I heard the same through the grapevine.
Typical plasma volume is about 5 liters, assume hematocrit of 45 = approximately 2 liters lost to hit hematocrit of 75 (all else being equal)
To put that into perspective:
About 10% of sweat comes from plasma volume.
If you're down 5 pounds (2.27kg= 2.27 liters of water), then .2L came from Plasma volume.
That means we're looking to lose approximately 20kg or 44 pounds to dehydrate plasma volume by 2 liters.
Now, if we combine some dehydration (certainly not helping) with severe vomiting / diarrhea or a condition like capillary leakage syndrome then you get a heck of a lot closer.
Ultimately, it's hard to say without knowing significantly more info.
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