Traditional Food Items in Ogimi, Okinawa: l-Serine Content and the Potential for Neuroprotection
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- Cox, P.A. & Metcalf, J.S. Curr Nutr Rep (2017) 6: 24. doi:10.1007/s13668-017-0191-0
Purpose of Review
Ogimi village is renowned for its aging population. We sought to determine if the l-serine content of their diet could account for their neurological health.
The most frequently consumed food items, including tofu and seaweeds, are rich in the dietary amino acid l-serine. l-serine content of the Ogimi diet >8 grams/day for Ogimi women significantly exceeds the average American dietary intake of 2.5 grams/day for women >70 years old.
Our hypothesis that the high l-serine content of the Ogimi diet is related to the paucity of tangle diseases among villagers is buttressed by in vivo results with non-human primates where dietary l-serine slowed development of neurofibrillary tangles and β-amyloid plaques by up to 85% and a human clinical trial finding that l-serine at 15 grams/day twice daily slows functional decline in ALS patients. Analysis of the Ogimi diet suggests that l-serine should be evaluated for therapeutic potential as a neuroprotective agent.
Keywordsl-Serine Dietary amino acids Tangle diseases Alzheimer’s disease ALS Neuroprotection
The population of Okinawa has achieved considerable fame for the overall longevity of its population, with Okinawan women having an average life expectancy of 85.08 years, exceeding the average Japanese women’s life expectancy of 83.99 years [1, 2]. Within Okinawa, the remote Ogimi village at the extreme north of the island has achieved the title of “longevity village” for its high number of centenarians. Despite considerable attention having been given to the Okinawan diet , there has not been an ethnobotanical survey performed for Ogimi village regarding dietary preferences.
Within the Okinawan political context, Ogimi village is a clear outlier due to its remoteness and its cultural isolation. Remarkably, the indigenous people in Ogimi have maintained their own language which is not understood by other Okinawans. Ogimi villagers learn Japanese in school, but maintain a distinct culture and religion.
The Ogimi diet has unique elements based on marine algae. Ogimi villagers who move to the Okinawan capital of Naha ask family members to supply them with those seaweeds. To explore a possible neuroprotective function for l-serine, we surveyed the Ogimi diet and analyzed the most frequently consumed items for l-serine content.
l-Serine as a Potential Neuroprotective Agent
l-Serine is abundant in human diets. The mean concentration in blood plasma is 0.12 mM and is not reduced in elderly Okinawans , while in intracellular tissue the mean is 0.98 mM . l-Serine is produced within the central nervous system  and is essential for cell cultures and cell proliferation . l-Serine, a precursor for l-glycine, is produced from 3-phosphoglycerate. Transport of l-serine across the blood-brain barrier is via the sodium-dependent system A and the sodium-independent alanine, serine, and cysteine preferring system (ASC) [8, 9, 10]. Made by astrocytes and neurons [11, 12], l-serine is a precursor to phosphoglycerols, sphingolipids, sulfur amino acids, and d-serine which serves as a cofactor at NMDA receptors. It is enzymatically degraded by serine dehydratase, with NH3 and pyruvate as the end nitrogen and carbon metabolites, respectively .
A deficiency in l-serine can trigger the genetic disease 3-phosphoglycerate dehydrogenase deficiency and 3-phosphoserine phosphatase deficiency . l-Serine is currently in a phase I clinical trial as a treatment for hereditary sensory autonomic neuropathy type 1 .
Indications that l-serine may be neuroprotective came from studies of human neuronal cell culture in which protein aggregation and cell apoptosis was triggered by the cyanobacterial toxin β-N-methylamino-l-alanine (BMAA) . BMAA contaminated the diet of the Chamorro people of Guam . Increasing l-serine in neuronal cell culture prevents BMAA misincorporation [18•].
Vervet monkeys with chronic dietary exposure to BMAA developed neurofibrillary tangles and plaques immunopositive for phosphotau and β-amyloid deposits consistent with Guam ALS/PDC [19••, 20]. Based on both in vitro and in vivo indications of a neuroprotective function for l-serine, an FDA-approved phase I clinical trial on 20 ALS patients was conducted. In addition to findings of safety, a statistically significant indication of efficacy was found: patients taking l-serine at 5 and 15 g/day had a 22% reduction in the rate of functional decline, while those at 30 g/day of l-serine experienced an 85% reduction in functional decline by the slope of ALSFRS-R [21••].In January 2017, the FDA approved a Phase IIa clinical trial (IND 133607) of L-serine for Alzheimer's patients.
