BEER!

Slate asks: How long could you survive on beer alone?

what will kill you is scurvy, from no vitamin c

But Archeoblog notes:

modern beer is different from beer from the past:

 Remember, however, that what we think of as beer and what really pre-industrialist people drank as beer are very different things. Staple beer was much more gruel-like and lower in alcohol than today’s recreational brews

I remember how African women in Zimbabwe brewed beer, which was grey and full of yeasty stuff.

MbuyaVaChinjanja blog discusses it here.

You heat some water in a drum and add mealie meal as if you are cooking sadza, Let it  cook for a good 4 hours. Cool the porridge down, add chimera and yeast  and leave over night. The next day you sieve, to remove the pulp/draff/dregs or maseses. Do not throw masese away because they attract a lot of flies but rather give is to the local fishmonger. Sprinkle a bit of brown sugar and people can start drinking on Saturday morning. Keep your mbira players away from this one.

Wikipedia has a more elaborate recipe (South African).

and a scholarly article on sorghum/maize beer here tells of it's high Vitamin B content and discusses the problems in it's commercial brewing.