1. Healthy people
on a regular diet do not likely develop phosphate deficiency.
2. Healthy people
on regular diet do not likely develop phosphate deficiency.
3. Healthy people
on the regular diet do not likely develop phosphate deficiency.
4. Healthy people
on regular diets do not likely develop phosphate deficiency.
I agree that the ending sounds a bit awkward. ...do not likely develop... I'd say "are not likely to" develop phosphate deficiency. Or, "are not usually deficient in phosphate/s(?)" since a deficiency doesn't exactly sound like something that would be "developed" but something that would be taken away, kind of the opposite of developing anything, if that makes sense.
#1- No. The plural-singular there sounds like the healthy people all share the same diet. This would sound better, in my opinion: "
A healthy person on a regular diet [is] not likely to develop phosphate deficiency.
#2- No. The missing "a" in "Healthy people on [a] regular diet..." sounds strange. It reminds me of UK speech, where they say someone is "at university" or "in hospital." It's not how people in the U.S. talk.
#3- No. "The" regular diet sounds like you are referring to a specific diet that has already been explained to us.
In fact, the word "regular" sounds a bit off, too. "Regular" can mean "typical" foods but also hints at the spacing of the meals, which seems a bit not what you mean. Other words that fit better might be typical, standard, normal, general...
#4- Yes. The plural "people" matches the plural "diets."
Also, when in doubt, I usually just do what Chase advises.