Design and participants
The ENSANUT used a probabilistic multistage stratified cluster sampling design. The sample size was designed to be representative of the country and regions, and their expansion factors and sampling weights were used in all analyses to render nationally representative estimates [7]. The surveys were conducted between October and May in 2005–2006 and 2011–2012. In 2006 data from 36,170 households was collected to obtain a sample of approximately 69,000 individuals. In 2012 data from 50,528 households was collected to obtain a sample of 89,000. In 2006, physical activity data were obtained on 17,183 adults who represented 55,165,527 people. In 2012, physical activity data were obtained from 10,729 adults who represented 65,252,418 people. Adults were defined as those between 20–69 years old according to the ENSANUT methodology. Detailed descriptions of the ENSANUT methodology are published elsewhere [8, 9]. All participants provided informed consent prior to participating. The National Public Health Institute Ethics Review Board of Mexico approved the study protocol.
Physical activity assessment using the international physical activity questionnaire
To assess physical activity, the Spanish version of the short form IPAQ was applied by trained personnel in face-to-face interviews. In 2006, the questionnaire was collected using a paper and pencil-based interview while a computer-based interview was used in 2012. The IPAQ assess the amount of moderate (3–5.9 metabolic equivalents (METs)), and vigorous (≥6 METs) physical activity accumulated in bouts of at least 10 minutes over the previous 7 days. Validity and reliability results from the short form IPAQ have been obtained in several countries, including Mexico [6, 10, 11].
The IPAQ questionnaire data was cleaned using established IPAQ protocols [12]. Based on responses obtained from the IPAQ, minutes per week of moderate physical activity (including walking) and vigorous physical activity were calculated for each participant. Moderate physical activity values were added to vigorous physical activity values to obtain MVPA minutes per week. Hereafter, we refer to the original MVPA data collected from the IPAQ as the “unadjusted” MVPA data.
Classification into WHO physical activity categories
Total minutes per week spent in MVPA was used to classified participants into three WHO physical activity categories [4]: “physically inactive” if they participated in <150 min/week of moderate intensity, or <75 min/week of vigorous intensity, or an equivalent combination of the two intensities, “sufficiently active” if they had 150–299 min/week of moderate intensity, or 75–149 min/week of vigorous intensity, or an equivalent combination, and “very active” if they participated in ≥300 min/week of moderate intensity, or ≥150 min/week of vigorous intensity, or an equivalent combination, considering bouts of at least 10 minutes. Participants were classified as very active if they accumulated at least twice the minimum WHO recommendation.
To assign participants to the appropriate physical activity category we multiplied their vigorous intensity minutes by two and added it to their minutes of moderate physical activity. We then used the moderate activity cut-points (<150 min/week for the physically inactive category, 150–299 min/week for the sufficiently active category and ≥300 min/week for the very active category) to determine the appropriate physical activity category.
Adjusting the international physical activity questionnaire data
The IPAQ and other self-reported questionnaire measures substantially over-report MVPA levels by comparison to those obtained objectively using accelerometers and other electronic movement sensing devices [13–17]. Recent physical inactivity surveillance studies have adjusted the over-reported questionnaire data, including those obtained using the IPAQ, so that they more closely reflect the truth [1]. To adjust MVPA for over-reporting, we developed an equation based in a validity study recently conducted by our group [11]. To achieve this, we obtained IPAQ and objectively measured MVPA on a sample of Mexican adults (19–69 years old) using accelerometers. Standardized protocols were used to verify the completeness of the accelerometry data, to clean it, and to determine the weekly volume of moderate (3–5.9 METs) and vigorous (≥6 METs) physical activity completed in bouts of at least 10 minutes [11]. MVPA was obtained by adding moderate physical activity (including walking) minutes per week plus vigorous physical activity minutes per week multiplied by two. With this information we developed the following equation to predict the objective MVPA with the estimated MVPA by IPAQ, and used it to adjust MVPA from ENSANUT 2006 and 2012:
Adjusted MVPA minutes per week were then classified using the moderate activity cut-point previously mentioned.
Sociodemographic and biological variables
Geographic areas
Differences in MVPA were considered based on whether participants lived in an urban (≥2, 500 residents) or rural (<2,500 residents) area and based on the region of the country they lived in. ENSANUT is representative of four geographic areas of the country including: North (Baja California, Southern Baja California, Coahuila, Durango, Nuevo Leon, Sonora, Sinaloa, Tamaulipas and Zacatecas), Central (Aguascalientes, Colima, Guanajuato, Hidalgo, Jalisco, Mexico (excepting D.F. and metropolitan areas), Michoacan, Nayarit, Querétaro, San Luis Potosi and Tlaxcala), Distrito Federal (D.F. and metropolitan areas), and South (Campeche, Chiapas, Guerrero, Morelos, Oaxaca, Puebla, Quintana Roo, Tabasco, Veracruz and Yucatan).
Socioeconomic status and education (SES)
A SES index was previously constructed and validated by the Center of Survey Research at the Mexican National Institute of Public Health [18] by combining 8 variables that assessed the household properties and available services including: construction materials of the floor, ceiling, and walls; sleeping rooms; water accessibility; vehicle ownership; household goods (refrigerator, washing machine, microwave, stove, boiler); and electrical goods (television, radio, telephone, and computer). The index was divided into tertiles and used as a proxy for low, medium, and high SES. Education level was stratified into three groups according to the highest level of education obtained: primary or less, secondary, and high school or higher [18].
Anthropometric measurements
Weight and height were measured to the nearest 0.1 kg and 0.1 cm, and the BMI was calculated as kg/m2. BMI status was based upon the WHO endorsed adult cut-points as: underweight (<18.5 kg/m2), normal weight (18.5-24.9 kg/m2), overweight (25.0-29.9 kg/m2), or obese (≥30.0 kg/m2) [19].
Statistical analysis
The sample design characteristics (sample weights, cluster and strata variables) were taken into consideration for all analyses. Means, interquartile ranges, and percentages were used to describe the MVPA levels (minutes/week), physical activity categories, and sociodemographic characteristics of the participants. Physical activity was described within the entire sample and within strata based on the sociodemographic characteristics. Kernel-density plots were used to compare the proportional amount of minutes per week of MVPA by survey (2006 vs. 2012) and a general linear model was used to compare minutes per week of MVPA by biological and sociodemographic characteristics. Bivariate logistic regression models were used to evaluate the association between physical inactivity and biological and sociodemographic variables. This was followed by a single multivariate model that simultaneously included all of the biological and sociodemographic variables.