Minimal data set.

Background and aim
The Systemic Immunity-inflammation Index (SII) is an emerging metric for assessing an individual’s immunological and inflammatory condition. Studies examining the relationship between SII and body mass index (BMI) are limited. This study aims to clarify the correlation between SII and BMI.
Methods
The current study investigated the association between SII and BMI using data from the National Health and Nutrition Examination Survey (NHANES). Information on SII and BMI was obtained from laboratory and demographic data. The analytical approach included techniques such as linear univariate analysis, linear multivariate analysis, and subgroup analysis to explore the intricate relationship between SII and BMI comprehensively.
Results
A total of 1568 participants who met the inclusion criteria were included in the study; their average age was 53.6 ± 16.7 years, and they were 40.8% female and 59.2% male. Several baseline indicators showed statistically significant differences across SII groups. Curve fitting revealed an inverted L-shape, non-linear relationship between SII and BMI. Multivariate linear regression analysis revealed that adjusted β values were 1.7 (95% CI: 0.87–2.54, p < 0.001) for T3 and 0.46 (95% CI: −0.36–1.28, p = 0.271) for T2 in comparison to T1. Threshold analysis indicated that SII < 677.842 was associated with a BMI increase β of 0.005 (95% CI: 0.002–0.007, p < 0.001). The stratified analysis demonstrated the positive association between SII and BMI was not influenced by gender, diabetes, or smoking (all p > 0.05).
Conclusions
In summary, these findings reveal a robust association between SII and BMI within the examined American population.

研究背景与目的
全身免疫炎症指数（Systemic Immunity-inflammation Index, SII）是一种新兴的用于评估个体免疫与炎症状态的指标。目前针对SII与体重指数（Body Mass Index, BMI）之间关联的研究较为有限。本研究旨在阐明二者之间的相关性。

研究方法
本研究利用美国国家健康与营养调查（National Health and Nutrition Examination Survey, NHANES）的数据，探究SII与BMI之间的关联。研究所需的SII与BMI相关信息取自实验室检测数据与人口统计学资料。本研究采用的分析方法包括线性单变量分析、线性多变量分析及亚组分析等，以全面揭示SII与BMI之间的复杂关联。

研究结果
本研究共纳入符合纳入标准的1568名受试者，其平均年龄为53.6±16.7岁，其中女性占比40.8%，男性占比59.2%。不同SII分组间的多项基线指标存在统计学显著差异。曲线拟合结果显示，SII与BMI之间呈倒L型非线性相关关系。多因素线性回归分析显示，相较于T1组，T3组的校正β值为1.7（95%置信区间（Confidence Interval, CI）：0.87~2.54，P<0.001），T2组的校正β值为0.46（95%置信区间：-0.36~1.28，P=0.271）。阈值分析表明，当SII<677.842时，BMI升高对应的β值为0.005（95%置信区间：0.002~0.007，P<0.001）。分层分析显示，SII与BMI之间的正相关关系不受性别、糖尿病或吸烟状况的影响（所有P>0.05）。

研究结论
综上，本研究结果揭示，在所纳入的美国人群中，SII与BMI之间存在显著关联。