Killing the starvelings

In my trib to the late lamented Hans Rosling, I mentioned his pioneering research on a weird East Africa myopathy called konzo. In that case, the symptoms were brought on by eating too much cassava Manihot esculenta and getting cyanide poisoning. In the news last week they ran a story about another way in which desperately hungry kids, who can't wait, get wasted.

Lychee Litchi chinensis is a fruit originally from Guangdong which you find on the dessert menu in Indian restaurants. It has been cultivated in China for at least 1000 years and its delicate aroma and sweet taste have seen it eaten with gusto by emperors and peasants. The trouble is that lychees don't want to be eaten, especially not eaten ripe, and so they mount a chemical offensive against vertebrates, including us, who want to scarf down a sackful. What's been happening over the last 30 years is a series of local (parts of India and Vietnam) epidemics of a non-inflammatory encephalopathy. So it's not meningitis, which is good, but nevertheless it's been killing about 100 poor Indian children every year just before the rains come. For a long time, nobody could figure out the cause, and suggested various contacts with rat or bat bodily fluids and an uncharacterised virus.

But, as with konzo and the toxins in cassava, epidemiologists are now convinced that it is due to eating too many unripe lychees on an empty stomach. The active principle in lychee is hypoglycin A a peculiar amino acid with a cyclopropanyl group in the side chain. It is relatively innocuous in itself but gets processed as if it was a normal long-chain amino acid into an active principal which a) inhibits the production of glucose [hence the name] but also b) blocks the degradation of fats, which is an alternative source of energy in the absence of glucose. When a hungry youngster raids a lychee orchard and fills up with fallen fruit, he is liable to refuse dinner and this also lowers circulating glucose. These kids are living on a metabolic knife-edge anyway and reduced sugar means that the brain is starved until convulsions and loss of consciousness ensues. Like with the Ames test, sometimes the liver's normal function can process cause something to become toxic. Similarly, sometimes the process of inflammation, normal a Good Thing, goes over-the-top and kills the young and fit as with the 1918 'flu epidemic. The cure for lychee sickness can be addressed on several levels: don't eat unripe lychees; have a mixed diet; don't be poor.

Some observers were sharp enough to note the similarity with Jamaican vomiting sickness, which occurs a long way from India but among children of a similar demographic. Lychee is a member of the family Sapinaceae, which includes such exotics as ackee Blighia sapida and soap-berry Sapindus mukorossi (from which the family name) but also all the maples Acer spp. and the horse chestnut Aesculus hippocastanum.  Ackee fruit also contain hypoglycin A, especially when unripe and can precipitate the same hypoglycemic encephalopathy. The advice above about avoiding unripe lychees can therefore be generalised to other related species . . . but don't go mad in your prohibitions: maple syrup is still fine. You probably know that horse chestnuts are mildly toxic but that's nothing to do with hypoglycin A: that species has a load of saponins which, among other effects, can punch holes in the membrane of red blood cells. We've seen before that chestnuts were key to the production of acetone in WWI; and in the way of unforeseen consequences, this led to the formation of the state of Israel.