2.1 Study setting and subjects
The data was drawn from inpatients’ electronic medical records (EMR) and dual X-ray absorptiometry (DXA) database stored at The Second Hospital of Jilin University, which is located in the northeast part of China. The Second Hospital of Jilin University has 62 clinical departments (i.e., endocrinology, bone and cardiology) and 8 medical technology departments (i.e., clinical laboratory and nuclear medicine). The hospital has more than 2,789 beds, 4,000 faculty members and staffs; its annual outpatients and inpatients are approximately 1.81 million and 0.11 million, respectively. The Second Hospital of Jilin University is a major diagnosis and treatment center for severe diseases (i.e., cancers and CVDs) for the whole Jilin Province in China. The linked for EMR and DXA databases capture patients demographics (i.e., age and sex), anthropometry data (e.g., body height and weight), clinical laboratory measurement records, prescription drug dispensation records, clinical diagnoses and BMD data.
We identified patients with complete and valid data on femoral neck and total hip BMD, TC, LDL-C, HDL-C and TG in 2017 in this study. We excluded patients: (1) with age < 50 years old at BMD test; (2) with early (< 40 years) menopause; (3) with a lipid-lowering therapy, synthyroid or hormone-replacement therapy; (4) with osteoporosis-related diseases such as cancer, thyroid disease, hypopituitarism, rheumatoid arthritis, chronic renal failure or renal dysfunction (creatinine > 442 µmol/L), chronic liver disease or liver dysfunction (aspartate aminotransferase or alanine aminotransferase > 80 U/L); (5) with a surgical history of bilateral salpingo-oophorectomy menopause or a history of bone surgery at the lumbar spine or hip. To partially exclude patients with osteoporosis-related medications, we only included the patients at their first DXA tests. This study was approved by The Second Hospital of Jilin University Life Science Ethics Committee (2017 Research Approval No.13).
2.2 BMD measurement
Femoral neck and total hip BMD were measured by a DXA fan-beam bone densitometer (Discovery Wi, Hologic, Bedford, MA, USA). The control spine phantom scan performed each day had a long-term (more than 5 years) coefficient of variation (CV) of < 0.5%. We defined osteoporosis as femoral neck or total hip T-score ≤ -2.5 [26]; the reference for calculating T-score was based on BMD data from Whites 20–29 years old in the third National Health and Nutrition Examination Survey (NHANES III) [27].
2.3 Blood draw and storage
Fasting blood samples (≥ 8 hours) were drawn from patients with spray-coated silica and a polymer gel evacuated sterile collection tubes (BD, Becton, Dickinson and Company, Franklin Lakes, New Jersey, USA) by nurses; these blood samples were centrifuged to obtain serum at clinical laboratory department.
2.4 Lipid biomarkers ascertainment
Within two hours following blood draw, serum samples were used to directly measure TC, LDL-C, HDL-C and TG; their levels were determined by a biochemical analyzer (7600 model, Hitachi, Tokyo, Japan). The CVs for TC, LDL-C, HDL-C and TG measurements were 2.5%, 3.1%, 5.1% and 1.9%, respectively.
2.5 Covariate ascertainment
The covariates were extracted from EMR and DXA databases. The covariates for this study included demographics variables (sex, age), anthropometric variables (height, weight, body mass index [BMI]), disease diagnoses (type 2 diabetes, neuropathy, hypertension, retinopathy, ischemic heart disease, ischemic stroke, diabetic nephropathy and osteoarthritis), medication for diseases (calcium channel blockers, angiotensin 2 receptor blockers, beta-blockers, diuretics, angiotensin converting enzyme inhibitors), liver function biomarkers (alanine transaminase, aspartate aminotransferase, total protein, albumin, total bilirubin, direct bilirubin, gamma-glutamyltransferase), bone biomarkers (alkaline phosphatase, calcium, phosphorus), thyroid function biomarkers (free thyroxine 4, thyroid-stimulating hormone), glycated hemoglobin A1c and fasting glucose. The covariates included in the analysis could be potential confounders that maybe associated with femoral neck or total hip BMD. Alanine transaminase, aspartate aminotransferase, total protein, albumin, total bilirubin, direct bilirubin, gamma-glutamyltransferase, alkaline phosphatase, calcium, phosphorus and fasting glucose were measured with a biochemical analyzer (7600 model, Hitachi, Tokyo, Japan) and free thyroxine 4, thyroid-stimulating hormone with a chemiluminesent microparticle immunoassay analyzer (Architect i2000sr model, Abbott, Chicago, USA) and glycated hemoglobin A1c with an automatic glycosylated hemoglobin analyzer (D-10, Bio-Rad, California, USA).
