In June of 1965, a 27-year-old gentleman by the name of Angus Barbieri checked himself into the Maryfield Hospital in Dundee, Scotland. His complaint was that he had become “grossly obese.” Upon admission, the young man weighed in at 456 lbs (207 kg). Barbieri’s doctors recommended a brief period of fasting, presumably in the hope that his stomach would shrink, reducing his appetite and his capacity for food. But at the end of the scheduled fast Barbieri was having no trouble starving himself, so he elected to continue in a supervised fashion.

Barbieri’s doctors prescribed a novel diet of non-caloric fluids, yeast, electrolytes, and vitamin/mineral supplements. Barbieri continued with this arrangement on an out-patient basis, returning to the hospital for periodic blood and urine tests. He extended his fast up to the recommended maximum of 40 days, and then beyond. Although his blood and urine tests showed some variations, there was nothing worthy of alarm. The doctors did not take stool samples during this time, but records indicated that the patient’s bowel movements became quite infrequent, with an average of 42 days between evacuations.

Once Barbieri felt he had reached his ideal weight, he celebrated the end of his self-imposed starvation with a breakfast of one boiled egg, a slice of buttered bread, and a cup of black coffee. It had been 382 days since he had last eaten solid food, and in that time he had shed 276 pounds (125 kg). He weighed in at 180 pounds (82 kg). During his fast of over a year, Barbieri lost an average of 11.56 oz per day⁠—about 3/4 lb (1/3 kg). Physicians noted no ill effects from the prolonged starvation. The 1971 edition of The Guinness Book of Records credited Barbieri’s 382-day fast as the longest a person had gone without solid food, a record which is unlikely to ever be broken considering Guinness’s current policy of declining to record any extremes that they deem dangerous. Although Barbieri did gain back a little weight⁠—reaching 196 lbs (89 kg), he kept most of the weight off for the rest of his life. He died in 1990.

The law of conservation of mass observes that matter cannot be created or destroyed, so it follows that a human must expel matter from their body in order to lose weight. A survey in 2013 asked health professionals how they believe the body rids itself of metabolized fat, and most respondents incorrectly surmised that “burned” fat is converted to energy or heat. Many others⁠—also incorrectly⁠—believed that most of the metabolites are converted into muscle, or excreted as feces. In fact, contrary to intuition, over 90% of lost weight leaves the body as carbon dioxide exhaled from the lungs, disappearing into thin air. The remainder is excreted in urine, sweat, and other bodily fluids.1

1 Plants and trees use this process in reverse to gain most of their mass⁠—they draw carbon carbon dioxide out of the air and add it to their own mass.