The relation of hydration status to declarative memory and working memory in older adults

Abstract

Objectives

Prior studies have suggested a relationship between dehydration and poor cognitive performance. The present study examined the relationships among hydration status, declarative memory and working memory skills, and blood pressure in a sample of older community dwelling females.

Design

Data was analyzed from a larger study; relationships among hydration status, blood pressure, and cognitive measures were assessed with correlation and meditational analyses.

Setting

Laboratory.

Participants

21 postmenopausal women (mean age 60.3, SD 8.03).

Measures

Hydration status was measured using bioelectrical impedance, baseline blood pressure was assessed using a Colin Pressmate, and cognition was examined using the Auditory Verbal Learning Test and Auditory Consonant Trigrams.

Results

Bioelectrical impedance total body water by weight was found to be related to working memory, r =.47, p =.04, and memory skills, r =.54, p =.01. Total body water by weight was also found to be related to diastolic blood pressure, r = −.56, p =.01, which in turn was related to working memory, r = −.67, p =.002, and declarative memory, r = −.57, p =.009, skills. When diastolic blood pressure was accounted for, the relationship between hydration status and cognitive skills was attenuated. A similar pattern of results was seen for systolic blood pressure, although findings did not reach statistical significance.

Conclusions

Results emphasize the importance of considering hydration status and blood pressure when interpreting cognitive performance in older adults.

Water is the basic element of our bodies and every system in the human body depends on it (1). For the average healthy adult, total body water accounts for 60-70% of total body weight (2). To keep the body functioning properly, a minimum of 2,000 ml of quality water is recommended each day (3). However, epidemiological data indicates that individuals in the United States, including those over the age of 60, do not drink enough water on a daily basis (4).

Older adults may be particularly vulnerable to mild dehydration. Total body water decreases with aging (5). Factors accounting for body water loss in aging include loss of striated muscle mass and increased adipose tissue, as well as decline in renal concentrating capacity, leading to less response to hormonal signals of fluid loss (6, 7). Aging is also associated with decreased perception of thirst, leading to delays in rehydration even under conditions such as environmental heat or physical exercise (i.e., involuntary dehydration; 6, 7). In addition, older adults often avoid fluids in the evening to prevent nocturia or incontinence and have more frequent use of diuretic medications (2, 8). There is evidence from both clinical and nonclinical samples that older adults may be chronically dehydrated (9, 10).

Disturbances in fluid balance due to severe dehydration can lead to a variety of neurological symptoms, including lethargy, confusion, delirium, dementia, seizures, and even coma (11, 12). Less is known about the cognitive effects of mild dehydration. Studies in young soldiers and endurance trained athletes, whose hydration status was experimentally manipulated, suggest mild dehydration can deleteriously affect attention, declarative memory, and psychomotor processing speed (13, 14, 15). Pilot work in our laboratories has documented similar findings, using naturally occurring hydration levels in community-dwelling older adults (16).

Another factor linked to poor cognitive performance in older adults is elevated blood pressure (i.e. hypertension), particularly in the cognitive domains of declarative learning and memory, attention, and psychomotor processing speed (17), and there is evidence that blood pressure is a mediator of the relationship between age and performance on reaction time and speed tasks (18). Possible mechanisms linking blood pressure to cognitive impairment include decreased cerebral blood flow and metabolism, cellular and neurochemical dysfunction, and, more chronically, an association with white matter disease and cerebral atrophy (19).

Interestingly, although few studies have examined the relationship of hydration status to blood pressure, there is evidence that dehydration is related to elevated blood pressure, particularly diastolic blood pressure, as well as increased resting heart rate (20). Hypohydration leads to decreased plasma volume, increased concentration of blood cells, lipids, and proteins in the blood stream and decreased blood viscosity, which, in turn, increases total peripheral resistance and, subsequently, arterial pressure. Although acute water-loading increases resting blood pressure up to 30 min after water ingestion, this may be the result of blood volume expansion rather than elevation in total peripheral resistance (21). A recent interventional study suggested that 6 months of increased fluid intake in older males resulted in lowered blood pressure for those who showed low levels of glomerular filtration rate at baseline (22). Thus, some data suggests that one mediator of the relationship between chronic hydration levels and cognitive performance might be elevations in blood pressure.

We attempted to replicate the findings from existing studies of dehydration and cognition in older adults and expand them to the cognitive domain of working memory. In addition, we explored whether blood pressure could explain, and therefore potentially mediate, the relationship between dehydration and cognition. We hypothesized that hydration status would be related to declarative memory and working memory skills. In addition, we hypothesized that the relation between hydration and cognition would be at least partially explained by diastolic blood pressure.

