The Dietary Reference Intake (aka RDA) is formulated for humans operating at one Earth gravity. Is there a similar reference or any research supporting specific recommendations for a human in prolonged freefall?
The "Nutrition and Human Spaceflight" section of NASA's Space Food factsheet has some good, basic information with links to specific research and studies. For example:
The nutrients astronauts need in space are the same ones all people need, but the amounts of some differ. Astronauts need the same number of calories for energy during spaceflight as they need on the ground.
Most of the vitamins and minerals they need are the same as on the ground. The amount of iron in an astronaut’s diet should be less than 10 milligrams per day for both men and women. Astronauts have fewer red blood cells while they are in space. Most of the iron absorbed from food goes into new red blood cells. If astronauts were to eat foods high in iron, the iron would be stored in their bodies and could cause health problems.
Sodium and vitamin D affect bone. The amount of sodium in the astronauts’ diet is limited because too much can lead to bone loss as well as other health problems. The body usually makes vitamin D when the skin is exposed to sunlight, but spacecraft are shielded to protect the astronauts from harmful radiation. On Earth and in microgravity, people need vitamin D for healthy bones. Vitamin D supplements are recommended for space travelers on the ISS, since the current space foods do not provide enough of this vitamin.
There's a great PDF provided by NASA on space food and nutrition from an educational stand point for grades k-8. It's also under public domain and does not have a copyright.
The beginning of the document goes into how Project Mercury, Gemini, Apollo, skylab and others prepared food for that period of time.
We also learn about the types of space foods (rehydratable food, thermostableized food, frozen food, etc) and how microgravity affects how food is eaten and packaged.
If you are interested in learning how to make space food there is an example of a Space tortilla recipe on page 54.
Ingredients % by Mass Wheat 61.79 Water 26.58 Glycerin 4.02 Shortening 3.71 Mono/Diglycerides 1.24 Salt 0.99 Baking Powder 0.87 Dough Conditioner 0.31 Fumaric Acid 0.19 Potassium Sorbate 0.15 Carboxymethyl Cellulose 0.12 Calcium Propionate 0.03 100.00%
Also according to this they reference USDA Food Guide Pyramid. Which indicates astronauts follow the same guideline.
This is obviously a good document for children interested in space food and nutrition because it's a lower level for better understanding.