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Two studies evaluate proposed link between Mediterranean diet, late-life cognitive function, AD risk

Wednesday, August 19 2009 | Comments

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What's This? In an attempt to replicate and expand upon 2006 findings linking higher adherence to a Mediterranean-type diet with a reduced risk of Alzheimer's disease (AD) and mild cognitive impairment, 2 new analyses of prospective cohort studies add to what is known about this association. According to Dr. David Knopman with the Mayo Clinic in Rochester, Minnesota, the 3 investigations collectively provide "moderately compelling evidence" that adherence to such a diet reduces the risk of late-life cognitive impairment.

The first of the 2 recently published analyses was conducted in the same cohort of Medicare beneficiaries evaluated in the 2006 investigation and included data for 1,880 community-dwelling Medicare beneficiaries residing in northern Manhattan. Information about diet and exercise was coupled with data from neurological and neuropsychological examinations administered periodically from 1992 through 2006 (mean follow-up, 5.4 years).

In a model that simultaneously evaluated the effects of dietary adherence and physical activity level on the incidence of AD, greater adherence to a Mediterranean diet and higher levels of physical activity were both independently associated with a reduced risk of AD.

Specifically, the hazard ratio (HR) for AD in participants with high adherence to a Mediterranean-type diet (relative to those with low adherence) was 0.60 (95% CI, 0.42-0.87; P=.007) in a fully adjusted model. The corresponding HR for AD in participants reporting high levels of physical activity (vs those reporting no physical activity) was 0.67; 95% CI, 0.47-0.95; P=.02). (Scarmeas N, et al. JAMA 2009;302:627-637.)

The second study, which was conducted in Bordeaux, France, evaluated the effects of a Mediterranean diet on changes in cognitive performance and risk of dementia in 1,410 adults aged 65 years or older.

The main outcome measures were scores on 4 neuropsychological tests: the Mini-Mental State Examination (MMSE), the Isaacs Set Test (IST; a measure of semantic verbal fluency and speed of verbal reproduction), the Benton Visual Retention Test (BVRT; a measure of immediate visual memory), and the Free and Cued Selective Reminding Test (FCSRT; a measure of verbal episodic memory).

When adherence to a Mediterranean-type diet was evaluated as a continuous variable, greater adherence to a Mediterranean-type diet was associated with fewer MMSE errors during follow-up assessment (median follow-up, 4.1 years) in an adjusted analysis (P=.04). However, there was no significant association between adherence and MMSE scores when adherence was modeled as a categorical value.

In addition, there were no significant associations between dietary adherence and IST, BVRT, or FSRT scores in adjusted analyses, nor was there a significant association between dietary adherence and incident dementia (HR, 1.12; 95% CI, 0.60-2.10; P=.72), although the authors acknowledged that the study was not sufficiently powered to detect the latter.

Sensitivity analyses of the patients who did not develop dementia during follow-up suggested a potential association between dietary adherence and FCSRT score during follow-up, as well as an association between adherence and MMSE score. (Feart C, et al. JAMA 2009;302:638-648.)

In an accompanying editorial, Dr. Knopman highlighted strengths and weaknesses of the investigations. He noted that--although the first analysis supported the findings from the 2006 investigation--these 2 studies shared a cohort, which prompts the question of whether findings from both studies can be generalized to other study populations.

Dr. Knopman added that, although the second analysis failed to show an association between adherence to a Mediterranean-type diet and cognitive decline, the IST and the BVRT might not have been ideal outcome measures for this purpose.

In addition, he cautioned, both studies controlled for a number of conditions and risk factors present among the participants in late life, but neither controlled for a lifetime history of vascular risk factors.

"The scientific value of these studies cannot be disputed, but whether and how they can or should be translated into recommendations for the public is the question. For now, it is reasonable to nibble on these findings and savor them, but not to swallow them whole," Dr. Knopman concluded. (JAMA 2009;302:686-687.)

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