“Aerobics: a series of strenuous exercises which help convert fats, sugars, and starches into aches, pains, and cramps” is a jocular one-line saying about aerobics. The line has been cited in print since at least 1989 and is of unknown authorship.
 
     
Google Books
Temple Maintenance:
Excellence, with Love
By James P. Gills
Tarpon Springs, FL : St. Luke’s Cataract and Intraocular Lens Institute
1989
Pg. 30:
“Aerobics is a series of strenuous exercises which help convert fats, sugars and starches into aches, pains and cramps,” quipped Mature Living magazine.
   
31 January 1990, Pittsburgh (PA) Press, “Fuller figure fitness classes attract loyal following” by Deborah Deasy, pg. N0, cols. 1-2:
She (Kay Johnston—ed.) knows how most older women react to aerobics and likes to show them the placard she totes to class: “Aerobics is a series of strenuous exercises which help convert fats, sugars and starches into aches, pains and cramps.”
 
Big Als Compound
TUESDAY, SEPTEMBER 16, 2008
Bonus day: 10 fitness quotes
(...)
(7) Aerobics: a series of strenuous exercises which help convert fats, sugars, and starches into aches, pains, and cramps. ~Author Unknown
 
Twitter
Cameron Morrissey
‏@ManagersDiary
June 2nd, 2009 - Five Funny Fitness/Diet Quotes:
Aerobics:  a series of strenuous exercises which.. http://bit.ly/XbNtZ
3:24 PM - 2 Jun 2009
   
Nat Med Talk (Natural Medicine Talk) 
What’s Your Fav Exercise Quote?
kind2creatures
01-21-2010, 10:18 PM
Aerobics: A series of strenuous exercises which help convert fats, sugars and starches into aches, pains and cramps. ~author unknown~
   
Google Books
Release Your Inner Old Fart
By Mark Leigh
London: Michael OMara Books Limited
2012
Pg. ?:
Aerobics: a series of strenuous exercises which help convert fats, sugars and starches into aches, pains and cramps.
   
Google Books
Gunny’s Rules: How to Get Squared Away Like a Marine
By R. Lee Ermey
Washington, DC: Regnery Publishing, Inc.
2013
Pg. ?:
“Aerobics: a series of strenuous exercises which help convert fats, sugars, and starches into aches, pains, and cramps.” —Anonymous
 
Twitter
American Proverb
‏@americccaann
Aerobics: a series of strenuous exercises which help convert fats, sugars, & starches into aches, pains, & cramps. ~Unknown health
10:02 AM - 23 Aug 2014