I have two meanings of the word 'diet' in my head. In one, a diet is what you typically eat from day to day. In the other, a diet is a weight-loss programme as in the 'Atkins diet'.
So your four sentences confused me, because I found myself flipping between these two definitions of 'diet'. An individual could try more than one weight loss diet, but they should only have one "regular" diet.
I stumbled a bit with the phrase "regular diet". Does this mean constant/ not varying over time or does it mean normal or does it mean average in size (as in ordering a regular coffee)?
Others have covered "do not likely".
If it were me, I'd go for something like "Healthy people who eat normally are not likely to develop a phosphate deficiency."