To investigate the impact of selenium levels on cognitive function.

Chinese aged 65 years and older from two sites in the Sichuan Province in southwest China and two sites in the Shandong Province in eastern China.

Subjects with hearing problems were not enrolled.

Recruitment

• The Chinese investigators and a team of interviewers who were employees of the provincial and county centers for disease control traveled to the area and conducted a complete census of residents aged 65 and older in the area 
• They enrolled eligible residents by going door-to-door, obtaining informed consent before conducting the interview and collecting biologic samples 
• The team of laboratory scientists collected samples of water, soil and food items from five locations within each village for analysis of selenium levels.

Design 

Cross-sectional Study

Blinding used

Not used 

Intervention

Not applicable 

Statistical Analysis

• Pearson's correlation coefficients were used to estimate correlations between nail selenium contents and selenium intake derived from the food frequency questionnaires and local food samples and between the selenium levels measured in nail and blood samples
• The study population was divided into quintiles according to nail selenium levels to best capture the association between selenium levels and cognitive function
• Analysis of variance models were used to compare differences in continuous variables and chi-squared tests were used to compare differences in categorical variables across the five quintile groups defined by nail selenium levels
• Multivariate analysis of covariance were used to first examine the association between selenium quintiles and all six cognitive test scores
• Following the significance of the multivariate analysis of covariance test, analysis of covariance models were used with each individual cognitive score as outcome variables.  A composite z score was created by using the average of standardized scores of the six cognitive test.
• The Wald-test statistic in mixed-effect models was used to detect significant correlation among cognitive scores from participants within the same site 
• With a nonsignificant correlation structure, regression models or analysis of variance models were conducted to identify variables associated with cognitive outcomes univariately. Analysis of covariance models were used with standardized cognitive test scores, including the composite z scores as the dependent variables and the quintile selenium levels as the independent variables adjusting for age, gender, education and other factors that were found to be related to either the selenium levels of the cognitive scores.
• To ensure that the associations between selenium and cognitive scores were not due to cardiovascular disease or cancer, the analysis was repeated excluding those subjects with a history of heart attack, stoke or cancer.

Timing of Measurements

• 500 subjects from Qionglai Sichuan Province were interviewed from December 2003 to January 2004
• 500 subjects from Gaomi Shandong Province were interviewed in May 2004
• 500 subjects from Jiange, Sichuan Province were interviewed in October 2004
• 500 subjects from Zichuan, Shangong Province were interviewed in May 2005.

Dependent Variables

Cognitive assessments including responses to the:

• Community Screening Instrument for Dementia (CSID)
• Consortium to Establish a Registry for Alzheimer's Disease (CERAD) Word List Learning Test
• CERAD Word List Recall Test
• Indiana University (IU) Story Recall Test
• Animal Fluency Test
• IU Token Test.

Independent Variables

• Selenium amounts in nail, food, water and soil samples
• Selenium amounts in blood samples from approximately 10% of randomly sampled subjects
• Food Frequency Questionnaire responses
• Apolipoprotein E genotype.

Control Variables

• Age
• Gender
• School Participation
• Years of Schooling
• Marital Status
• Household Composition
• Birthplace
• Migration History
• Alcohol Consumption
• Smoking History
• History of: Cancer, Parkinson's Disease, Diabetes, Hypertension, Stroke, Heart Attack, Head Injury and Bone Fracture
• Height
• Weight
• Blood Pressure.

• Initial N: 2,000 rural elderly Chinese
• Attrition (final N): 2,000
• Age:  
   

  Quintile	Mean Age
  1	72(5.6)
  2	72.2 (5.6)
  3	71.8 (5.5)
  4	71.9 (5.4)
  5	71.6 (5.7)

  
   
• Ethnicity: Chinese
• Other relevant demographics:
  
   

  Quintile	% Female
  1	52.9
  2	44.1
  3	47.4
  4	58.4
  5	65.2

• Anthropometrics:
  
  

  Quintile	% ε4 Carrier	% No ε4	Selenium Intake µg/day (SD)	Blood Selenium ng/ml (SD)
  1	14.5	85.5	10.4 (5)	56.7 (19.6)
  2	18.7	81.3	14.1 (10.9)	107.8 (36.7)
  3	18.7	81.3	18.6 (12.7)	119.3 (21.9)
  4	17.5	82.5	25.2 (19)	132.2 (32.7)
  5	13.3	86.7	39.4 (28.6)	140.2 (26.7)

  
   
• Location: 
  • Qionglai Sichuan Province
  • Gaomi Shandong Province 
  • Jiange, Sichuan Province 
  • Zichuan, Shandong Province.

