In this episode of “You Write the Top, I’ll Write the Bottom,” we find one writer on yahoo.com writing about a PhD dietitian (whatever that means), and another writing about dieticians:
The preferred spelling in the American Heritage Dictionary is dietitian, although dietician is acceptable. What’s not acceptable? Using both spellings.
Terribly Write 
July 16, 2014 at 2:53 pm
The most useful advice: don’t trust the experts. Especially when they cite only one.
I tried – unsuccessfully – to find the article. Their /food page is quite nice – especially if you’re hungry.
July 16, 2014 at 5:25 pm
The article is in the new, “improved” Health: https://www.yahoo.com/health/14-foods-nutritionists-never-eat-91317726342.html