Participants
This hospital-based case-control study was carried out on 84 newly-diagnosed COPD patients and 252 healthy controls [non-COPD patients] in Alzahra University Hospital, Isfahan, Iran, in 2015. Cases were individuals with forced expiratory volume in 1 second (FEV1)/forced vital capacity (FVC) < 70% or FEV1 < 80%. Controls were individuals without a history of COPD who were hospitalized in the same hospital. Case and control groups were individually matched in terms of age (± 5), and sex. Individuals older than 30 years of age and those whom COPD diagnosis was based on physician diagnosis and spirometry test were not included in the case group. Participants were excluded if they had stroke, dementia, or any condition that would preclude the possibility for an interview. Moreover, other chronic diseases including chronic liver cirrhosis, renal failure, uncontrolled thyroid disease, inflammatory bowel disease, rheumatoid arthritis, severe heart failure, cachexia, cancer in the past 3 years, chronic infections (HIV, tuberculosis, etc.), systematic period of treatment by steroids drugs for a long time, and other pulmonary problems such as fibrosis were considered as non-inclusion criteria due to their effects on patients’ dietary patterns or inability of these people to respond to questions. All cases and controls provided their written informed consent. The study was ethically approved by the Medical Ethics Committee of the Tehran University of Medical Sciences, Tehran, Iran.
Assessment of dietary intakes
Usual dietary intakes of participants over the past year were assessed using a validated 168-item FFQ. The FFQ consisted of 168 food items with a standard serving size for each food item (18–21). A trained interviewer administered the FFQ through face to face interviews. All reported consumptions were converted to grams per day using household measures (22). Subsequently, daily intakes of energy and nutrients were computed for each person using the US Department of Agriculture food composition database that was modified for Iranian foods.
To assess the adherence to the low carbohydrate diet, we divided the study participants into deciles of fat, protein and carbohydrate intakes, expressed as a percentage of energy. For fat and protein, highest intake received 10 points and lowest intake received 1 points. For carbohydrate, the order of the received points was reverse; those with the lowest carbohydrate intake received 10 points and those with the highest carbohydrate intake received 1 points. Then, the points for each of the three macronutrients were summed to create the overall diet score, which ranged from 3 (the lowest fat and protein intake and the highest carbohydrate intake) to 30 (the highest protein and fat intake and the lowest carbohydrate intake). Therefore, the higher the score, the more closely the participant's diet followed the pattern of a low-carbohydrate diet. Cut points were taken from control groups.
Assessment of pulmonary function
A trained technician assessed pulmonary function with spirometry test and calculated FEV1, FVC, and FEV1/FVC. Other respiratory symptoms, including chronic cough, sputum production, and breathlessness, were assessed. Chronic cough refers to coughing for more than 3 weeks (23). Sputum production for more than 3 months in 2 consecutive years is an epidemiologic definition of sputum production (24, 25). Breathlessness was assessed using a visual analogue scale. A visual analogue scale is a 100-mm horizontal line with descriptive words on both sides for individuals to explain their breathlessness rate using picture observations (26).
Assessment of other variables
Required information about age, gender, marital status, education level, cigarette smoking (current smoker, ex-smoker, never smoker), familial history of pulmonary disorders, and history of drug and supplement use were collected through the use of a pre-tested questionnaire. Body weight was quantified by digital scale to the nearest 100 g with minimal clothes and bare feet. Height was measured without shoes with shoulders in a normal position. BMI was calculated as weight in kilograms divided by height in square meters. The long form of the International Physical Activity Questionnaire (IPAQ) was administered to examine participants’ daily physical activity (27). Physical activity was calculated based on metabolic equivalent for task (MET), number of days per week, and amount of time per day (minutes) and was finally expressed as MET.min/week.
Statistical methods
Participants were categorized into quintiles based on LCD score. General characteristics and dietary intakes of participants across quintiles of LCD score were compared using one-way ANOVA for continues variables and chi-square tests for categorical variables. The association between LCD score and COPD was assessed by using logistic regression in different models. Model 1 adjusted for age, sex, and energy intake (kcal/d). Additional adjustment was conducted for university education (yes/no), and smoking status (yes/no). Finally, we adjusted the analysis for dietary intakes of red and processed meats, whole grain, and sugar-sweetened beverages. All confounders were chosen based on previous publications. The statistical analyses were carried out by using SPSS version 21. P values were considered significant at < 0.05.