Women with breast cancer are at increased risk of comorbidities and premature mortality, potentially due to accelerated biological aging. Telomere attrition has been proposed as a biomarker of this process, which could be mitigated through interventions targeting behavioral factors such as diet. In recent years, avocado has drawn attention in nutritional research due to its unique nutritional profile.

Main objective: To evaluate the effect of consuming one avocado per day on biological aging-measured by telomere length-in breast cancer survivors, compared to a habitual diet (less than two avocados per week). Secondary objectives include changes in telomerase activity and biomarkers of inflammation and oxidative stress. Additional objectives include classical cardiovascular disease markers (glucose metabolism, lipid profile, blood pressure); anthropometric measurements; symptoms of depression and anxiety; quality of life and fatigue; and diet quality.

Methodology: A randomized controlled parallel-group trial involving 120 breast cancer survivors. Participants will be randomly assigned to either the intervention group (one avocado per day) or the control group (habitual diet with fewer than two avocados per week) and followed for 4 months. At baseline and the end of the intervention, a general questionnaire will be administered; blood and urine samples will be collected; anthropometric and blood pressure measurements will be taken; and diet, physical activity, symptoms of depression and anxiety, quality of life, and fatigue will be assessed. Mean changes from baseline to the end of the intervention in the primary outcome (telomere length) and secondary outcomes (inflammation, oxidative stress, classical cardiovascular disease markers, anthropometric measures, symptoms of depression and anxiety, quality of life, and diet) will be compared between the intervention and control groups using linear regression models.

Inclusion Criteria:

• Women diagnosed with primary breast cancer in stages I, II or III;
• 40-65 years old at screening;
• Cancer treatment (radiotherapy and chemotherapy) completed ≥ 6 months but not more than 10 years at the time of recruitment;
• Currently consuming less than 2 avocados per week;
• Signed the informed consent letter.

Exclusion Criteria:

• Metastasis;
• Ductal carcinoma or lobular carcinoma in situ;
• Breast cancer recurrence;
• Cancer diagnosis other than breast cancer or non-melanoma skin cancer;
• Body mass index ≥40kg/m2;
• Currently pregnant or breastfeeding, or planning pregnancy in the following 6 months;
• Allergy to latex;
• Unwilling to consume avocado;
• Immunodeficiency or HIV-positive status;
• Alcohol abuse (>50g/day);
• Unstable use of plant sterols, mineral supplements, use of fibre supplements, fish oil, or antioxidants in the 4 weeks prior to enrolment, or plans to change this during the study;
• Currently participating in any other randomized controlled trial;
• Difficulty or impossibility of an adequate follow-up;
• Inability or unwillingness to give written informed consent or to communicate with study personnel, or illiteracy;
• Patients with an acute infection or inflammation (e.g., pneumonia) will be allowed to participate in the study 3 months after recovery.