C-reactive protein (CRP), a systemic inflammatory biomarker, is positively associated with the development of disease and inversely correlated with regular physical activity. CRP has been previously reported to have a diurnal rhythm with higher levels upon awakening and lower levels thereafter.
PURPOSE: To characterize the pattern of salivary CRP and evaluate associations with sedentary and active behaviors in U.S. Navy men.
METHODS: Healthy, active duty men (n = 17; mean ± SD age = 36.1 ± 6.0 yr) self-collected samples in a free-living setting using oral swabs on 2 consecutive workdays at Wake, Wake + 30 min, Wake + 60 min, 1600, and 2100 for a total of 10 samples. Following our prior published study, CRP variables and summary parameters were computed. Stability across both days was evaluated via Pearson product-moment correlational analyses. Stable measures were then correlated to self-reported percent time (%time) spent sedentary, or active, in a typical workday for the previous week. Three non-responders, defined as having an absolute reactivity (AR) > 0, were excluded from analyses.
RESULTS: Stability was high between both days at all time points (r value range = .75-.92, all p ≤ .001). CRP was highest at Wake, decreased on average by 42.8 ± 5.7% at Wake + 30, and then plateaued for the rest of the day. The stabilities of Wake mean (r = .89, p < .001) and AR ([Wake + 30] - Wake), a measure of CRP pattern (r = .48, p = .053), were determined. Mean ± SE of Wake mean was 12,460 ± 3968 pg/mL and AR was -8298 ± 3123 pg/mL. Wake mean was strongly associated with %time walking (r = -.57, p < .01). Wake mean also showed an intuitive yet nonsignificant positive association with %time sitting, and a negative association with both %time standing and %time performing heavy labor. Hypothesized associations between %time and AR were not performed due to the borderline stability of AR.
CONCLUSIONS: In a military population, the salivary CRP pattern was described as diurnal with robust stability across 2 consecutive days. The negative correlation between Wake mean and %time walking suggests that walking is a powerful modality to reduce systemic inflammation. Subsequent analyses will comprehensively characterize the CRP pattern (area under the curve and other summary parameters) and evaluate additional biobehavioral correlates.