2.1 Study population:
The study population of the present study was selected from the participants of the Northern Finland Birth Cohort 1966 (NFBC1966) [29]. A total of 12,055 mothers with expected delivery dates between 1 January and 31 December 1966 from Oulu and Lapland (the two northernmost provinces of Finland) had been recruited for the study. Originally 12,231 males and females whose expected year of birth was 1966 were included in this study. The follow-ups were conducted at 1, 14, 31, and 46 years of age [30]. Data from the 31-year and 46-year follow-ups were used in the present study.
Information about alcohol consumption, background factors, and lifestyle were gathered using a postal questionnaire in 1997 when the participants were 31 years old. The survey was repeated, and the Paired Associative Learning (PAL) test was conducted, at follow-up in 2012–2014 when the participants were 46 years old.
From the original study population, 5,585 participants conducted the PAL test at the age of 46 years (Fig. 1).
The cross-sectional study exploring the association between alcohol use at the age of 46 years and cognition at the age of 46 years included 4,417 participants, after excluding those with missing information on alcohol consumption (N = 1,040) and covariates (N = 128) at the age of 46 years (Fig. 1).
Longitudinal study 1 exploring the association between alcohol use at the age of 31 years and cognition at the age of 46 years included 4,819 participants after excluding those with missing information on alcohol consumption (N = 692) and covariates (N = 74) at the age of 31 years (Fig. 1).
Longitudinal study 2 exploring the association between change in alcohol use between the ages of 31 years and 46 years and cognition at the age of 46 years included 3,957 participants after excluding those with missing information on alcohol consumption (N = 1,581) at the age 31 and 46, and covariates (N = 47) at the age of 31 years.
2.2 Alcohol drinking measures:
Alcohol use was measured by questionnaire at the ages of 31 years and 46 years. The items included questions on the frequency and amount of use of different types of alcohol (mild drinks or beer, wine, and spirits).
The frequency of alcohol use was measured on a 10-point scale (1 drinking never and 10 drinking daily). Quantities consumed on a typical occasion were asked about separately for all three beverage types: mild drinks in the number of 0.33 l bottles; wine in the number of glasses (= 12 cl) or 0.75 l bottles; and spirits in the number of 4 cl shots or 0.5 l bottles. The mild drink quantity was measured on a 9-point scale (1 drinking none and 9 drinking 15 bottles or more), and for wines and spirits, the typical quantity was measured similarly.
Gram per day alcohol use by the participants was assessed by forming the following variables: Intake of total alcohol in grams of 100% alcohol per day, intake of beer in grams per day, intake of wine in grams per day, and intake of spirit in grams per day. We assumed one bottle of mild drink to contain one standard unit of alcohol (= 12 g of ethanol), wine to contain 13% alcohol, and spirits to contain 38% alcohol. From the questionnaire answers, daily consumption of ethanol was calculated by multiplying the typical frequency of use by quantity at both 31 years and 46 years.
For the frequency of drinking light drinks, wine, or spirits, the cut-off was weekly drinking (drinking less frequently than once a week vs. drinking weekly or more frequently). The cut-off point for the amount usually used when drinking for light drinks was 6 bottles (under 6 bottles vs. 6 bottles or more), and for wines, one bottle (under one bottle of wine vs. a bottle of wine or more), and for spirits six drinks (under six drinks vs. six drinks or more).
2.3 Visual memory:
The Paired Associative Learning (PAL) task from the Cambridge Neuropsychological Test Automated Battery (CANTAB) was used to assess visual memory. PAL has been found to differentiate between Alzheimer's disease and mild cognitive impairment [31]. PAL has been successfully used in different cultures and educations in both longitudinal and cross-sectional studies [32]. This study used the PAL test in particular because it can reliably assess the type and degree of functional loss and the specificity of age-related cognitive decline [33].
In the PAL test, we assessed visual memory using the primary outcome variables, ‘total errors adjusted score’ and ‘first trial memory score’. Total Errors (Adjusted) (TEA) reflects how quickly the participant learns when they have multiple attempts at each problem, while First Trial Memory Score (FTMS) reflects how many patterns the participant correctly places on the first attempt at each problem.
Both PAL TEA and PAL FTMS were used as continuous variables.
2.4 Covariates:
Covariates were selected from the 31-year follow-up data. We used educational level, marital status, diet, and physical activity as covariates.
Educational level is associated with cognitive functioning [34]. The question addressing the education of the NFBC1966 participants was: ‘What is your basic education?’ (Less than 9 years of basic school, 9 years of basic school, matriculation examination). In the analysis, we combined ‘less than 9 years of basic school’ and ‘9 years of basic school’ as ‘No matriculation examination’.
The question addressing the marital status of the NFBC1966 participants was: ‘What is your relationship status?’ (Married since, cohabiting since, single, legal separation or divorced since, widowed since). In the analysis, we combined 'married’ and ‘cohabiting’ as ‘In a relationship’, and ‘single’, ‘legal separation or divorced’ and ‘widowed’ as ‘Not in a relationship’.
Diet might be associated with cognitive functioning [35,36,37]. Diet was categorised as: ‘Healthy (Consuming vegetables, roots, and salad 3 times per week or more)’ and ‘Unhealthy’ (Consuming vegetables, roots, and salad 2 times per week or less)’.
Physical activity might [38,39,40,41] or might not [42,43] be associated with cognition. Physical activity was categorised as: ‘Active (1 hour or more brisk physical activity at a time causing at least some breathlessness and sweating)’ and ‘Inactive (Less than 1 hour of brisk physical activity at a time causing at least some breathlessness and sweating)’.
2.5 Statistical analysis:
Statistical analysis was performed using R version 1.1.456. We evaluated the association between cognition and alcohol use using two different cognition variables: PAL FTMS and PAL TEA. The association between the PAL tests was analysed using linear regression, and a β with 95% CI was reported. We considered < 0.01 p-value as statistically significant, as there were so many comparisons.
All continuous variables were normalised using z-scores. We assessed crude models and models adjusted with education, relationship status, diet, and physical activity. All analyses were conducted separately in males and females, as alcohol consumption differs between the two sexes [44] and performance in visual memory tests, including the PAL test, has also been reported to be different in males and females [45,46].
2.6 Attrition analysis:
To evaluate the representativeness of our study population, an attrition analysis was performed while studying the associations with alcohol use in the longitudinal study 1 dataset.