In the film Interstellar, it looks like the only food human beings eat any more is corn. They seem to be pretty healthy (unless they get ill from the Blight). Is it really possible for people to live healthily on nothing but corn?
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.
Corn (maize) is the main source of food in many societies. One can live for a normal human lifetime on a diet of mostly corn if the minority foods in that diet supply what corn lacks. It is the same with diets of mostly wheat, or mostly rice, or mostly potatoes, which have also been what countless millions of persons have lived on.