Plans for assessment and collection of outcomes Demographic variables

A general self-administered questionnaire will be used to assess participants’ demographic characteristics, including age, sex, marital status, education, medical history and their medications, family history of migraine, supplementation, and smoking at the beginning of the study. Moreover, the socioeconomic status of subjects will be evaluated through specifically designed questions such as the number of family members, home or car ownership, and participant’s job using a self-administered questionnaire.

Migraine symptoms assessment

An expert neurologist will assess migraine characteristics, including severity, frequency, and duration of migraine attacks. Headache severity will be assessed using a visual analogue scale (VAS) questionnaire on a 0–10 scale with 0 representing "no pain" and 10 "the worst imaginable pain”. The Persian version of this questionnaire, validated by previous investigators, will be used [21]. The patients will note the feeling that represents their perception of pain. The frequency and duration of migraine headaches will be evaluated as the number of attacks per month and the mean duration of attack in each day, respectively.

Dietary intake assessment

Six dietary recalls (four weekdays and two weekend days) will be obtained from all participants at weeks 2, 4, 6, 8, 10, and 12. All participants will be asked to report their dietary intake based on household measures. Then, these reports will be converted to grams using standard protocols [22]. To calculate nutrient intake throughout the trial, we will use the United States Department of Agriculture (USDA) nutrient databank.

Physical activity assessment

In addition, two 1-day physical activity (PA) records (including a working day and a non-working day) will be collected from all participants at weeks 2 and 8. PA levels will be reported as metabolic equivalent hours per day (MET/h/day).

Anthropometric measures and blood pressure

Body weight will be measured to the closest 100 g using a digital scale at the state of minimum clothing and without shoes. Height will be assessed to the nearest 1 mm using a non-stretched tape in a standing position with no shoes. Waist circumference (WC) will be measured to the nearest 1 mm at the midpoint between the lowest rib and the iliac crest, with the subject standing, after a normal exhalation. Body mass index (BMI) will be calculated as weight in kilograms divided by height squared in meters. Blood pressure (BP), including systolic and diastolic BP, will be assessed using a standard sphygmomanometer. Measurements will be taken twice, and the average value will be reported as the final BP.

Assessment of quality of life

There is some evidence indicating that quality of life is impaired in migraine patients [23]. In the present study, we will use two widely used questionnaires, including the Headache Impact Test (HIT-6) and the Migraine-specific Quality of Life (MSQ), to investigate the effect of selenium supplementation on the participants’ quality of life. As different aspects of functioning are examined in each questionnaire, using these two concurrently is common in clinical trials [24, 25]. These questionnaires have been translated into Persian by prior investigators, with their validity and reliability confirmed among Iranian adults [26, 27].

The HIT-6 questionnaire evaluates the influence of headaches on migraine patients’ well-being and daily performance [28]. This questionnaire includes six items, each with five response options, scored between 6 and 10, including never (six points), rarely (eight points), sometimes (10 points), very often (11 points), and always (13 points) options. The total score of HIT-6 ranges from 36 to 78. It is categorized as no impact (36–49), moderate impact (50–55), substantial impact (56–59), and severe impact (≥ 60) on patients' quality of life.

The MSQ (version 2.1) explores the impact of migraine headaches on patients’ daily functioning over the past four weeks [29]. This questionnaire has three domains: 1) role restrictive domain describing the reduction of daily activities (7 items), 2) role preventive domain describing the influence of migraine headaches on normal work and social activities (4 items), and 3) emotional functioning domain that evaluates the emotions associated with headache (3 items). Each item includes six response options, scored between 1 and 6: none of the time (score 1), a little bit of the time (score 2), some of the time (score 3), a good bit of the time (score 4), most of the time (score 5), and all of the time (score 6).The total score of each domain is computed as a sum of item responses and rescaled from 0 to 100, where higher scores indicate better quality of life.

Mental health assessment

To assess psychological disorders, a 21-item Depression, Anxiety, and Stress Scale (DASS-21) will be used [30]. The validity and reliability of the Persian version of this questionnaire have been also approved by Samani et al. [31]. Each subscale (depression, anxiety, and distress) consists of seven questions, with responses rated from 0 (does not apply to me at all) to 3 (applies to me most of the time). Subscale scores range from 0 to 21, with higher scores indicating increased psychological distress. To align with DASS-42, subscale scores will be multiplied by two for interpretation.

Plans to promote participant retention and complete follow-up

To enhance participant retention, the potential health benefits of selenium will be discussed, and regular telephone contact by the principal investigator (AB) will be maintained.

Data management

Data coding, security, and storage will be monitored by one of the researchers. Electronic data entry will occur at the participating site, with secure storage of forms. Data entry and values will undergo double-checking.

Confidentiality

Each participant will receive a unique identification code to store research data. The code list will be accessible only to researchers and protected by the principal investigator throughout the trial. Participants’ personal details will remain confidential in all publications.

Plans for collection, laboratory evaluation, and storage of biological specimens for genetic or molecular analysis in this trial/future use

A 5 ml blood sample will be collected at the baseline and end of the intervention. The serum will be obtained by centrifugation immediately after sample collection, then will be maintained at -80 degrees Celsius for future tests. NO levels will be measured by the Griess method, serum levels of TAC by the CUPric Reducing Antioxidant Capacity, TOS by the modified Ferrous Oxidation-xylenol Orange, and MDA levels by the Thiobarbituric acid reactive substance using commercial kits (Kiazist Life Sciences, Iran).