Given both the longevity and the apparent paucity of progressive neurodegenerative disease among the Ogimi villagers, an ethnobotanical study was designed to determine the most frequently consumed components of the Ogimi diet and to test those components for l-serine content.
The diet of Ogimi village, with a focus on the most frequently consumed food items, was studied through ethnobotanical surveys of the aged villagers in six different week-long expeditions to the village. Using participant observation techniques , villagers were initially surveyed to develop a comprehensive list of the 25 most common or highly salient food items. Villagers collecting marine algae or terrestrial plants were photographed, and the manufacture of tofu from soy beans by villagers was documented, as was the manufacturer of flour from the gametophytes of Cycas revoluta (Cycadaceae) which contains BMAA . Interviews were conducted in Japanese but plant and animal species were referred to by their Ogimi names. Photographs of these food items were printed on 8 × 10 cardstock, one photograph per sheet, with the Ogimi name written in Hiragana above the photograph. Fifty different villagers were shown each photograph and questioned as to their personal frequency of consumption and the amount they consumed. Median frequency of consumption was then calculated for each food item. Samples of each food item were analyzed for amino acid content.
Extraction of Food Items
Solid food items, obtained from Ogimi village, were placed into 50-ml plastic Falcon tubes and frozen at −80 °C. The sclerotesta from cycad seeds was removed and the gametophyte exposed before freezing in 50-ml Falcon tubes. After freeze-drying, subsamples were placed inside microcentrifuge tubes with the addition of 5–10 glass beads, then macerated using a bead beater (Omni bead beater 24, Omni International, Kennesaw, GA, USA) for 20–30 s. Once disrupted, a subsample was weighed into a 1.6-ml screw-capped glass vial. 6 M HCl was added at a concentration of 50 mg/ml. For liquid samples, 500 μl was placed into a 1.6-ml screw-capped glass vial and an equal volume of 12 M HCl was added. Samples were then hydrolyzed at 110 °C for 16 h. Once cooled, 200 μl of the hydrolysate were centrifuge filtered (0.22 μm; Millipore Ultrafree-MC, Billicore, MA, USA) and dried in a SpeedVac. Once dried, the residue was stored at −20 °C.
Analysis of Collected Materials by Amino Acid Analyzer
Dried residues were re-suspended with 200 μl of 20 mM HCl and analyzed by amino acid analyzer (Hitachi 8900). Samples (10 μl) of the re-suspended extract or a 1/10 dilution with 20 mM HCl were injected into a Hitachi Amino Acid Analyzer L8900 equipped with a Hitachi Reaction column (PN 855-3533) at 135 °C, a high-speed physiological fluid analysis analytical column (Li-form resin #2622SC 6 mm ID × 40 L 060928C; PN855-4515), precolumn (PN855-3643), guard column #2619 4 mm ID × 5 L 2007.07.19 070639 (PN855-5268), and ammonia filter column (ion exchange 4.6 × 40 column #2650 L, PN 855-3523). Pre-made buffers (Hitachi) were used as follows: (B1) PF-1/AN0-8711, (B2) PF-2/AN0-8712, (B3) PF-3/AN0-8713, (B4) PF-4/AN0-8714, (B5) 5% methanol, (B6) PF-RG/AN0-8715, (R1) ninhydrin solution, (R2) ninhydrin-buffer of lithium acetate dihydrate, (R3) 5% ethanol. Separation was made with a flow rate for pump 1 of 0.53 and 0.45 ml/min for pump 2 and a 152 min gradient elution (0.0 min = 100% B1, separation column and guard column temp 35 °C, 50% R1, 50% R2; 1.5 min = column 32 °C; 15.6 min = 81% B1, 19% B2, column 57 °C; 36 min = 60 °C; 45 min = 32 °C; 57 min = 70 °C; 68 min = 15% B1, 75% B2, 10% B3; 69 min = 58 °C; 76.1 min = 60% B2, 40% B3; 89 min = 65 °C; 95 min = 20% B2, 80% B4; 98.1 min = 25% B2, 75% B4; 112.1 min = 100% B4; 125 min = 70 °C; 125.1 min = 100% R3; 129.1 min = 100% B6; 132.1 = 100% B1; 137.1 min = 50% R1, 50% R2, 35 °C).