Height and weight were measured using a wall-mounted stadiometer (to the nearest 0.1 cm) and an electronic scale (to the nearest 0.1 kg) and stored in the DXA database. BMI was calculated as weight (kilograms) divided by the square of the height (meters). Disease diagnoses (including type 2 diabetes, neuropathy, hypertension, retinopathy, ischemic heart disease, ischemic stroke, diabetic nephropathy and osteoarthritis) and medication for diseases (including calcium channel blockers, angiotensin 2 receptor blockers, beta-blockers, diuretics, angiotensin converting enzyme inhibitors) were also extracted from EMR, they were either self-report or clinically diagnosed with specific criteria.
2.6 Statistical analysis
We showed continuous variables as means (standard deviations [SDs]) or medians (inter-quartile ranges), categorical variables as frequencies (percentages). Statistical differences between osteoporotic and non-osteoporotic patients for characteristics of patients were tested with the student t test or chi-square test.
Multivariable logistic regression models were used to test the associations of TC (per SD increase), LDL-C (per SD increase), HDL-C (per SD increase) and TG (per SD increase) with osteoporosis. Due to the high correlations between lipid biomarkers (Pearson r up to 0.70), we included each lipid biomarker in one model. Models were further adjusted for sex, age, BMI, type 2 diabetes, neuropathy, calcium channel blockers, angiotensin converting enzyme inhibitors, alanine transaminase, albumin, total bilirubin, gamma-glutamyltransferase and alkaline phosphatase; variables considered but not included in the adjusted model were hypertension, retinopathy, ischemic heart disease, ischemic stroke, diabetic nephropathy, osteoarthritis, angiotensin 2 receptor blockers, beta-blockers, diuretics, aspartate aminotransferase, total protein, calcium, phosphorus, free thyroxine 4, thyroid-stimulating hormone, glycated hemoglobin A1c and fasting glucose, because they failed to meet P < 0.10 criteria under univariate analyses with osteoporosis. Because total bilirubin and direct bilirubin are highly associated (r = 0.80), to avoid the effect of colinearity, direct bilirubin was not included in the multivariable logistic regression models. There were 1.25%, 1.46%, 1.66%, 1.46% and 1.46% missing data for alanine transaminase, albumin, total bilirubin, gamma-glutamyltransferase and alkaline phosphatase. According to Downer et al.’s methods [28], these missing data were classified into an independent category while we adjusted them in the model.
TC, LDL-C, HDL-C and TG were classified as normal and abnormal groups according to their clinical reference values. Multivariable logistic regression models were used to test the associations of each lipid biomarker group with osteoporosis. Models were adjusted for sex, age, BMI, type 2 diabetes, neuropathy, calcium channel blockers, angiotensin converting enzyme inhibitors, alanine transaminase, albumin, total bilirubin, gamma-glutamyltransferase and alkaline phosphatase.
Multivariable linear regression models were used to test the associations between lipid biomarkers and femoral neck or total hip BMD. Models were adjusted for sex, age, BMI, type 2 diabetes, neuropathy, calcium channel blockers, angiotensin converting enzyme inhibitors, alanine transaminase, albumin, total bilirubin, gamma-glutamyltransferase and alkaline phosphatase. All analyses were conducted with SPSS (version 24.0, IBM, Inc., New York, USA) and two-sided P values < 0.05 were considered statistically significant.