The host takes what it needs - except the essential element that the Symbiote needs a constant supply of. The Symbiote's alien biochemistry means the same things the host needs are sustaining - but there is still one essential craving, need and urge driven by a chemical deficiency.
The Symbiote's cravings drive the host to get the Arsenic it needs.
Arsenic is highly poisonous to the human host:
Arsenic exerts its toxicity by inactivating up to 200 enzymes,
especially those involved in cellular energy pathways and DNA
synthesis and repair. Acute arsenic poisoning is associated initially
with nausea, vomiting, abdominal pain, and severe diarrhoea.
Encephalopathy and peripheral neuropathy are reported. Chronic arsenic
toxicity results in multisystem disease
... There are no evidence based treatment regimens to treat chronic
That being said, there's a small amount of arsenic in everything, including a few molecules here and there bound up in every cell of the human body. We can all live with this tiny amount.
There's no way to take in enough to satisfy the Symbiote without killing the host except by eating vast quantities of people.
Chelation therapy may be used. This treatment uses certain chemicals,
including dimercaptosuccinic acid and dimercaprol, to isolate the
arsenic from the blood proteins.
The Symbiote chemicaly-binds the tiny quantities of Arsenic then extracts it from the blood. This prevents the host from ever getting a fatal dose.
Doesn't have to be Arsenic it can be any of the poisonous elements eg. metals.
Edited by request of nzaman, JBH.
The rest of the biomass is exhaled as gasses (oxygen, carbon dioxide), urinated out (acetic acid in vast quantities), and defecated in the form of egg-like mineral deposits. Among the minerals, just occasionally will appear something special, a perfectly smooth pearlescent object of extraordinary beauty and an eerie almost supernatural radiance.
These objects become much prised for their apparent ability when worn on a clasp about the neck of a lady (one sufficiently rich to be able to afford it), to apparently transform even the most plain-lookin wearer into a world-class beauty. legends say that long life is bestowed on the wearer - but that's just a myth right?
What is not immediatley apparent from the start is - what happens when they begin to hatch.....