In Interstellar, due to crop blight they can only grow corn now. But is corn really sufficient enough to fulfill the nutritional needs of the human body? All the characters seem healthy enough, how is that possible with a corn-only diet?
In a world focus solely on farming to survive, it's likely that agricultural science, and genetic engineering of any and all crops is common place. Anything that natural corn can't produce, would get modified to include it. Corn itself is already heavily modified from its natural state, by years of selective breeding. Natural corn doesn't grow such thick husks.
But two key points. One, the blight is unrealistic:
There's a fundamental tenet of plant pathology called the disease triangle. It says that for any disease to spread widely it needs three things: a susceptible host, a pathogen, and the right environmental conditions. Take any one away, and the disease won't spread.
"It's very unlikely that a single superbug would spread across our diverse planet, or even do as well in Alaska as Peru."
But still, why corn and okra? I asked. "Corn and okra are not related so it’s odd that they picked those two," Kleinhenz laughed. "Maybe they needed one familiar and one more unusual crop! But corn and okra are actually grown together all over the [real] world, and part of the reason they’re grown together is they’re not related, so they don’t support the same diseases."
And Two, the blight has not yet killed off Okra. This is the urgency that old Professor Brand talks about. That Okra is dying and soon, not yet, they will be left with just Corn. And those that don't due of starvation (or malnutrition), will suffocate eventually, in a generation or two. Meaning decades.
Brand: Blight. Wheat seven years ago. Okra this year. Now there's just corn.
Coop: And we're growing more than we ever have.
Brand: But like the potatoes in Ireland...
and the wheat in the Dust Bowl...
...the corn will die.
And right after:
Brand: Earth's atmosphere is 80 percent nitrogen. We don't even breathe nitrogen. Blight does. And as it thrives, our air gets less and less oxygen.
The last people to starve will be the first to suffocate.
And your daughter's generation... will be the last to survive on Earth.
Keep in mind 25 years Earth local time after Coop left, we start seeing pulmonary issues due to the blight. Coops grandson either has oxygen deprivation, nitrogen sickness, or complications from malnutrition. People are starting to die.
Copied from Science Fiction & Fantasy
Nutritionally speaking, the answer is no. If you tried to survive on a diet of nothing but corn you'd soon die of malnutrition, initially succumbing to diarrhea and mental deficiencies, depression, skin lesions and over a longer period of time, developing full-blown pellagra due to a lack of niacian/Vitamin B3. The best guess (based on pig studies) is that you'd be dead within 6-8 months and incapable of self-care within about 4 months.
That being said, you could stave off these complications by synthesising a simple niacin supplement, something that even a first year chemistry student should be able to accomplish with a $20 home-chemistry set so the short answer is yes, with a few vitamin supplements added to their diet, you could feed a family indefinitely on nothing but corn.
The world of Interstellar
Within the film you don't actually see them eating a corn-only diet.
Their nearest neighbour is growing okra.
At the ball game, Coop says that there'll be "candy and soda".
Coop is drinking beer which implies that yeast is readily available. Yeast would also supply the needed vitamins to stave off pellagra.
In the breakfast scene (with the broken lander) Coop is making grits which would usually require milk, an excellent source of Vitamin B3 and much-needed fatty acids. On the table we also see white sugar, brown sugar and pancakes.