The grain is artificial in nature and is composed of small robots similar to nanites. The grain and other machinery on the planet were all created and left behind by a previous civilization thousands of years ago.
Velexian grain can be consumed whole or ground in a mill and used to create foodstuffs like bread. The grain has healing properties but can be harmful or even fatal to individuals with artifical implants such as artificial hearts. The grain only works for a short period of time before the host body breaks it down and the nano-technology is deactivated.
In 2369 the USS Enterprise-D visited Velex to mediate peace talks between the Klingon Empire and the Hidran people after many years of conflict. A meal was served featuring bread made from Velexian grain. In an attempt to make a diplomatic breakthrough with the Hidran Ambassador Urosk, Worf, son of Mogh ate part of the bread and gave some to Urosk, who ate some as well. A short time later the nano-technology caused Urosk to reject the protective mask that had been implanted to enable him to breathe in a standard oxygen-nitrogen atmosphere. Unable to breathe Urosk died, but not before killing a Klingon officer. During this time, Geordi La Forge ate some of the bread as well, which caused his body to reject his VISOR and associated implants, forcing Doctor Beverly Crusher to remove them. They were later replaced after the nano-technology in La Forge had deactivated. Lieutenant commander Data also consumed a few kernels of the grain to analyze it. In this case the nano-technology caused Data to suffer a feedback loop in his brain that led him to believe the Klingons were preparing to attack the Federation, and were in league with Captain Picard and certain others. Data took the Enterprise on a course to Klingon space before he was stopped by Picard and Commander Riker. After the grain deactivated, Data returned to normal and would not stop apologizing to his colleagues. Riker told Picard that he was very fortunate to not have eaten any of the bread on Velex as it likely would have caused him to reject his artificial heart and kill him in the process.
Further analysis of the grain showed that long term use of the grain was also taxing to a person's immune system. Barbara Hollitt - who led the science team stationed on Velex - recommended the Velexian grain to her company to market as an occasional supplement. (TNG novel: Foreign Foes)