Food

They do that already in the Army. They are called 'ration packs' Ration packs are scientifically designed to keep you alive for one day. After a week of eating ration packs you die of boredom!!

What other things can we possibly do in the time we normally spend for eating. Life is to enjoy and relish things and food is one of them. Moreover our body stops functioning in the natural way if we resort to such shortcuts. We have enough of medical problems all around why shoul we bring in more?

Science already invented a consecrated bread,( a very thin bread with 2/1" dia) which will give all the supplements to the body to survive. But it is yet to gain popularity, as people are addicted to foods and they cannot compromise with taste they got used to.

Food gives us our energy. Our digestion process is inefficient in that it only extracts a very small fraction of the matter that we ingest. In the far future if scientists were able to replace all of our biological components with mechanical ones and power it all by a core that generated energy by way of nuclear fusion. We could probably run for several centuries on a single glass of heavy water. We are probably at least a few hundred years away from having that level of technology though.