuvant chemotherapy can improve the pathological and/or radiological response rate in patients with hormone receptor positive (HR+), HER2-negative, stage II/III breast cancer. Secondary endpoints include quality of life, cognition, toxicity ≥grade 3 and the immunomodulating effect of FMD. The FMD consists of a low-caloric diet of 600kcal/day starting 3 days prior to, and on the day of chemotherapy administration every 4 weeks, for a total of 5 FMD cycles during neoadjuvant chemotherapy (4 times dose dense Adriamycin plus cyclophosphamide followed by 12 times weekly paclitaxel). Patients will be supported and monitored by a centralized dietician during each FMD cycle. An additional MRI will be conducted after 4ddAC to evaluate efficacy mid-way through chemotherapy. Furthermore, an optional tumor biopsy will be done to determine the immunomodulatory effects of the FMD which will be determined using cutting-edge RNA sequencing and multispectral imaging techniques. Pathological response will be graded on the surgical specimen according to Miller & Payne 4/5, indicating a 90-100% tumor cell loss. Radiological response will be determined by MRI (RECIST1.1). In total 240 patients are needed to detect an increase from 20% to 37.5% in pathological response rate in the FMD arm allowing for 15% dropout, using a two-sided Z test with pooled variance and 5% significance level with a power of 80%. Randomization will be performed in Castor EDC with variable block sizes and stratified by clinical stage (stadium II versus III) and BMI (< 25kg/m² versus ≥ 25kg/m²). Inclusion criteria are HR+, HER2- breast cancer stage II-III, BMI > 18.5 kg/m², WHO status 0-2, without a history of malignancy in the past 5 years, invasive or ipsilateral non-invasive breast cancer. Patients will be excluded in case of pregnancy, lactation or metabolic disease that affects gluconeogenesis and adaptability to FMD. The trial is supported by grants from the Dutch Cancer Society and WCRF. The study is open since February 2023 for inclusion for 4 years and plans to recruit 240 patients in total in 20 participating hospitals in the Netherlands. Clinical trial information: NCT05503108.