Obviously Jesus did not consult with health experts before delivering his message about salt, or else he would have known it was bad for us.
Or perhaps, given that in his culture there was no way of preserving food via refrigeration, he would have deemed eating spoiled meat more of a health hazard than sodium and just gone ahead with his analogy. My money's on the latter.
But the truth is that our culture has a variety of different understandings of salt than did the ancient Near East. Everything from the way it's used to the way it looks to the way it was processed to be ready for use is very little like what it was in Jesus' day.
There were two main methods of getting salt -- one was from a mine, where rock salt was dug from the earth like anything else valuable and underground. The other was from the shores of the Dead Sea, a body of water so salty that people don't sink in it. As the tide ebbed and flowed, pools of water would be left behind that would evaporate. When they did, the minerals found in the Dead Sea water remained, and those mineral clumps could be carefully washed to leave behind only the desired salt. The second method was the one by which most poorer people obtained salt, since they could just pick up the mineral clumps themselves if they were on the shore or buy them from those who had. Salt from mines was purer and more expensive, and very often beyond the reach of most people.
The only problem with washing the Dead Sea mineral clumps to get salt was that if the washing used too much water, the clump would turn into a sludgy mess and it would be impossible to separate the salt from the other elements. The sludge was useless and thrown out, which us what Jesus refers to. Each clump was different and the job a hard one.
So when Jesus told his followers they were the salt of the earth, he communicated several important ideas that we might miss. For one, in order for them to be salt they would have been carefully refined and prepared for use, "washed" by God until all the unwanted minerals were gone what remained was only the valuable and useful salt.
Many of us might think of baptism when we talk about this, but we should also remember that our refinement and purification by God is a lifelong process. Baptism is a symbol of it, but it goes on over and over again. We should also remember that we, unlike the salt, can stop or stall the process. When we do, we may lose our saltiness and reduce our value in God's work in the world. A Christian who is no different from the people around her in any way loses her ability to create change. A church that makes itself like the world around it, whether for good or poor reasons, will likewise lose its ability to have an impact on that world.
Broadly speaking, our roles in the world are not significantly different from the roles salt played in food preparation in Jesus' day. Salt gave flavor to food, because it tasted different from the food itself. Salt was a preservative that could slow or even arrest the process of decay. Salt that could do neither of these things was of no value; Christians that cannot be different from the world around them or show a better way of living that both follows God more closely and evokes the fullest measure of human life are in the same boat.
Which leads us to the last and probably most important difference. Just as Jesus said, salt that has lost its saltiness is of no value. It is thrown away. And when we lose our saltiness, we too are of little or no use in God's plan for the world. But we are not thrown away.
Instead, because of God's love for us and Jesus giving himself on the cross, we may find ourselves "re-salted," a chance the flavorless salt never had. In fact, if our purpose is to be the "salt of the earth," then it is just as plain that one of Jesus' purposes was to salinate us when we go bland.
And that indeed sounds like good news to me.