Answer
Nitrogen fixing would allow us to build proteins using atmospheric N_{2} and therefore we would not need to eat high-protein foods. However, we would need to include a source of carbon in our diet, as well as minerals and water. A typical meal might include: carbohydrates as a carbon source, with fruits and vegetables providing minerals.
Work Step by Step
Consider that we would not need to consume protein, because we would be able to us the nitrogen gas from the air to build proteins. In addition, remember that carbon would be a much needed resource. Carbon is abundant in carbohydrates, which can be provided by fruits and vegetables.