The association of egg consumption with blood pressure levels and glycated hemoglobin in Spanish adults according to body mass index

Study design and participants

This was a cross-sectional analysis of data from baseline measurements of the EVIDENT II trial (NCT02016014), a multicenter, randomized double-blind clinical trial that aimed to develop and validate a smartphone application and to evaluate the effect of adding this tool to a standardized intervention designed to improve adherence to the Mediterranean diet and to physical activity36. The study included six groups of the Research Network on Preventive Activities and Health Promotion (REDIAPP) in Bilbao, Cuenca, Zaragoza, Valladolid, Barcelona, ​​and Salamanca (Spain). This trial included adult men and women (aged 18 to 70 years) free of advanced cardiovascular disease, cancer, and other major physical or mental disorders. Face-to-face and individual interviews, as well as anthropometric measurements, were performed in a research center by previously trained investigators. Recruitment, data collection and measurement procedures have been described elsewhere36. The study was approved by the Ethics Committee of Salamanca University Hospital (Spain), and all participants gave written informed consent according to the general recommendations of the Declaration of Helsinki.

Of the 833 participants who were initially examined, 127 were excluded because of a lack of data on HbA1c, 30 were excluded because of a lack of dietary data, and 8 were excluded because of missing data on any of the covariates that were considered. Thus, the present analyses were based on a subsample of 668 individuals (80.2%) in which all dataset variables were measured. The characteristics of the participants included in the present analysis were overall very similar to those from the total initial population (Table S1, Supplementary material).

Study variables

Exposure

Egg consumption was obtained with a 137-item Food Frequency Questionnaire (FFQ-137) that has been validated in a population of elderly people at high cardiovascular risk in Spain37. An incremental scale with 6 levels, from “1 to 3 times / month” to “2 to 3 times / day”, was used to collect information on food consumption frequencies. To convert participants’ responses on egg consumption frequency to consumption in g / day, it was considered i) that 1 standard egg weighs 60 g and ii) that 1 egg was consumed according to the midpoint of the frequency of each category. For example, individuals who reported consuming eggs 1 to 3 times / month were considered to consume 2 (the midpoint between 1 and 3) 60 g / month eggs, which is equivalent to 4 g / day (2 × 60 g / 30 days = 4 g / day). In addition, because body weight (BW) is an important variable to consider when studying diet-related cardiometabolic risk factors, we used the unit of measurement in g / day / kg of BW, the same currently used by the European Food Safety Authority ( EFSA), to recommend dietary guidelines16.38.

Outcomes

Three measurements of systolic (SBP) and diastolic (DBP) blood pressure were performed with a validated Omron M10-IT model sphygmomanometer (Omron Healthcare, Kyoto, Japan), and the average of the last two measurements was considered for each participant. Mean arterial pressure (MAP) was calculated as DBP + (0.333 x [SBP-DBP]). The measurements were made on the participant’s dominant arm in a seated position after at least 5 min of rest with a cuff of appropriate size, as determined by measurement of the upper arm circumference and following the recommendations of the European Society of Hypertension39.

To evaluate HbA1c, blood samples were obtained from the cubital vein between 8.00 and 9.00 am after the individuals had fasted and abstained from smoking, alcohol, and caffeinated beverages for the previous 12 h. Blood samples were collected at the respective health centers, and all samples were analyzed at the city hospital that participates in the external quality assurance programs of the Spanish Society of Clinical Chemistry and Molecular Pathology.

Covariates

Information was also collected on potentially confounding covariates of the association between egg consumption and the outcomes, including sociodemographic variables (age [continuous, years]sex [male, female]educational level [none, primary or secondary studies, university studies]), total energy intake (continuous, in Kcal / day, obtained with the FFQ-137 questionnaire), adherence to the Mediterranean diet (continuous, obtained with the validated 14-point Mediterranean Diet Adherence Screener [MEDAS score]40), tobacco consumption (nonsmoker or former smoker, current smoker), alcohol intake (nondrinker, current alcohol drinker), and moderate-to-vigorous leisure-time physical activity (continuous, in minutes / week, measured with the International Physical Activity Questionnaire –IPAQ). BMI (continuous, kg / m2) was obtained with objective measures of body weight divided by height squared. According to the cutoffs for BMI defined by the World Health Organization, individuals were classified as normal weight (<25 kg / m2), overweight (≥ 25 to <30 kg / m2) or obese (≥ 30 kg / m2).

