Population
The National Health and Nutrition Examination Survey (NHANES) is designed to assess Americans’ health and nutritional status. The NHANES began in the early 1960s, and in 1999, the survey became a continuous program. A nationally representative sample of 5000 people is surveyed annually, and two-year surveys constitute each cycle. The National Center for Health Statistics (NCHS) Ethics Review Board approved the program, informed consent was obtained from all participants, and survey data are publicly available on the internet.
A total of 48484 participants in NHANES 2007–2016 were recruited initially. Among them, 7013 subjects aged 65 years or older were included in our study. Then 2168 and 8 participants were excluded for unclear NPPS usage and follow-up results, respectively. Finally, a total of 4837 subjects were used for data analysis (Fig. 1).
Nonfood probiotics, prebiotics, and synbiotics
Currently, probiotics are not regulated by The Food and Drug Administration (FDA), so they are available as dietary supplements only, while the FDA regulates part of prebiotics as medications. Therefore, according to the search terms in O'Connor’s study, (20) a keyword phrase mining process was conducted on all dietary supplement names, dietary supplement ingredients, medication names, and medication ingredients from the database to identify NPPS use. The NPPS was a dichotomized variable in which participants had one or more pro-, pre-, and synbiotics assigned “yes”.
Covariables
Age, gender, ethnicity, ratio of family income to poverty, and educational level information were acquired from the demographics file. Ethnicity was classified as Mexican American, Non-Hispanic Black, Non-Hispanic White, and other. The ratio of family income to poverty is an index of household financial status. Participants with a ratio of family income to poverty ≤ 1.3 was defined as low income, 1.3 ~ 3.5 was defined as median income, and >3.5 was defined as high income. (21) Education levels were categorized as less than high school and equal to or above high school.
The dietary inflammation index (DII) is an assessment tool developed to estimate the inflammatory potential of an individual’s diet. (22) In a word, a dietary parameter with proof of anti-inflammation effect would obtain a score of “-1”. In contrast, a food parameter would be scored “+1” if it was reported to increase inflammatory biomarkers level and scored”0” for no effect on inflammation biomarkers. Less than 30 food parameters can keep its predictive ability. (23) Twenty-eight food parameters from NHANES 24-h dietary recall interviews were used in our study to calculate the DII, including carbohydrates, protein, total fat, alcohol, fiber, cholesterol, saturated fat, monounsaturated fatty acids (MUFAs), polyunsaturated fatty acids (PUFAs), niacin, vitamin A, thiamin, riboflavin, vitamin B6, vitamin B12, vitamin C, vitamin D, vitamin E, Fe, Mg, Zinc, Selenium, folic acid, β -carotene, caffeine, energy, n-3 fatty acids, and n-6 fatty acids. (24) The healthy eating index (HEI) was devised to measure the quality of American diets, and a higher HEI reflects a better diet quality. In our study, HEI 2015 edition was calculated based on the first-day total nutrient intakes obtained from the NHANES 24-h recall interview, Food and Nutrient Database for Dietary Studies (FNDDS), and Food Pattern Equivalence Database (FPED). (25) Subjects who smoked less than 100 cigarettes in life were classified as non-smokers, otherwise the smoker. (26)
Hypertension can be diagnosed if participants had been informed of hypertension previously, measured at least three times in four blood pressure measurements ≥ 140/90 mmHg, or were on anti-hypertensive agents. Diabetes was diagnosed based on the following criteria: ever been diagnosed diabetes, glycohemoglobin >6.5%, fasting glucose ≥ 7.0 mmol/L, random blood glucose ≥ 11.1mmol/L, tow-hour OGTT blood glucose ≥ 11,1mmol/L, or using diabetes medications or insulin currently. Hyperlipidemia was defined as having total triglyceride ≥ 150mg/dL, total cholesterol ≥ 200mg/dL, low-density lipoprotein ≥ 130mg/dL, high-density lipoprotein (HDL)< 40mg/dL for males, HDL<50mg/dL for females, or using lipid-lowering medications. Body mass index (BMI, kg/m2) is a continuous variable. Chronic kidney disease (CKD) was diagnosed for participants with Chronic Kidney Disease Epidemiology Collaboration(CKD-EPI) equation calculated estimated glomerular filtration rate(eGFR)<60ml/min/1.73m2, or previously diagnosed weak/failing kidneys. (27) Cardiovascular disease (CVD) was defined as coronary heart disease (CHD), congestive heart failure (HF), heart attack, stroke, or angina. The responses to the Medical Conditions questionnaires defined CHD, HF, heart attack, stroke, angina, and cancer. Trained interviewers collected medication information in household interviews. Having angiotensin-converting enzyme inhibitors (ACEI), angiotensin-II receptor blockers (ARB), or β blockers at baseline was defined as taking medication.
Endpoints and follow-up period
According to the National Death Index (NDI), we defined all-cause and cardiovascular mortality endpoints. The follow-up period was from the household interview to either death endpoints or the end of follow-up (December 31, 2019).
Statistics
All analyses were performed in R 4.2.1. Missing values (Supplementary Fig. S1) were mainly in DII (n = 542, 11.21%), HEI (n = 542, 11.21%), and the ratio of family income to poverty (n = 535, 11.06%). To retain the samples as many as possible, we adopted multiple imputations by “mice” R package to impute the missing values. All cases of all variables were included, and the repetition counts were 20.
According to the rule of “the least common denominator”, appropriate interviewer weights from the home interview (WTINT2YR) were used to obtain US nationally representative estimates. (20, 21) Continuous variables were presented as weighted medians with 95% confidence intervals (CI), and categorical variables were weighted proportions. Comparisons for baseline characteristics were made based on the usage of NPPS. Weighted Wilcoxon rank-sum test and Rao& Scott’s second-order correction test were used to analyzing group differences for continuous and categorical data, respectively. Two side P value<0.05 was considered statistically significant.
Weighted Cox proportional hazards regression models estimated the association between NPPS and all-cause and cardiovascular mortality. Model 1 was unadjusted, and the covariables in 3 additional models were as follows: age, gender, ethnicity, the ratio of family income to poverty, and education level (Model 2); Model 2 plus DII, HEI, and smoking (Model 3); Model 3 plus hypertension, diabetes, hyperlipidemia, CVD, cancer, CKD, BMI, and medication (Model 4). Then, subgroup analyses were made according to gender, the ratio of family income to poverty, education levels, DII, HEI, smoking, hypertension, diabetes, hyperlipidemia, BMI, CVD, cancer, CKD, and medication. Among them, DII and HEI were dichotomized based on weighted mean values (1.45, 95%CI 1.36 to 1.54 and 55.66, 95%CI 55.01 to 56.31, respectively) while BMI was classified as ≥ 25kg/m2 and<25kg/m2. The covariables in subgroup analyses were Model 4 minus the corresponding subgroup variables. Finally, the complete-case analysis was used to avoid imputation bias for missing data (n = 3619).