Relationship Between Blood Lipid And Cognitive Impairment In Elderly In China

BackgroundCognitive impairment is a frequent disorder among the elderly. It affects the individual’s ability to function independently. With the aging of Chinese population, dementia and mild cognitive impairment (MCI) become a significant public health problem, now approximately 5.9 million and 23.9 million Chinese aged 65 years and older with dementia and MCI are currently living in China respectively. Recently, the effects of cholesterol, including total cholesterol (TC), triglycerides (TG), low density lipoprotein cholesterol (LDL-C) and high density lipoprotein cholesterol (HDL-C) on cognitive impairment among the elderly have attracted particular attention. However, whether cognitive function is affected by cholesterol levels is poorly understood; results to date have been inconsistent. Most of the existing studies only focused on the total population; therefore, there is still some ambiguity with respect to potential differences in the relationships of cognition and lipids in men and women. Although the association between cholesterol and cognitive function is strongly age-dependent, yet few of previous studies focused on the oldest old.ObjectivesThe purpose of this study is to investigate the serum cholesterol level and the prevalence of cognitive impairment, and further to explore the association of cholesterol with cognitive performance among Chinese elderly in longevity areas. The two following questions were addressed that whether the relationship between cholesterol and cognitive impairment positive or negative and whether the relationship maintained between different genders or age sub-groups.MethodsA cross-sectional community-based study was implemented in 2012 in eight longevity areas in China. A total of 2228 elderly aged 65 years and over (mean 86.8±12.2 years) participated in this study. A variety of potential risk factors for cognitive function, such as sociodemographic characteristics, life style, and physical measurements were included in the questionnaire. Blood plasma analyses included TC, TG, HDL-C, fasting plasma glucose, creatinine and hemoglobin were determined. Cognitive function was assessed using the Mini-Mental Status Examination (MMSE) and scores ranged from 0 to 30. Cognitive function was classified into a dichotomous variable according to the MMSE score:normal cognition (score:24-30) and cognitive impairment (score:0-23).Differences in characteristics were assessed by Student’s t or Cochran-Cox test for continuous variables, and by Chi-square tests for categorical variables between the participants with cognitive impairment and without.Generalized additive models were performed to explore the shape of the association between cholesterol and cognitive impairment.Then the multilevel logistic regression model was run to evaluate the risk of cognitive impairment; the cholesterol was examined as 1 mmol/L increments and in tertiles using the lowest tertile as the reference. We adjusted for demographic variables including age, gender, marital status, residence and education level in the basic model. Further adjustment were included in the final models for current cigarette smoking, current alcohol drinking, central obesity, sleep quality, anemia, hypertension, type 2 diabetes mellitus, and CKD were included in the final model. All the analyses were repeated, stratified by gender and age group (65-79 years, and 80 years and older) including the adjustments described for the final model.Results1. Characteristics of study participants:A total of 2228 participants aged 65 years and above were included in this study, of which 1000 were men and 1228 were women. The mean age of the participants was 85.8±12.0 years,61.0% of then were not in marriage,61.0% didn’t receive any formal education,16.1% were current cigarette smokers,14.1% were current alcohol drinkers,38.9% had general or bad sleep. A majority of participants suffered from chronic conditions, i.e.,38.1% of the participants had central obesity,30.2% had dyslipidemia,52.8% of them were anemic, 15.2% of them had type 2 diabetes,53.4% hypertension and 30.4% CKD.2. Serum cholesterol level:In the total population, the average level of TC, TG, LDL-C, HDL-C is 4.27 mmol/L,0.99mmol/L,2.54mmol/L,1.28mmol/L respectively, and 4.10mmol/L,0.92mmol/L,2.41mmol/L,1.26mmol/L in men; 4.47mmol/L, 1.07mmol/L,2.67mmol/L,1.31mmol/L in women; 4.42mmol/L,1.12mmol/L, 2.61mmol/L,1.30mmol/L in younger elderly; and 4.23mmol/L,0.94mmol/L, 2.52mmol/L,1.28mmol/L in the oldest old.3. The prevalence of cognitive impairment:The average MMSE is 24.2 points in the total population, and 26.7 points in men,22.0 points in women,28.6 points in younger elderly,21.8 in the oldest old. The prevalence of cognitive impairment is 31.5% in total population,17.9% in men,43.4% in women,6.7% in younger elderly, 45.0% in oldest old.4. The association of serum lipids with cognitive impairment:Each 1 mmol/L increase in TC, LDL-C and HDL-C corresponded to a decreased risk of cognitive impairment in multilevel logistic regression models. Compared with the lowest tertile, the highest tertile of TC, LDL-C and HDL-C had a lower risk of cognitive impairment. The adjusted odds ratios and 95% CI were 0.73(0.62-0.84) for TC,0.81(0.70-0.94) for LDL-C and 0.81(0.70-0.94) for HDL-C. There was no gender difference in the protective effects of high TC and LDL-C levels on cognitive impairment. However, for high HDL-C levels the effect was only observed in women.5. The association of serum lipids with cognitive impairment between different genders:when serum was taken as continuous variables, there was a significant decreased risk of cognitive impairment with higher TC and LDL-C in men, the OR was 0.63(0.53-0.76) and 0.77 (0.60-1.00) respectively;with higher TC, LDL-C and HDL-C in women, the OR was 0.78 (0.65-0.94),0.78 (0.65-0.94) and 0.57 (0.38-0.87) respectively.Compared with individuals in the lowest tertile of TC, both men and women with the highest tertiles of TC had a lower risk of cognitive impairment. However, a decreased risk of cognitive impairment was only revealed in women but not in men in those with the highest tertiles of LDL-C and HDL-C.6. The association of serum lipids with cognitive impairment between different age groups:There was no significant association between cholesterol and cognitive impairment among young elderly (under age 79).High TC, LDL-C and HDL-C levels were associated with lower risk of cognitive impairment in the oldest old (aged 80 and older). In the oldest-old (age≥80 years) groups, the highest tertiles of TC, LDL-C and HDL-C were associated with a better cognitive performance compared to those with the lowest tertile.ConclusionsThese findings suggest that relatively higher cholesterol levels are associated with better cognitive performance in Chinese elderly, specifically in the oldest old. With further validation, low cholesterol may serve a clinical indicator of risk for cognitive impairment in the elderly.Additional prospective cohort studies researches are needed to confirm the noted age difference in the association of cholesterol with cognitive impairment in the elderly in present study.