Protein sparing modified fasts (PSMF) are the most rapid yet safe weight control diets. Known by names like Optifast these usually take the form of five liquid "meals" daily-usually prepared
by mixing a dry powder with water. These liquid diets usually provide about 500 calories daily along with some minerals and electrolytes. They contain a carefully calibrated amount of carbohydrate (in order to reduce protein loss through gluconeogenesis) but still,  these diets are strongly ketogenic.

Because ketosis suppresses hunger, many patients find these diets surprisingly painless. Because ketosis causes large shifts in fluids and electrolytes in the body, patients need close medical monitoring during their time on the fasts. They also usually require additional prescription electrolyte replacement.

PSMF's typically produce very rapid weight loss. Some studies show that obese patients placed on PSMF's lost about thirty pounds of body fat in six weeks. One particularly famous patient was Oprah Winfrey whose first major public weight loss occured in the late 1980's using a PSMF. Unfortunately, Ms. Winfrey's success after stopping the fast was rather typical for some patients: rapid weight regain accompanied by irrepressible hunger. Indeed, largely because of Winfrey's public re-grain, PSMF's fell into widespread public disfavor by the early 1990s.

Today, PSMF's have a limited but important role in medical weight control. These diets are generally reserved for patients who have an urgent need for weight loss, often an upcoming surgical proceedure (including bariatric surgery) or very serious weight related medical
problems. Used appropriately, these diets can produce results that no other diet can match.