Threshold effect and sex characteristics of the relationship between chronic inflammation and BMI

Chronic inflammation is an important pathway for obesity to harm health, the aggravation of chronic inflammation occurs without clinical symptoms. BMI is closely related to chronic inflammation, and it is a predictive factor of chronic inflammation, but the following questions remain unanswered: Are the effects of chronic inflammation on different BMI intervals consistent? Are the effects of BMI on chronic inflammation consistent between male and female? This study aimed to explore the threshold effect, and sex characteristics of the relationship between chronic inflammation and BMI. Methods: People with normal weight, overweight, and obesity were selected as subjects for cross-sectional study. BMI, hs-CRP, adiponectin and irisin was tested. Multiple regression analysis and generalized additive models were used to examine the association between hs-CRP and BMI. Results: 119 adults were recruited (normal weight: n = 30, 28.1 ± 7.65 years, BMI: 22.04 ± 1.55; overweight: n = 29, 27.45 ± 7.47 years, BMI: 26.11 ± 1.22; and obesity: n = 60, 28.82 ± 6.05 years, BMI: 33.68 ± 3.57). After adjusting for age and sex, BMI was found to be positively associated with the chronic inflammatory marker hs-CRP (β = 0.45; P < 0.001), which had a threshold effect on hs-CRP. Positive correlation with hs-CRP was observed for BMI of > 24.6 (β = 0.54; P < 0.001) but not for BMI of 18.5–25.4 (β = −0.02; P > 0.05). The pro-inflammatory effect caused by BMI increase in female (β = 0.56; P < 0.001) was higher than that in male (β = 0.38; P < 0.001). When BMI was greater than 33, a positive correlation with hs-CRP was observed in female (β = 0.97; P < 0.001) but not in male (β = 0.14; P > 0.05). Conclusions: BMI has a threshold effect on chronic inflammation, BMI greater than 24.3 is positively correlated with hs-CRP. BMI in 18.5–24.3 is not correlated with hs-CRP. Furthermore, when the BMI greater than 33, hs-CRP is not positively correlated with BMI in male, whereas the pro-inflammatory effect of BMI increase becomes greater in female. Highlights: • BMI has a threshold effect on chronic inflammation. BMI in 18.5–24.3 is not correlated with chronic inflammation, and BMI greater than 24.3 is positively correlated with chronic inflammation. • The pro-inflammatory effect caused by BMI increase in female is higher than that in male. In particular, when the BMI is greater than 33, chronic inflammation is not positively correlated with BMI in male, whereas the pro-inflammatory effect of BMI increase becomes greater in female. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12902-023-01396-1.