Statistical analysis

To differentiate the lowest and the highest egg consumption, quartiles of consumption were established in the statistical analyzes. The lowest intake category (from 0 to 0.14 g of egg / day / kg of BW) was used as the reference category. To give a more applicable sense of the unit of measurement of egg consumption, if an individual with a BW of 70 kg and a standard egg weight of 60 g is considered, the 1st quartile varies from 0 eggs to approximately 1 egg consumed per week ( 0.14 g / day / kg of BW = 0.14 × 70 kg × 7 days / 60 g = 1.2 or ≈ 1 egg / week).

Statistical analysis included a description of the study population and the variables analyzed in total and by quartiles of egg consumption. First, the normal distribution of continuous variables was examined using both statistical (Kolmogorov – Smirnov test) and graphical (normal probability plots) methods. Then, chi-square tests were used for categorical variables, and ANOVA was used for continuous variables to compare the mean differences of each outcome variable according to the categories of egg consumption. Pairwise multiple comparisons were examined using the post hoc Bonferroni test. Then, the Pearson correlation test was applied considering the continuous variables of egg intake (g / d / kg of BW), SBP, DBP and MAP (mmHg) and HbA1c (%).

Linear regression models were used to analyze the association between egg consumption in quartiles of consumption (independent variable) and each of the blood pressure parameters (SBP, DBP and MAP) and HbA1c (dependent variables). Initially, an unadjusted model (Model 1) was performed separately for each dependent variable. In sequence, this model was adjusted for age (continuous, years), sex (male, female) and education level (primary or secondary studies, university studies) (Model 2). The next model was adjusted for all covariates of Model 2 adjusted for body mass index (continuous, Kg / m2), total energy intake (continuous, kcal / day) and adherence to the Mediterranean Diet (continuous, MEDAS score) (Model 3). Finally, the last model included all covariates of Model 3 and, in addition, was adjusted for smoking status (nonsmoker or former smoker, current smoker), alcohol intake (nondrinker, current alcohol drinker) and moderate-to-vigorous physical activity (continuous , minutes / week) (Model 4). To examine whether there was a linear trend in the association between egg consumption and the dependent variables, the aforementioned unadjusted and adjusted models were repeated after replacing the categorical variable in quartiles with the continuous variable of egg consumption in g / day / kg of BW.

Logistic regression models were also performed considering the dependent variables hypertension (SBP ≥ 140 and / or DBP ≥ 90 mmHg or using antihypertensive drugs) or type 2 diabetes (HbA1c ≥ 6.5% or using antidiabetic drugs). Similar to the procedure used in linear models, we repeated the unadjusted and adjusted models using the same set of potential confounders.

As other studies reported effect modification in the association between egg consumption and cardiometabolic risk by sex5 and body weight status19, we tested whether there was a first-order interaction with these variables. For each of them, we used the 2-log likelihood test, which compares the models without and with the corresponding interaction term. No interaction was found between egg consumption and sex and BMI considering all the dependent variables analyzed (p for interaction> 0.10 for all variables tested) (data not shown).

Finally, we performed analysis of covariance (ANCOVA) models for each dependent variable considering the quartiles of egg consumption and body weight status as fixed factors. These models included age, sex, education level, smoking status, alcohol intake, total energy intake, adherence to the Mediterranean Diet and moderate-to-vigorous physical activity as covariates. Although no interaction was observed between egg consumption and BMI, to make the results more transparent and to reinforce the main message of the study, estimated marginal means (± 1 standard deviation) of blood pressure parameters and HbA1c are represented in Fig. 1 according to the body weight status of the participants.

All analyzes were carried out with the IBM SPSS program (version 28), and a p value <0.05 was considered statistically significant.

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