Study plan and participants

The sample of this cross-sectional study consisted of women university students living in Niğde, Türkiye. Students were recruited using convenience sampling, and data were collected between May and September 2024. A priori sample size estimation was conducted using G*Power software based on relevant literatüre.^{(Reference Büyükkayacı Duman, Yıldırım and Vural2,Reference Onieva-Zafra, Fernández-Martínez and Abreu-Sánchez19,Reference Monday, Anthony and Olunu29)} Assuming a two-tailed alpha level of 0.05, a statistical power of 0.90, and an effect size derived from previous studies, the minimum required sample size was calculated to be 170 students. Women aged 19–35 years with regular menstrual cycles were included in the study. Women younger than 19 or older than 35 years; those diagnosed with polycystic ovary syndrome; pregnant or breastfeeding; those using dietary supplements, following a special diet, or taking medications that may affect body weight; those who did not provide informed consent; women with a known thyroid disorder; and those with a history of oral contraceptive or hormone replacement therapy use were excluded from the study. Within the scope of the study, 402 students were reached, but some of them (those with missing data, pregnant students, and oral contraceptive users) were excluded for various reasons. The sample consisted of 362 students who fulfilled the inclusion criteria.

The presence of PD was evaluated based on the following criteria.^{(Reference Jolin, Rapkin, Berek and Novak30,Reference Atasü and Şahmay31)}

• • Pain occurring with every menstrual period.

• • Pain experienced in the lower back, pelvic area, or abdomen.

• • Pain starting on the day before menstruation or on the first day of menstruation.

• • Pain subsiding within 48–72 hours after the start of menstruation.

Study data were collected through face-to-face interviews using a questionnaire form (Supplementary File 1). The questionnaire included general information about the students, a Food Consumption Record, the International Physical Activity Questionnaire Short Form, and the Visual Analog Scale (VAS).

General information

The general information section of the questionnaire included questions regarding the students’ age, smoking status, alcohol consumption, menstrual pain, menstrual regularity, menstrual cycle length, duration of menstrual bleeding (days), amount of menstrual bleeding (light/moderate/heavy), and the presence and characteristics of PD.

Dietary intake

To determine the dietary energy and nutrient intakes of the students, a 3-day dietary record was used. At the first visit, participants completed a 24-hour dietary recall, which constituted the first day of the 3-day dietary assessment. At the end of the visit, students were asked to keep food consumption records for two additional days beginning on the first day of menstruation. Students were given a one-page printed version of the food consumption record form to record the foods they consumed. Detailed information about the portion sizes, cooking methods, and additional ingredients used (e.g., butter, cream, sauces) for all foods and beverages consumed at each meal was requested to be recorded. If eating out, the name of the restaurant and the food eaten should be recorded. The portion sizes of the consumed foods were determined using the ‘Food and Nutrition Photo Catalogue: Measures and Quantities’ catalogue.^{(Reference Rakıcıoğlu, Acar-Tek, Ayaz and Pekcan32)} For the purpose of identifying the content of ready-made or out-of-home meals, the ‘Standard Recipes for Institutional Catering’ book was utilised.^{(Reference Merdol33)} The averages of the three-day food consumption records were analysed by the BEBIS dietary analysis program (Version 8.1) based on the national food composition database.⁽³⁴⁾

Dietary phytochemical index

The DPI proposed by McCarty was used to assess the phytochemical intake of students through their diet. This index is calculated by dividing the dietary energy from phytochemical-rich foods (kcal) by the total dietary energy intake (kcal) and multiplying by 100. The foods included in the calculation of the phytochemical content of the diet, due to their high phytochemical content, are fruits/fruit juices, vegetables (excluding potatoes but including other tubers)/vegetable juices, legumes, nuts, seeds, whole grains and foods combined with these, as well as wine, beer, cider, soy, and olive oil.^{(Reference McCarty28)}

International Physical Activity Questionnaire Short Form

The extent of students’ engagement in physical activity was quantified via the Short Form of the International Physical Activity Questionnaire (IPAQ), devised by Craig et al.^{(Reference Craig35)} and linguistically and culturally validated for Turkish respondents by Öztürk et al.^{(Reference Öztürk36)} The instrument assesses activities that participants engaged in for no less than 10 minutes over the last seven days. It also records the number of days and the duration of vigorous physical activity (VPA), moderate physical activity (MPA), or walking (W) performed during the same period. Additionally, it records the time spent being sedentary (sitting, lying down, etc.). Students’ physical activity levels were evaluated according to the Metabolic Equivalent (MET), with HPA, MPA, and walking assigned 8.0, 4.0, and 3.3 MET, respectively. Weekly MET totals were computed by summing the duration and number of days each activity was performed. According to these totals, physical activity was categorised into low (<600 MET-min/week), moderate (600–3000 MET-min/week), and high (>3000 MET-min/week) levels.

Visual analog scale

To determine the intensity of pain, the Visual Analog Scale (VAS) developed by Fredy^{(Reference Freyd37)} and validated in Turkish by Yaray et al.^{(Reference Yaray, Akesen, Ocaklioğlu and Aydınlı38)} was used. A 0–10 cm line was used on this scale, and students were asked to indicate the intensity of their pain on the scale. The intensity of pain was classified into mild (1–3 points), moderate (4–6 points), or severe (7–10 points), following the criteria established by Onieva-Zafra et al.^{(Reference Onieva-Zafra, Fernández-Martínez and Abreu-Sánchez19)} The VAS scores were subsequently used to classify pain severity and to examine the association between DPI and menstrual pain intensity.

Anthropometric measurements

Height (cm Students’ height was measured using a non-stretchable tape measure while their head, hips, and heels were against the wall and their eyes were aligned with their earlobes (in the Frankfort plane), with values recorded to the nearest 0.1 cm.^{(Reference Lee and Nieman39)}

Body Weight (kg): Body weight was measured with students fasting and wearing light clothing using a Tanita (BC 401) scale, with values recorded to the nearest 0.1 kg. BMI was calculated using the measurements of body weight and height [body weight (kg)/height (m)²]. According to WHO criteria, BMI values were classified as underweight (<18.5 kg/m²), normal weight (18.5–24.9 kg/m²), overweight (25.0–29.9 kg/m²), and obesity (≥30.0 kg/m²).⁽⁴⁰⁾

Waist Circumference (cm): Waist circumference was measured with students’ arms at their sides and feet together, with the measurer standing in front of them. The assessment was conducted at the midpoint between the lower rib margin and the iliac crest with a rigid tape measure.^{(Reference Gibson41)}

Ethical considerations

The study protocol was approved by the Ankara University Ethics Committee before data collection (Approval Number: 1381084). All students provided voluntary informed consent to participate in the study, which was conducted in accordance with the Declaration of Helsinki. This cross-sectional study was designed and reported in accordance with the STROBE (Strengthening the Reporting of Observational Studies in Epidemiology) guidelines.

Statistical analysis

Statistical analyses were performed using IBM SPSS Statistics for Windows, version 26.0 (IBM Corp., Armonk, NY, USA). The normality of data distribution was assessed using visual and analytical methods. Descriptive data are presented as frequencies and percentages or as mean ± SD, as appropriate. Pearson’s chi-square test or Fisher’s exact test was used for comparisons of categorical variables. Student’s t-test and one-way ANOVA were used to compare continuous variables. Logistic regression analysis was performed to examine the associations of the DPI with PD and pain severity. Statistical significance was set at p < 0.05.^{(Reference Hayran and Özdemir42)}