Key Findings

Selenium Distribution and Correlation in Selenium Measures 

• Nail selenium levels were significantly correlated with the selenium levels measured in the blood (r=0.60, P<0.0001)
• The selenium intake derived from food frequency questionnaires and selenium measures of local food samples also correlated significantly with selenium measures in nail samples (r=0.51, P<0.0001) and in blood samples (r=0.46, P,0.0001)
• Vitamin E was also measured in the blood samples, but it was not correlated with the blood selenium level (r= -0.04, P=0.552).

Factors Associated With Cognitive Function

All scores except those of the Animal Fluency Test showed significant differences by selenium quintiles:

• Using the composite z score as the outcome variable, it was found that increasing age, female gender, no school attendance, non-drinkers, non-smokers, BMI and lower diastolic blood pressure were univariately associated with lower cognitive function
• No significant correlation among individuals within the same site was detected by use of mixed-effect models with each cognitive score as an outcome variable (P>0.1143 for all cognitive scores)
• Subsequent analysis using all significant variables found that marital status, household composition, alcohol, cancer, hypertension and heart attack were not associated with any of the cognitive scores
• Selenium levels accounted for the following variances in test scores:

• APOE ε4 carriers had significantly lower CSID (P=0.0135) and IU Token Test (P=0.0251) scores
• Increasing age, female gender, no education and lower body mass index were significantly associated with lower cognitive scores in all models
• Increasing selenium quintiles were associated with better cognitive scores except those form the Animal Fluency Test
• Estimated difference in CSID scores between participants in the highest and lowest quintiles in nail selenium levels is 0.54 (standard deviation), while the effect of an increase of 10 years in age on the CSID score was estimated to be 0.45 (standard deviation). Similar results were obtained after excluding subjects who reported having cancer, stroke and heart attack from the analysis
• Significant positive associations were found between selenium intake and cognitive scores (P<0.0001 for all scores) after adjustment for age, gender, education, smoking, BMI, cancer and APOE genotype
• When the same models were conducted in the 200 individuals with blood samples, decreasing blood selenium levels were significantly associated with lower CSID scores (P<0.0001), lower IU Token Test scores (P=0.238) and marginally associated with lower composite z scores (P=0.0603).

In this study, decreasing selenium levels measured in nail samples were associated with lower cognitive scores when controlling for age, gender, education, BMI and APOE status. The effect of the lowest selenium quintile compared with the highest quintile on the CSID score was equivalent to an increase in 10 years in age in this cohort. The stability of the rural population and the high correlations among different selenium measurements suggests that the results reflect the lifelong selenium exposure on cognitive function.

In this population, selenium had a consistent, dose-response relation with cognitive performance, such that higher selenium levels were associated with better cognition. The largest effects were seen for the CSID, which is a multifactorial cognitive screening test and the IU Token Test which measures working memory and language comprehension. The CERAD Word List Learning Test task and the IU Story Recall Test were also related to selenium levels.

Selenium is recognized as an important dietary micronutrient in humans and is hypothesized to impact the aging process. In animal studies, selenium deficiency has been shown to increase protein oxidation in mice and to shorten the life span in transgenic Drosphila. Selenium's effect on aging has also been investigated in term of DNA damage. Animal studies also show that the brain has a unique feature in that it stores selenium. This suggests that long-term exposure to selenium may be needed to impact brain function later in life. Because the majority of participants were lifelong residents of the same towns and villages, selenium measures in the participants reflect lifelong exposure.

The effect of APOE in Alzheimer's disease and cognitive function has been of particular interest in Asian populations because the frequency of ε4 is lower in these populations than in most but not all of European and North American populations. Significantly lower cognitive performance in ε4 carriers was found in the CSID and the IU Token Test scores in this study. Although the ε4 allele has been reported to be associated with memory-dominated functions, the associations have been inconsistent about the impact on different brain regions and functions.

In this study, a lower BMI was associated with lower cognitive scores. In past studies, the relationship between BMI and cognitive function or the risk of Alzheimer's disease have been inconsistent. These inconsistencies may be due to the variation in time lapse between BMI measurements and outcome measures in various studies.

The authors note the following strengths:

• Selenium levels were measured in nail samples, dietary intakes and blood samples, increasing measurement validity
• The study design ensured an extensive range of selenium exposure in the cohort
• The majority of the study participants were lifelong residents of the towns where they were interviewed and the participants were known not to take vitamin supplements. 

The authors note the following limitations:

• The reported association was found in a cross-sectional examination of selenium levels and cognitive function
• Although, the stability of this population makes a reciprocal effect of low cognitive function on selenium levels unlikely, longitudinal evaluation of the cohort will help to establish whether selenium levels affect the rate of cognitive decline associated with aging.