The dose makes the poison.
– Paracelsus
Fun fact: Paracelsus believed that the principles sulphur, mercury, and salt contained the poisons contributing to all diseases. (Wikipedia)
Considering that our cells work under specific physico-chemical conditions with constant supplies, there is no surprise that a lack or an excess is dangerous for this balance.
Also consider the fact that we are complex organisms which have enhanced radically the survival of these cells: it is a giant multi-system of buffers, distribution of nutrients and evacuation of wastes.
Sodium of salt and potassium are a lot used in a pair.
But the main effect of excess of salt is raising blood pressure, which damages hearts and vessels, especially when they are already rigidified (by sugar/cholesterol/calcium déposit)
Excess of salt and wastes ? Removed by the kidneys (recycling most of the water).
Storage of calcium in the bones (needed for nervous system among others)
Excess of sugar in blood? Stored in the liver and muscles, and if too much, transformed as fat (for later), but in case of diabetes it does damages everywhere by glycation.
About protein, well, no “classic” storage, either used directly of eliminated or converted as fat (that why it is better to do sports and grow muscles, they can be degraded back as a source of amino-acids later)
Iron? Mostly recycled in the spleen (but women are subject to bigger needs of intake
)
Fibers are used to feed the good bacteries in our guts (who help us to digest and transform the nutrients so we can use them). Note that we have a secondary autonomous nervous system in our guts to manage the digestion!
And globally (animal) fat is healthier that sugar, even with cholesterol (in fact, its our big bowel which produce cholesterol, sometimes in excess, not the food intake)
I used to explain my colleagues that we were like coral, cells around mineral skeleton, the only difference was that we added a system of locomotion to go and find food because we can’t wait for it to come to us like in the sea.
And we are like an embedded ecosystem.
Our biggest problem is not the lack but the excess …
We are marvelous system of recycling and survival, made to get by with only little supplies.
But our brain is still working in survival mode and brings us pleasure when we “find” rare and energetic nutrients like salt, sugar and fatty things, which in fact are now abondant in food … and pushing the machine to its limits !
And for the toilets analyzing the urin … it exists U-Scan | Withings 
The topic is huge and you can do dozens of videos ^^