Associations of body mass index, fasting insulin, and inflammation with mortality: a prospective cohort study

Obesity is often considered to increase the risk for premature mortality. Higher fasting insulin and c-reactive protein are associated with higher body mass index (BMI) and all-cause mortality, so may confound the association between obesity and mortality. Our objective was to determine the independent associations between BMI, fasting insulin, c-reactive protein, and all-cause mortality in a general population sample.

This prospective cohort study included non-institutionalized US adults (≥20 years) from the National Health and Nutrition Examination Surveys 1999-2000 to 2013-2014. The main exposures of interest were BMI, fasting insulin, c-reactive protein. Mortality data were obtained through linking participants to the National Death Index (ending December 31, 2015).

There were 12,563 participants with a median age of 45 years (range 20-85) and 47.9% were male. The median BMI was 27 kg/m² (IQR 24-32), median fasting insulin was 54 pmol/L (IQR 35-87), and median c-reactive protein was 1.9 mg/L (IQR 0.8-4.4). In a Cox model adjusted for age, biological sex, cigarette smoking, and ten chronic conditions, higher BMI parameterized with quadratic and linear terms was not associated with mortality. When fasting insulin and the natural logarithm of c-reactive protein were included in the model, an inverse association between BMI and mortality was present (compared to the referent category of 5th percentile: 1st percentile, HR 1.10, 95% CI 1.06-1.13; 99th percentile, HR 0.48, 95% CI 0.34-0.69). In contrast, higher levels of fasting insulin and c-reactive protein were associated with an increased risk of mortality (for fasting insulin: 1st percentile, HR 0.98, 95% CI 0.97-0.99; 99th percentile, HR 1.83, 95% CI 1.48-2.26; for c-reactive protein, 1st percentile, HR 0.87, 95% CI 0.84-0.90; 99th percentile, HR 2.77, 95% CI 2.12-3.62).

Higher fasting insulin and higher c-reactive protein confound the association between BMI and the risk of all-cause mortality. The increase in mortality that has been attributed to higher BMI is more likely due to hyperinsulinemia and inflammation rather than obesity.