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. 2017 Oct 16;17(1):239.
doi: 10.1186/s12877-017-0639-5.

Low plasma tryptophan is associated with olfactory function in healthy elderly community dwellers in Japan

Yusuke Adachi  1 , Yoshiki Shimodaira  2 , Hidehiro Nakamura  2 , Akira Imaizumi  2 , Maiko Mori  2 , Yoko Kageyama  2 , Yasushi Noguchi  2 , Asuka Seki  2 , Yuki Okabe  2 , Yuko Miyake  2 , Kaori Ono  2 , Shu Kumagai  3
Affiliations

Low plasma tryptophan is associated with olfactory function in healthy elderly community dwellers in Japan

Yusuke Adachi et al. BMC Geriatr. .

Abstract

Background: Decreased circulating tryptophan (Trp) levels are frequently observed in elderly patients with neurodegenerative disease including Alzheimer's disease. Trp may serve as a potential biomarker for monitoring disease risk in elderly people. We aimed to investigate the association between low plasma Trp levels and olfactory function, which is known to predict age-related diseases including dementia in elderly people.

Methods: A total of 144 healthy elderly Japanese community (≥ 65 years old) dwellers from the Health, Aging and Nutritional Improvement study (HANI study) were the subjects of our analysis. Low Trp levels were classified using the lower limit values of the reference interval according to a previous report. Olfactory function was assessed using a card-type test called Open Essence, which includes 12 odour items that are familiar to Japanese people. The elderly subjects with low circulating Trp levels were compared to a control group with normal plasma Trp levels.

Results: We conducted the analyses using 144 people aged 65 years or older (mean age 73.7 ± 5.5 years; 36.1% men). The subjects showed normal serum albumin levels (4.4 ± 0.2 g/dL) and no daily living disabilities. Low plasma Trp levels (low Trp group) were found in 11.1% of the study population. The low Trp group showed a significantly lower correct-answer rate for the items india ink, perfume, curry and sweaty smelling socks than control group (P < 0.05). There was also a significant association between low Trp levels and low olfactory ability, after adjusting for age and sex.

Conclusions: Lower plasma Trp levels were associated with a decrease in olfactory function in functionally competent older individuals. Because olfactory dysfunction predicts age-related diseases, low plasma Trp levels may represent a clinical sign of disease risk in elderly people.

Keywords: Low plasma tryptophan; Olfactory function; Plasma amino acid; Tryptophan.

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Conflict of interest statement

Ethics approval and consent to participate

This study was conducted in accordance with the Declaration of Helsinki, and the study protocol was approved by the Ethics Committees of the University of Human Arts and Sciences (Saitama, Japan) and Ajinomoto Co., Inc. (Kanagawa, Japan). Written informed consent was obtained from all patients.

Consent for publication

Not applicable

Competing interests

Yusuke Adachi, Yoshiki Shimodaira, Hidehiro Nakamura, Maiko Mori, Yasushi Noguchi, Yuki Okabe, Asuka Seki, Yuko Miyake, and Kaori Ono are employees of Ajinomoto Co., Inc. The authors declare that they have no competing interests.

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