If you are deficient in B12 then by all means remedy that deficiency. According to my understanding of human biology, anything that you take in excess of your body's needs will be eliminated or stored in or as fat and that process will use up energy and possibly result in other vitamin/mineral deficiencies.

And yes I would include vitamin D3 and elderberry extract as likely foreign substances but cannot be absolutely sure that they wouldn't count as foodstuffs. If they are seen as organic, then the body may use them for nutrition. If inorganic, they will be shown the exit at the earliest possible opportunity.

I fully agree with the Natural Hygiene principle that only the body can heal itself and in times of acute illness, the correct "treatment" is to stop eating altogether, go to bed and remain there, and drink only pure water. If you have flu, you will automatically do the first two anyway but may require prompting to drink the water. No food should be taken until 24 hours after the fever breaks. Any attempt to "manage" the disease will interfere with the body's inherent healing ability and can result in serious damage to organs or worse. That's what I believe but obviously I'm not suggesting that anyone follow this method based on my opinion. I do believe that it's worth looking into.

Who's to say that today's wonderdrugs are not just the calomel of our time?