Methods

Participants

Participants in the present study were 21 postmenopausal women ages 50 to 78 (mean 60.3, SD 8.03) who participated in a larger study of stress and cognition (23). Participants were eligible for the study if they reported no immune or endocrine-relevant health conditions (e.g., diabetes, cancer within the previous 5 years, recent surgeries, cardiovascular disease), were non-smokers, did not drink more than 7 alcohol drinks per week, did not exercise more than 10 hours per week on average, had a body mass index<30, were not using psychotropic medication or blood pressure medication, and had hydration data available. Although the larger study included male participants, once we controlled for use of blood pressure medication, very few men were left in the sample, and the males differed significantly from the females in hydration status (p < .001), potentially complicating interpretation of results. Therefore, we opted to report only on qualifying females from the larger study.

Measures

Hydration Assessment: Total body water, extracellular body water, intracellular body water, and total body water by weight (%TBW/kg) were assessed using the Multiscan 5000 multi-frequency bioelectrical impedance monitor (Bodystat Ltd), which passes a mild electrical current (800A) across a range of frequencies (5 to 500 kHz) through the participant's body via four electrodes, two placed on the right hand and two placed on the right foot. Impedance from the 5 kHz signal is used to determine extracellular water and the 200 kHz signal is used to determine total body water. Bioelectric impedance has shown results similar to those produced by other hydration assessment methods, including hydrodensity (24) and sodium bromide and deuterium oxide isotope dilutions (25). Bioelectric impedance measures of hydration status show good 1-week test-retest reliability (26), suggesting that %TBW/kg can serve as a good estimate of naturally occurring chronic hydration status.

Cognitive Assessment: Declarative memory was assessed using the Auditory Verbal Learning Test (AVLT), a well-validated 15-word list learning and recall task (27). The words are presented 5 times to participants, who are asked to recall as much of the list as possible. Then an intrusion list follows, with one trial to recall as many words from the intrusion list as possible. After the intrusion list, participants are asked to recall as many words from the original list as possible (immediate memory), which served as the measure of interest for the present study.

Working memory was assessed using Auditory Consonant Trigrams (ACT), which was administered immediately after the immediate recall trial of the AVLT. In the ACT, participants hear a three-digit number, and are then required to count backward from a number given by the examiner until they hear a signal to stop, at which time they must correctly recall the three-digit number. Performance on this task correlates with performance on measures of attention, memory, and executive functioning, and patients with memory and executive impairment show impairments on the task (27). The measure of interest for the present study was total number correct out of 20 trials.

Blood Pressure Assessment: Systolic (SBP) and diastolic (DBP) blood pressure were measured automatically every 2 min with a Colin Pressmate 8800 automated blood pressure monitor (Colin Medical Instruments) throughout the session. The occluding cuff was placed over the upper part of the non-dominant arm, directly over the brachial artery. Average blood pressure values calculated across the last 15 min of a 30-min resting baseline period were used as the dependent variables. Of note, no one was present with the participant during the baseline period blood pressure collection data.

Procedure

After completing informed consent, height (cm) and weight (kg) measurements were recorded. A tetrapolar electrode arrangement was applied to the right hand and foot after alcohol preparation. Electrodes were placed on the dorsal surface of the right foot 2 cm proximal from the metatarsophalangeal joint of the third toe, on the anteriodorsal surface of the right ankle over the axis of the medial and lateral malleoli, on the dorsal surface of the right hand 1 cm proximal from the metacarpophalangeal joint of the third finger, and on the dorsal surface of the right wrist 7.5 cm proximal from the hand electrode. Participants then completed a 10-min supine rest period, after which they continued to lie still while bioelectrical impedance assessment of body water was taken for approximately 5 min. Following bioelectrical impedance assessment, participants were fitted with a blood pressure cuff and then sat upright for a 30-minute rest period. Blood pressure measures during the last 15 min of the rest period were averaged to provide a baseline blood pressure measure. After a brief reading task (related to the larger study protocol), the experimenter administered the cognitive assessment battery, consisting of the learning trials of the AVLT, the ACT, and then delayed recall trials of the AVLT.

Results

Age was not related to %TBW/kg, r = -.14, p = .50, DBP, r = -.16, p = .41, SBP, r = .08, p = .75, or the cognitive variables (working memory r = .12, p = .58; AVLT immediate recall r = - .02, p = .91). Therefore age was not controlled for in any subsequent analyses.

