HUGE logical problem in the movie

Such reasonable observations invite the suggestion that Altman, who's clearly no fool (nor, evidently, especially interested in making a 'conventional' Sci-Fi flick), has created a kind of dreamscape, à la 3 Women (1977), with the kind of contradictory logic that might entail. The (rather unnecessary) technique of blurring the edges of the screen image are even reminiscent of dream sequences in countless films.

The lion and the calf shall lie down together, but the calf won't get much sleep.

First, right in the beginning, when Essex and his girl are entering the city, they go through something that looks like a market (Essex has to grab Vivia a couple of times because she keeps getting distracted by the various merchandise). Well, some of the stuff sold / bought looked like fish.

But the problem is not the lack of food, indeed that doesn't play much of a role; instead, there is a sense of hopelessness that has engulfed the human community, out of several reasons perhaps, one of which was definitely the ice age & scarcity of food. Thing is that the very instinct of survival seems to have vanished, which is why people need to play with death in order to regain the meaning of life, or some part of it. The populace is growing old, there are no more kids, a general sense of carelessness has invaded the community, which is why Quintet became the only thing that mattered. Which is why those people are so stupefied when they see Essex' wife pregnancy. Which is why Deuca doesn't believe that Ambrosia slept with Essex / Redstone.

So there may have been enough food, but it didn't matter anymore. I don't know precisely where the sense of doom came from (maybe because the people knew that it was cold, and was only going to get colder, until all life would cease, so fighting for life would only bring a short delay of the unavoidable? maybe because of the isolation? maybe simply because the human race got tired of itself?), but it is all over the place, controlling everyone's actions. By rendering them not idiotic, just extremely disinterested.
So no, I don't think that a major suspension of disbelief is required.

http://tinyurl.com/BVSC-ChanChan
http://tinyurl.com/salga-la-luna

They eat Soylent Green! ;-)

Two things have happened that you are missing.

1) This was mentioned before, but humanity has entered into a stage of depression and people as a whole have lost the collective will to live.

2) Because of the eco-disaster that has plunged the world into an ice age, the population density of the human race has probably fallen too far to maintain itself. In other words, enough people have died that birth mortality factors that substantial populations like ours can ignore are now a significant factor. In short, more people are dying than are being born.

Think about it; how often do you hear about a couple having trouble conceiving a child? In a city like the size of New York and all of its boroughs, there is a population in the millions, so it is not a major factor. Now look at it in a city the size of Mineral Point, WI (just an example) with a population of 2538. Pre-natal and post natal mortality suddenly turns into a major factor.

I could go into more of the science of populations, but hopefully you get my point.