Q. Can there be a nutritional or physical reason for the sudden onset of a whistling “S” in an adult’s speech?
A. The answer is almost certainly physical rather than nutritional, unless the sufferer happened to eat a cavity-causing diet that led to dental work.
The flaw in pronouncing a sibilant S, which speech therapists call sigmatism, can follow even minor changes to the way the tongue aligns with the teeth. As a result of either too small a gap or too large a gap between the biting edges of the front teeth, the air forced through the gap in pronouncing the S sound can produce an extra whistle.
There's more, of course, but I just thought I'd pass it on if anyone was interested. :)