Consistent with hypotheses, better hydration was related to better performance in recall on the AVLT, r = .54, p = .01, and better working memory performance on the ACT, r = .47, p = .04. In addition, poor hydration was related to higher baseline DBP, r = -.56, p = .01 and SBP, r = -.65, p < .003. Blood pressure was related to poorer immediate recall (DBP r =-.57, p = .009, SBP r = -.46, p < .05) and poorer working memory (DBP r = -.67, p = .002, SBP r = -.64, p < .003).

Following procedures outlined by Baron & Kenny (28) to test for DBP mediation of the hydration – immediate recall relationship (see Figure 1), we first regressed DBP on hydration levels (path a), which was significant, B = -1.44, SE = .50, p = .01. We then regressed immediate recall on hydration levels (path c), which was also significant, B = .43, SE = .15, p = .01. Finally, we regressed immediate recall on both hydration and DBP; the relationship of DBP to immediate recall (path b) was significant, B = -.18, SE = .06, p = .009, while the previously significant relationship between hydration and immediate recall (path c') was no longer significant, B = .41, SE = .54, p = .46. Last, the ab pathway was tested for mediation using the multiplicative Goodman test as an unbiased test of the mediation effect (29, 30) and was significant, z = 2.09, p = .03. A similar model was run for SBP, which showed significant paths for a (p = .003) and c (p = .01), but not for b (p = .58), and the ab pathway for mediation was not significant, z = -1.69, p = .09.

Figure 1.

Mediational Model for Working Memory (unstandardized betas shown), ∗ p <.05, ∗∗ p < .01, ∗∗∗ p < .005

To assess for DBP mediation of the hydration – working memory relationship (see Figure 2), we first regressed DBP on hydration levels (path a), which was significant, B = -1.44, SE = .50, p = .01. We also regressed working memory on hydration levels (path c), which was also significant B = 1.12, SE = .50, p = .04. Finally, we regressed working memory on both hydration levels and DBP; the relation of DBP to working memory (path b) was significant, B = -.64, SE = .17, p = .002), and the previously significant relationship between hydration levels and working memory (path c') was no longer significant B = .41, SE = .54, p = .46). The test of the ab pathway for mediation using the Goodman test was significant, z = 2.29, p = .02. A similar model was run for SBP, which showed significant paths for a (p = .003), c (p = .04), and b (p = .04), and with the significant c path losing significance with SBP in the model (p = .63); however, the test of the ab pathway for mediation did not quite reach significance, z = 1.91, p = .056.

Figure 2.

Mediational Model for Immediate Recall (unstandardized betas shown), ∗ p <.05, ∗∗ p < .01, ∗∗∗ p < .005

Discussion

Overall, findings are consistent with prior studies of the relationship of mild dehydration to cognitive performance. In this sample of older community-dwelling females, poor hydration status as measured by bioimpedence was related to worse immediate recall on a word list and to poorer working memory. In addition, findings suggest that DBP mediates the relationship of naturally occurring chronic dehydration to both declarative memory and working memory performance. Poorer hydration was related to higher DBP, which in turn was related to poorer immediate recall of the word list and worse working memory; accounting for DBP significantly minimized the relation between hydration and cognitive ability. Although a similar pattern of findings emerged for SBP, the relationships were not quite as strong as in the DBP models.

Results emphasize the importance of considering hydration status and blood pressure when interpreting cognitive performance in older adults. However, there are several reasons for the lack of basic and applied research assessing the effects of hydration status on cognitive performance, ranging from expensive and time consuming assessment that requires technical skills (i.e., radioactive isotope dilutions; 25) to quick and easy to use techniques that lack in accuracy or specificity (i.e., urine color; 30). The bioelectrical impedance technique affords a fast, simple, valid, and reliable method for evaluating individual differences in extracellular, intracellular, and total body water, all of which are of growing importance in behavioral medicine research.

One limitation of the present study was that hydration and blood pressure were assessed in the same session, which did not allow for definitive conclusions regarding possible mechanistic relationships among the variables of interest. Future prospective studies should include repeated measurement of hydration, blood pressure, and cognitive status over time to better determine the nature of the relationships suggested by our findings. Future studies could also manipulate fluid intake over 1 to 2 weeks to determine whether chronic hydration enhancement leads to improvements in blood pressure and/or cognitive performance.

Other study limitations include the use of only female participants who were relatively healthy and well-educated, and the relatively small sample size. For example, prior studies have found increased blood pressure and decreased hydration with increased age, which we may not have seen due to the many health-related exclusionary criteria utilized in our study. However, findings of even mild to moderate relationships between hydration and cognition in such a healthy sample may suggest stronger implications for individuals with age-related diseases characterized by dehydration, such as diabetes, or who chronically experience cognitive, physical, or environmental barriers to adequate hydration.

Financial disclosure: None of the authors had any financial interest or support for this paper.