Adipose tissue in obesity, diabetes, and metabolic syndrome is in a state of chronic inflammation (1-5). Quantitative C-reactive protein (CRP), an acute-phase reactant produced dominantly by liver, is a plasma protein that circulates in increased amounts during inflammation and often damages the tissue (6,7). Results of studies showed that the increased CRP levels were associated with functional impairment, coronary heart disease, stroke, and mortality (8). So, reducing the CRP levels can reduce mortality and morbidity due to cardiovascular disorders.
Physical activity can prevent cardiovascular disorders (9). This effect can be partly mediated through reducing inflammation, including serum CRP levels (10). Results of some population-based studies showed a negative association between physical activity and the circulating levels of CRP (7,10-14). However, the pattern of this association with respect to different intensities and metabolic equivalents (METs) of physical activity and several tertiary factors, such as adiposity and glycaemic status, is not well-understood. Results of some studies showed that the impact of physical activity is mediated through its weight-lowering effect (15,16). A study reported that the circulating levels of CRP can be markedly suppressed, independent of total adiposity or fat mass, by intense regular physical exercise (17).
The present study was carried out to examine the association among different intensities of physical activity (moderate and severe), total physical activity (using METs for the intensity of activities), duration of sedentary behaviours, and the serum CRP levels after adjustment for glycaemic status and markers of adiposity, including waist-circumference and body mass index (BMI) in a large population-based sample of Iranian adults. Physical activity was defined by an international standard questionnaire, named global physical activity questionnaire (GPAQ).
MATERIALS AND METHODS
The study was based on data collected in the third national surveillance of risk factors of non-communicable diseases (SuRFNCD-2007). Details of the survey were reported elsewhere (18). In brief, a cluster-sampling scheme was applied to randomly select a representative sample of Iranian adults aged 25-64 years. The number of clusters selected from each province was proportional to the urban/rural size of that province. Trained healthcare professionals conducted household interviews and physical examinations. All interviews were conducted in Persian. Data were recorded in standardized sets of questionnaire. Blood sampling was done within a few days of the interview.
The survey received ethical approval from the Center for Disease Control of Iran, and written informed consent was obtained from all the participants.
Assessment of physical activity
The second version of the GPAQ was used in the survey (19). This questionnaire, developed by the World Health Organization (WHO), contains 16 questions about physical activity in a typical week and assesses physical activity in three domains: work, transportation, and recreational activities. The evaluation of physical activity in these domains is one of the factors that make the GPAQ distinct from other sets of questionnaire, such as the less-sophisticated, short version of the international physical activity questionnaire (IPAQ) (http://www.ipaq.ki.se). It also determines the intensity of activity (i.e. vigorous or moderate) in each domain and the time spent on sedentary behaviours, such as watching TV. Sedentary behaviours were defined as activities, such as sitting at a desk, travelling in car/bus/train, reading, working with computer, and watching television.
To measure energy expenditure, the concept of METs was used (19). MET is the ratio of a person's working metabolic rate and the resting metabolic rate (19). One MET is defined as the energy cost of sitting quietly and is equivalent to a caloric consumption of 1 kcal/kg/hour. …