A colleague of mine and I have just been discussing the meaning of fold changes and though this question has been asked before non of the answers are actually as straight forward as needed so lets try here to solve this once and for all.

I list here what I understand so far and will update it from your answers.

What is the correct way to understand a fold change value of a gene or protein?

 - A foldchange describes the difference of two values (eg. difference of expression in gene/protein A between healthy and diseased case)
 - Biostatistical porgrams/packages calculate it via: "Log(FC)" = mean(log2(Group1)) - mean(log2(Group2))
 -  log2 fold changes are used/plotted in graphs as those are nicer to show because they center around 0,  giving reductions a negative value and increments a positive value
 - log2 fold change values (eg 1 or 2 or 3) can be converted to fold changes by taking 2^1 or 2^2 or 2^3 = 1 or 4 or 8
 - To convert the fold change into change in % or anything that is actually tangible/understandable in "real life terms" ... need answers here! (= actual question I want to ask)

Lets get this solved once and for all, im looking forwards to your posts!
 
 
 
Here are great posts explaining more about fold changes:

[graphical representations][1] 

[conversion of log2 fold change to fold change][2]


  [1]: https://www.biostars.org/p/312980/
  [2]: https://www.biostars.org/p/100460/
Percent change = 2^(fold change) * 100, where the baseline then correspond to 100%

log2FC = 2 : 2^2 * 100 = 400, where the baseline then correspond to 100%

Log2FC = -2 : 2^-2 * 100 = 25, where the baseline then correspond to 100%

Hope this helps