120px|right'''β-methylamino L-alanine''', or '''BMAA''', is a neurotoxin found in the seeds of the cycad. This non-proteinogenic amino acid (very similar to the non-essential amino acid alanine) is produced by cyanobacteria of the genus ''Nostoc'' that live on the plant's roots.

==Neurotoxicity==
BMAA is considered a possible cause of the Chamorro people of ''lytico-bodig''.<ref name=Miller>Miller, G. July 2006. [http://dx.doi.org/10.1126/science.313.5786.428 Guam's Deadly Stalker: On the Loose Worldwide?] ''Science'' '''28''' (313): 428-431.</ref>

===Sources of BMAA===
In the 1950s, ALS/PDC prevalence ratios and death rates for Chamorro residents of Guam and Rota were 50&ndash;100 times that of developed countries, including the United States. No demonstrable heritable or viral factors were found for the disease, and a subsequent decline of ALS/PDC after 1955 on Guam, led to the search for responsible environmental agents. The use of cycad (''Cycas circinalis'') seeds in food and traditional medicine had decreased as the Chamorro population became more Americanized following World War II.<ref name=Spencer>Spencer, P.S., P.B. Nunn, J. Hugon, A.C. Ludolph, S.M. Ross, D.N. Roy, and R.C. Robertson July 1987. Guam amyotrophic lateral sclerosis-parkinsonism-dementia linked to a plant excitant neurotoxin. ''Science'' '''31''' (237): 517-522.</ref>

In addition to eating the seeds directly, BMAA may have found its way into human diets by way of biomagnification. Fruit bats, a Chamorro delicacy, may feed on cycad seeds and concentrate the toxin in their flesh. Three museum specimen bats, collected in Guam in the 1950s, contained hundreds of times more BMAA, gram for gram, than cycad seeds.<ref name=Miller />

===BMAA neurotoxic effects===
Degenerative locomotor diseases had been described in animals grazing on cycad species, fueling interest in a possible link between the plant and the etiology of ALS/PDC . Subsequent laboratory investigations discovered the presence of BMAA. BMAA induced severe neurotoxicity in rhesus macaques, including:<ref name=Spencer />
*limb muscle atrophy
*nonreactive degeneration of anterior horn cells
*degeneration and partial loss of pyramidal neurons of the motor cortex
*behavioral dysfunction
*conduction deficits in the central motor pathway
*neuropathological changes of motor cortex Betz cells

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'''β-甲氨基-L-丙氨酸'''，（英文'''β-methylamino L-alanine'''，簡稱'''BMAA'''），是一種神經毒素，能在蘇鐵科植物種子中找到。這一種非蛋白胺基酸，與非必要胺基酸丙氨酸（alanine）十分相似，是由念珠藻屬的藍藻產生，而這些藍藻大都生長於植物的根部。

==神經毒性==
根據一些研究，BMAA 被視為導致「帕金徵群癡呆綜合症」（''ALS/PDC''）<ref>注：ALS/PDC 的英文全名是「''Amyotrophic lateral sclerosis / Parkinsonism dementia complex of Guam''」</ref>的可能成因，在關島的查莫羅人罹患這症狀的機率十分高<ref>Cox PA, Banack SA, Murch SJ, Rasmussen U, Tien G, Bidigare RR, Metcalf JS, Morrison LF, Codd GA, Bergman B. Diverse taxa of cyanobacteria produce β-N-methylamino-L-alanine, a neurotoxic amino acid. ''Proceedings of the National Academy of Sciences of the United States of America'' '''2005''', ''102'', 5074-5078. [http://dx.doi.org/10.1073/pnas.0501526102]</ref>，當地人稱之''Lytico-boding''，「Lytico」意指肌肉萎縮，「Boding」則是肌肉僵硬的意思。<ref name="Miller">Greg Miller. Guam's Deadly Stalker: On the Loose Worldwide? ''Science'' '''July 2006''', ''28'' (313), 428-431. [http://dx.doi.org/10.1126/science.313.5786.428]</ref>香港中文大學生物化學系副教授陳竟明指，人體會從食物吸收蛋白胺基酸，代謝物會被排出體外，但神經細胞可能未能識別胺基酸的代謝物BMAA，遺留體內而損害神經系統。<ref name="ming">[http://www.mingpaonews.com/20070130/gba1.htm 髮菜無益 調查指三成假貨真貨含神經毒素, 香港明報, 30/1/07]</ref> 但中大腦神經科主任黃家星指，尚未確定BMAA對人類有害，難以估計安全的食用量。<ref>''中大引述文獻：食髮菜會癡呆'', 香港《蘋果日報》,30/1/07</ref>

===BMAA的來源===
1950年代，在羅塔島和關島查莫羅人，ALS/PDC的患病率和死亡率，皆高於已發展國家的50至100倍，包括美國。當時亦不能確實証明，是遺傳和病毒引致居物發病，但1955年後，關島患ALS/PDC的比率卻不明地減少，這引起學者和環保組織的關注和調查。研究發現查莫羅人以蘇鐵科植物種子，來製造傳統藥物，但因為第二次世界大戰後，關島由美國統治，引進現代化的醫療藥物，令查莫羅人降低對傳統藥物的依賴，間接使居民減少吸收蘇鐵種子中的BMAA。<ref name="Spencer">PS Spencer, PB Nunn, J Hugon, AC Ludolph, SM Ross, DN Roy, and RC Robertson. Guam amyotrophic lateral sclerosis-parkinsonism-dementia linked to a plant excitant neurotoxin. ''Science'' '''July 1987''', ''31'' (237), 517-522.</ref>

其後的研究指，BMAA能夠透過狐蝠的風俗，而狐蝠則以蘇鐵種子為主要食糧，故BMAA大量累積在狐蝠體內，食用至一定份量便會產生毒性。1950年代在關島收集的蝙蝠樣本顯示，蝙蝠體內含有的BMAA，比每克的蘇鐵種子高出數百倍。<ref name="Miller" />

===BMAA的神經毒性效應===
一些動物食用蘇鐵科植物而出現的機能退化，令植物和ALS/PDC病源的可能關聯得以肯定，其後終在實驗室發現BMAA的存在。而BMAA就在恆河猴（''rhesus macaques''）身上產生強烈的毒性，症狀包括<ref name="Spencer" />：

*四肢肌肉萎縮。
*脊髓的前角細胞產生非反應性退化。
*大腦皮質和錐體細胞（''pyramidal neuron''）退化或流失。
*皮質的貝茲細胞（''Betz cell''）產生神經病理學上的異變。
*中樞傳導路徑（''central motor pathway''）的傳導能力不足。
*行為機能障礙。