Study design
Data collection for this cross-sectional study occurred from February to July 2019 and consisted of a single in-person visit to the Healthy Lifestyle research facility located in downtown Phoenix, Arizona, USA. Study outcomes were arterial stiffness (Cf-PWV); blood pressure [systolic blood pressure (SBP), diastolic blood pressure (DBP), central systolic blood pressure (CSBP), and central diastolic blood pressure (CDBP)]; cardiometabolic markers [total cholesterol, HDL and LDL cholesterol, and triglycerides], and inflammation [high-sensitivity C-reactive protein (hs-CRP)]. Anthropometric measures [body weight, body mass index (BMI), and waist and hip circumferences], and physical activity [Godin-Shephard Leisure-Time Physical Activity Questionnaire] [9] were also recorded. To increase the validity of inferences and to reduce bias, the research team, including the research nurse and phlebotomist were blinded to group allocation, and data were analyzed by the principal investigator.
Participants
Healthy, non-smoking adults (18–65 years old) adhering to either a vegetarian/vegan or omnivore diet were recruited from a campus population. A vegetarian/vegan self-identified as ‘eating no meat, poultry, or fish’ and were distinguished from individuals who reported consuming ‘meat, fish, and/or poultry on occasion but not daily.’ Pregnant or recently pregnant (within the past three months) women, lactating women, individuals with chronic health conditions as well as those taking medication for these conditions, and individuals engaged in > 150 min per week of moderate to vigorous intensity physical activity were excluded from the study. To create comparable groups, omnivores were matched to vegetarians using broad BMI (18.5 to < 25.0, 25.0 to < 30.0, and ≥ 30.0 kg/m2) and age (18–25, 30–40, and 50–65 years) categories. Written informed consent was obtained for all participants, and the study was approved by the Institutional Review Board at Arizona State University.
Study outcomes
Arterial stiffness and blood pressure
Participants were instructed to lay supine in a dimly lit, temperature-controlled room for 10 min. A non-invasive SphygmoCor XCEL (AtCor Medical, Sydney, NSW, Australia) system assessed Cf-PWV. A blood pressure cuff was placed around each participant’s upper thigh and brachial artery of the non-dominant arm. Carotid and femoral pulse rates were identified and recorded, and measured distances from the femoral pulse to the topmost point above of the blood pressure cuff, from the sternal notch to the topmost point above the blood pressure cuff, and from the carotid pulse to the sternal notch were entered into the SphygmoCor XCEL software. A tonometer was placed directly above the carotid pulse, and firm yet stable pressure was applied allowing the SphygmoCor XCEL device to compute Cf-PWV. Brachial artery and central aortic blood pressures were also recorded. Measurements were obtained in triplicate, and the mean of the last two measurements was recorded.
Cardiometabolic markers and inflammation
One 10 mL fasting (except water for 12 h) blood sample was acquired from each participant’s antecubital vein. Total cholesterol was measured using a cholesterol fluorometric assay kit (Cat. No. 10007640, Cayman Company, Ann Arbor, MI, USA), HDL cholesterol was assessed with a quantitation kit (Cat. No. MAK045-1KT, Millipore-Sigma, Burlington, MA, USA), LDL cholesterol was calculated using the Friedewald formula [10], triglycerides were measured using a Colorimetric Assay Kit (Cat. No. 10010303, Cayman Company, Ann Arbor, MI), and an ELISA kit (Cat. No.10011236) assessed hs-CRP.
Anthropometry measurements
Height (cm) was recorded using a stadiometer (SECA 217, Tiger Medical, Irvington, NJ, USA). Weight (kg) was obtained using a research-grade, calibrated total body composition analyzer (Cat. No. TBF-300, Tanita, Arlington Heights, IL, USA). Waist and hip circumferences were obtained with a SECA ergonomic measuring tape (Cat. No. 2011717009, Tiger Medical, Irvington, NJ, USA). For waist circumference, the measuring tape was positioned horizontally around the abdomen at the midpoint between the lowest rib and the top of the iliac crest. For hip circumference, the measuring tape was positioned around the widest portion of the posterior, parallel to the floor. Measurements were obtained twice and averaged.
Statistical analysis
Power analysis indicated that a sample size of 58 was suitable to detect a 0.6 m/s difference in PWV at a 5% level of significance assuming a standard deviation of 0.8 [11]. Data are represented as mean ± SD. Data that violated the Gaussian distribution were transformed prior to analyses (Cf-PWV, HDL cholesterol, hs-CRP, total cholesterol/HDL ratio, and triglyceride/HDL ratio). Group differences for Cf-PWF, brachial artery and central aortic blood pressures, and cardiometabolic markers were analyzed using univariate analyses controlling for age, adiposity, and gender. Additionally, a multivariate ANOVA was conducted for the cardiometabolic factors that were correlated (SBP, DBP, LDL cholesterol, total cholesterol, and triglycerides), and a two-way ANOVA was utilized to assess Cf-PWV by diet plan and gender. Descriptive data were compared using the Mann–Whitney U test or the Chi-Square test. Spearman coefficients were computed to compare relationships between variables. All analyses were performed using the Statistical Package for the Social Sciences, version 27 (IBM Corp., Armonk, NY, USA), and statistical significance was established at a p-value ≤ 0.05.