As the film begins, Spurlock is in physically above average shape according to his personal trainer. He is seen by three physicians (a cardiologist, a gastroenterologist, and a general practitioner), as well as a nutritionist and a personal trainer. All of the health professionals predict the "McDiet" will have unwelcome effects on his body, but none expected anything too drastic, one citing the human body as being "extremely adaptable". Prior to the experiment, Spurlock ate a varied diet but always had vegan evening meals to appease his girlfriend, Alexandra, a vegan chef. At the beginning of the experiment, Spurlock, who stood 6 feet 2 inches (188 cm) tall, had a body weight of 185 pounds (84 kg).
Spurlock followed specific rules governing his eating habits:
- He must fully eat three McDonald's meals per day: breakfast, lunch, and dinner.
- He must consume every item on the McDonald's menu at least once over the course of the 30 days (he managed this in nine days).
- He must only ingest items that are offered on the McDonald's menu, including bottled water. All outside consumption of food is prohibited.
- He must only Super Size the meal when offered; he may not request to Super Size items.
- He will attempt to walk about as much as a typical United States citizen, based on a suggested figure of 5,000 standardized distance steps per day, but he did not closely adhere to this, as he walked more while in New York than in Houston.
On February 1, Spurlock starts the month with breakfast near his home in Manhattan, where there is an average of four McDonald's locations (and 66,950 residents, with twice as many commuters) per square mile (2.6 km²). He aims to keep the distances he walks in line with the 5,000 steps (approximately two miles) walked per day by the average American.
Day 2 brings Spurlock's first (of nine) Super Size meal, at the McDonald's on 34th Street and Tenth Avenue, which happens to be a meal made of a Double Quarter Pounder with Cheese, Super Size French fries, and a 42-ounce Coke, which takes 22 minutes to eat. He experiences steadily increasing stomach discomfort during the process, and then vomits in the McDonald's parking lot.
After five days Spurlock has gained 9.5 pounds (4.3 kg) (from 185.5 to about 195 pounds). It is not long before he finds himself experiencing depression, and he claims that his bouts of depression, lethargy, and headaches could be relieved by eating a McDonald's meal. His general practitioner describes him as being "addicted". At his second weigh-in, he had gained another 8 pounds (3.6 kg), putting his weight at 203.5 pounds (92.3 kg). By the end of the month he weighs about 210 pounds (95 kg), an increase of about 24.5 pounds (about 11 kg). Because he could only eat McDonald's food for a month, Spurlock refused to take any medication at all. At one weigh-in Morgan lost 1 lb. from the previous weigh-in, and a nutritionist hypothesized that he had lost muscle mass, which weighs more than an identical volume of fat. At another weigh-in, a nutritionist said that he gained 17 pounds (7.7 kg) in 12 days.
Spurlock's girlfriend, Alexandra Jamieson, attests to the fact that Spurlock lost much of his energy and sex drive during his experiment. It was not clear at the time whether or not Spurlock would be able to complete the full month of the high-fat, high-carbohydrate diet, and family and friends began to express concern.
On Day 21, Spurlock has heart palpitations. His internist, Dr. Daryl Isaacs, advises him to stop what he is doing immediately to avoid any serious health problems. He compares Spurlock with the protagonist played by Nicolas Cage in the movie Leaving Las Vegas, who intentionally drinks himself to death in a matter of weeks. Despite this warning, Spurlock decides to continue the experiment.
On March 2, Spurlock makes it to day 30 and achieves his goal. In thirty days, he has "Supersized" his meals nine times along the way (five of which were in Texas, four in New York City). His physicians are surprised at the degree of deterioration in Spurlock's health. He notes that he has eaten as many McDonald's meals as most nutritionists say the ordinary person should eat in 8 years (he ate 90 meals, which is close to the number of meals consumed once a month in an 8-year period).
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. (January 2014)
The documentary's end text states that it took Spurlock 5 months to lose 20.1 pounds (9.1 kg) and another 9 months to lose the last 4.5 pounds (2.0 kg). His then-girlfriend Alex, now his ex-wife, began supervising his recovery with her "detox diet", which became the basis for her book, The Great American Detox Diet.
The movie ends with a rhetorical question, "Who do you want to see go first, you or them?" This is accompanied by a cartoon tombstone, which reads "Ronald McDonald (1954–2012)", which originally appeared in The Economist in an article addressing the ethics of marketing to children.
A short epilogue was added to the film. It showed that the salads can contain even more calories than burgers if the customer adds liberal amounts of cheese and dressing prior to consumption. Also, it described McDonald's discontinuation of the Super Size option six weeks after the movie's premiere, as well as its recent emphasis on healthier menu items such as salads, and the release of the new adult Happy Meal. McDonald's denied that these changes had anything to do with the film.