September 6, 2011 — Cutting back on sodium might help older adults maintain their cognitive function, particularly in those who aren't physically active.
In a study of more than 1200 older adults with normal cognitive function at the outset, researchers found that a high intake of sodium combined with low levels of physical activity was associated with a decline in global cognitive function over 3 years.
"Importantly, this association was independent of hypertension and global diet quality," the researchers say. "The independent effect of sodium intake from other nutrient intakes, including energy and lipids, suggests that sodium intake alone may affect cognitive function in sedentary older adults above and beyond the effects of overall diet," they note.
The study is published online August 19 in the Neurobiology of Aging.
In comments to Medscape Medical News, first author Alexandra J. Fiocco, PhD, assistant professor, Department of Psychology, Ryerson University, Toronto, Ontario, said that "clinicians should ensure that their patients' sodium intake falls below the recommended level (maximum 2300 mg/day)."
Further, she said the study results suggest that it is important to focus on multiple lifestyle domains, such as exercise and diet, instead of singling out one factor when creating health-promotion strategies in the clinic.
David L. Katz, MD, MPH, FACPM, FACP, director of the Yale University Prevention Research Center in New Haven, Connecticut, who was not involved in the study, said that "the interaction between sodium and physical activity cited here is certainly novel."
"The basic notion," he told Medscape Medical News, "is that higher sodium intake is potentially injurious to the (aging) brain, and physical activity is protective. The effects of physical activity seem to win out, so that habitual, moderate exercise may essentially 'immunize' the brain against adverse effects of higher sodium intake. However, the combination of more physical activity and less sodium is clearly better than either alone — and far better than neither," Dr. Katz added.
The well-established negative impact that high sodium intake has on cardiovascular health has led to the development of worldwide population salt-reduction strategies. Given the link between cardiovascular factors, such as hypertension, and brain health, Dr. Fiocco's team wanted to examine the effects of sodium intake on cognitive function.
Given the plethora of research associating exercise and brain health, they decided to look at the interaction between sodium intake and physical activity on cognitive function and change over time.
They studied 1262 older adults participating in the Quebec Longitudinal Study on Nutrition and Successful Aging (NuAge). At recruitment, participants were between 67 and 84 years of age, cognitively normal, and living independently.
A validated semiquantitative food frequency questionnaire was used to estimate energy and nutrient intake, including daily sodium intake. The Modified Mini-Mental State Examination (3MS) was administered annually, and the Physical Activity Scale for the Elderly was used to assess physical activity.
Of the 1262 subjects, 420 fell into the low sodium intake tertile (median, 1781 mg/day), 421 fell into the middle tertile group (median, 2648 mg/day), and the remaining 421 made up the high sodium tertile group (median, 3919 mg/day).
Level of Physical Activity Key
After controlling for age, sex, education, waist circumference, diabetes, and overall diet, there was an association between sodium intake and cognitive change over time in those with low levels of physical activity.
In the low physical activity group, those with low sodium intake displayed better cognitive performance over time than those with medium and high levels of sodium intake. The mean decline in 3MS for the low, mid, and high sodium tertile groups was 0.57, 2.07 and 1.72, respectively.
The findings remained unchanged after additional adjustment for intakes of energy, calcium, cholesterol, and total lipids, and total Canadian Healthy Eating Index score, the researchers say.
They failed to see an association between sodium intake and cognitive health among the highly physically active adults. "One potential explanation for this finding is that the impact of physical activity outweighs the impact of sodium intake on cognitive function, making it more difficult to find an association," Dr. Fiocco said.
There was also no significant association between sodium intake and cognitive function at baseline in either the high or low physical activity groups.
"A possible explanation for this null effect is selection of the sample," they explain. "At time of recruitment," they point out, "participants had to be in good general health with no subsequent exclusions for health reasons, be free of disabilities in activities of daily living, and show no signs of cognitive impairment. This relatively high health status of the group may have diminished the sensitivity of the associations at baseline."
Dr. Fiocco and colleagues say it is important to note that people who experienced a decline in global cognitive function over the study period "displayed normal age-related decline and did not display clinically significant rates of decline, defined as a 5-point decline per year on the 3MS."
According to previous research, a potential mechanism underlying the association between sodium intake and cognition is blood pressure levels, which are associated with white matter lesions observed in dementia patients, the investigators note.
"However, the present findings do not support this hypothesis," they note, "due to the independent effect of sodium intake on cognitive function from hypertension."
"Although a cohort effect may be speculated, another explanation is that sodium intake impacts brain health via alternative pathways, such as compromising integrity of the blood–brain barrier or via function of the hypothalamic paraventricular nucleus," they add.
Animal studies, they point out, have shown that increases in levels of sodium in the brain affect cardiovascular and renal function, which is mediated by the paraventricular nucleus.
Additional studies, the researchers say, are needed to delineate underlying mechanisms at play in the link between sodium intake and cognitive function.
"The provisos about this study," Dr. Katz noted, "are that it is observational, citing an association between sodium and cognitive decline, not actually testing cause and effect, and that it is limited to a particular demographic and of fairly short duration."
"The authors," he told Medscape Medical News, "are suitably humble about study limitations, while providing convincing, potential mechanistic explanations for their findings.
"There is a theoretical basis for these findings to be valid, further substantiated by the mechanisms on which the authors expound."
The study authors and Dr. Katz have disclosed no relevant financial relationships.
Neurobiol Aging 2011. Published online August 19, 2011. Abstract
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