Amino acid concentrations were determined from retention times of individual l-amino acid standards, corresponding to the 20 canonical amino acids (Sigma Chemical Co., St. Louis, MO). Using peak area and retention time, the amino acid concentrations were determined on a gravimetric basis for solid food samples and on a volumetric basis for liquid samples.
Amino acid profiles (mg/100 g) for marine algae and other frequently consumed dietary items in Ogimi village, Okinawa
Ranked frequency of consumption of the 25 top food items in the Ogimi diet compared to rank content mg/g of l-serine
Serine (mg/100 g)
Citrus depressa juice
l-Serine-rich dietary items are consumed in religious festivals within Ogimi village, which occur several times a month. In the matriarchal society of Ogimi, women from the highlands bring C. depressa (kugani) fruits to these religious rituals, while coastal women bring marine algae. A meal is served consisting of tofu (23,510 μg/g l-serine), Saccharina japonica (kombu), and H. charoides (moi) which is boiled and mixed with bonito flakes and carrots or other vegetables, producing a gelatin-like substance called in Ogimi moi dofu. The amino acid profile of moi dofu (Fig. 1) shows that it is rich in l-serine and other amino acids.
The importance of fermented soy products, in the form of tofu, and the frequency of consumption of Glycine max as edamame, which appear to be the major source of calories in the Ogimi diet (as opposed to rice in the rest of Japan), and particularly the importance and diversity of marine algae consumed by villagers, appear to distinguish the Ogimi diet. “The Okinawan people have always included a number of different varieties of seaweed in their everyday diet and in large amounts. Konbu [sic] is particularly used, which is not locally produced, nevertheless features in both traditional festival menus and in the everyday diet” ( pp. 163–164). The import of kombu (Saccharina japonica) from Hokkaido to Okinawa in general and Ogimi in particular stretches back nearly two centuries.
It is claimed that edible seaweeds in general, and kombu and wakame (Undaria pinnatifida) in particular, are good sources of protein [36, 37, 38] and offer a complete complement of essential amino acids [39, 40]. Although seaweeds are particularly rich in glutamic acid, alanine, and phenylalanine , we here note they are also rich sources of l-serine. Hypnea charoides and Ulva lactuca have been found by previous investigators to be good sources of l-serine [42, 43]. Wakame may also serve to reduce serum and lower triacylglycerol , while the fruits of C. depressa, which is the most frequently consumed food item in Ogimi village, have clear hypolipidemic effects, which together with the antioxidants present in the fruit reduce the risk of lifestyle disease .
Food preferences and taboos embedded within indigenous religions are strong indicators of cultural importance [22, 46]. During frequent indigenous religious ceremonies in Ogimi, participants consume a meal which is extraordinarily high in l-serine. Although Ogimi villagers have not been aware either of the high amino acid content of their indigenous diet, nor of recent research suggesting that l-serine in particular has neuroprotective activity, both their religion and cultural practice ensure that they consistently receive one of the highest l-serine-containing diets in the world. The people of Ogimi are known to be long-lived. A wide range of potential factors could contribute to their longevity. Due to the unique nature of their diet, rich in seaweeds and tofu, the high serine content of their diet may offer neuroprotection and contribute to their neurological health in this community.
We thank Brain Chemistry Labs Emeritus Director Mr. Masayuki Kishimoto and his brother Mr. Masaharu Kishimoto, both of whom were raised in Okinawa, and Mr. Chiba Koetsu for facilitating and funding our research in Okinawa. The Senior Citizen Coordinator for Ogimi village Mr. Miyagi scheduled our visits with Ogimi elders and assisted with translations from the Ogimi language. Mr. Susumu Onodera provided simultaneous Japanese translation and significant cultural advice. We thank the Mayor and Council of Ogimi village for permission to conduct our interviews and all of the 50 village elders who graciously agreed to interviews. Marilyn Asay assisted in the statistical analysis and manuscript preparation.
Compliance with Ethical Standards
Conflict of Interest
The Institute for Ethnomedicine has applied for patents for the use of l-serine to treat neurodegenerative illness (US 13/683,821).
Human and Animal Rights and Informed Consent
Other than interviews, this article does not contain any studies with human or animal subjects performed by any of the authors.
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