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Differential Relations Between Cognition and ¹⁵N Isotopic Content of Hair in Elderly People With Dementia and Controls

^a OPTIMA, Radcliffe Infirmary, Oxford, United Kingdom
^b Research Laboratory for Archaeology, University of Oxford, United Kingdom

Jonathan H. Williams, OPTIMA, Radcliffe Infirmary, Woodstock Road, Oxford OX2 6HE, United Kingdom E-mail: jonathan.williams{at}pharmacology.ox.ac.uk.

Background. Previous researchers have suggested that a vegetarian diet or one rich in fish may protect against Alzheimer's disease (AD). However, assessing diet is difficult in AD patients. ¹⁵N:¹⁴N isotopic ratios (¹⁵N) of body proteins can estimate long-term dietary habits in a way that does not depend on memory. ¹⁵N is high in people who eat a lot of fish and low in vegetarians.

Methods. To choose between the vegetarian and fish hypotheses of AD, we compared dietary questionnaire reports and ¹⁵N of hair samples from AD patients and controls.

Results. Patients' cognitive scores related directly to reported frequency of eating fish and to hair ¹⁵N_AIR, but inversely to reported frequency of eating beans. Homocysteine levels related inversely to hair ¹⁵N_AIR in controls, but not in patients. Dietary questionnaire reports accounted for slightly more variance in ¹⁵N_AIR in patients than controls. Therefore, our questionnaire assessed dietary habits as reliably for individuals with AD as for cognitively unimpaired controls.

Conclusions. A diet rich in fish may ameliorate AD, possibly by lowering homocysteine, but more vegetarian diets do not. In fact, eating beans correlated with worse cognition in AD patients. Further studies should test if restricting the intake of beans slows the progression of AD.