A summa cum laude graduate in neuro-physiology from Boston University, Connelly specialized in anesthesiology, internal medicine, intensive care and cardiovascular medicine. Whilst working with critically ill intensive care patients at Massachusetts General Hospital Connelly began researching products to help prevent their loss of muscle mass. It was while working as a Senior Fellow at Stanford University that he created a protein powder he dubbed METAMYOSYN. METAMYOSYN contains whey protein, calcium caseinate, egg albumen, milk protein isolate and other protein sources, and is the base ingredient of MET-Rx.

Main article MET-Rx

Founded in 1991, MET-Rx pioneered a new category of bodybuilding supplements known as Meal Replacement Powders (MRPs), a combination of protein, combined with carbohydrates (usually from maltodextrin), vitamins, minerals and added amino acids. Like MET-Rx, they are usually low in fat and are used to supplement 1-2 "meals" per day. Connelly sold all interest in the company to Rexall Sundown[1] (a wholly owned subsidiary of Royal Numico) for $108 million in 2000. [2]

Connelly is the namesake of UCLA's Connelly Laboratory for Applied Nutritional Sciences at the UCLA School of Medicine, Division of Clinical Nutrition. He is also a visiting professor at UCLA's School of Medicine with teaching responsibilities in the area of the physiology of nutrition and muscle metabolism, and recently published the book Body Rx: Dr.Scott Connelly's 6-Pack Prescription

More recently Connelly launched a new product K9-Rx a dietary supplement for dogs, described on its company website as:

"the highest quality, most innovative and complete nutritional supplement food available. It is the product of years of research into canine physiology, together with a close scrutiny of the pet food industry today."[3]