I did the 5/2 diet before I started my new job.
I loved it, for the first time in a very long time I felt the difference between eating because I was hungry, bored, or emotional.
The downside? For me the drop in blood sugar made it difficult for me to concentrate. My old job did not require extensive mental work, so I could get around this. My new job, however, requires me to be mentally alert most of the time, so the 5/2 diet was not doable.
Hence, I expect the effects of low calorie and low blood sugar to be more pronounced - worse fatigue, reduced ability to concentrate, zoning out, etc. There may be other effects - I am not a dietician so I don't know.