When I write reports for patients and other healthcare providers, I always try to avoid use of casual terminology and stick to medical terminology. It looks more professional that way because the terms have a scientific basis. While the word “fat” is a colloquial term, the words “obese” and “obesity” are not. They have specific scientific meanings in the medical community. Don’t believe me? Grab a medical dictionary. I just looked up the terms “fat” and “obese/obesity” in the two most popular medical dictionaries: Mosby’s and Stedman’s. Mosby’s does not even have an entry for the word “fat’ as a descriptive term and Stedman’s only briefly noted that it is a common (i.e., colloquial) term for obese. However, both dictionaries contain extensively detailed scientific entries on obesity. Neither dictionary contains the term “unhealthy weight” which is vague since it can also apply to people who are underweight.
The terminology is becoming so diluted that I have even heard that some overweight people are being referred to as “persons of size.” That term means absolutely nothing since everyone is technically a person of size.