Whereas the SUBSTANCE of Lembas indeed has a high energy density, its ACCIDENTS remain those of ordinary unleavened bread. It's all in Aquinas (http://www.seton-parish.org/parish/Eucharist.htm#3.), all in Aquinas: bless me, what do they teach them at those schools!
What Benedict XVI (or indeed Professor Tolkien) would have to say about Lembas...