As the film begins, Spurlock, age 32 at the time the movie was filmed in 2003, is physically above average, as attested to by three doctors (a cardiologist, a gastroenterologist, and a general practitioner), as well as a nutritionist and a personal trainer. He enlists all three to track his health during the month-long binge. All of the health professionals predict the "McDiet" will have unwelcome effects on his body, but none expects 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 then-girlfriend (now wife), Alexandra, a vegan chef. At the beginning of the experiment, Spurlock, who stands 6 feet 2 inches (188 cm) tall, had a body weight of 185.5 lb (84.1 kg).
Spurlock starts the month with breakfast near his home in Manhattan, where there are an average of four McDonald's (and 66,950 residents, and twice as many commuters) per square mile (2.6 kmē). He also elects to ride in taxis more often, since 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. Spurlock has several stipulations which govern his eating habits:
He must fully consume three McDonald's meals per day (at breakfast, lunch, and dinner time).
He must sample 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. This includes bottled water. Any and all outside consumption of food is prohibited.
He must SuperSize the meal when asked, but only when asked. He is not able to SuperSize by his own accord.
He will attempt to walk about as much as a typical U.S 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 relatively more while in New York than Houston.
Day 2 brings Spurlock's first 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 52 minutes to eat. He experiences steadily increasing stomach aches during the process, and promptly throws up in the McDonald's parking lot.
After five days Spurlock has gained almost 10 pounds (4.5 kg) (from 185.5 to about 195 pounds). It is not long before he finds himself with a feeling of depression, and he claims that his bouts of depression, lethargy, and headaches are relieved by a McDonald's meal. One doctor describes him as "addicted." He has soon gained another 8 pounds (3.5 kg), putting his weight at 203.5 lb (92 kg). By the end of the month he weighs about 210 pounds (95.5 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, but it was hypothesized by a nutritionist that he had lost muscle mass, which weighs more than an identical volume of fat.
Spurlock's girlfriend, Alexandra Jamieson, attests to the fact that Spurlock has lost much of his energy and sex drive during his experiment. It was not clear at the time if Spurlock would be able to complete the full month of the high-fat, high-carbohydrate diet, and friends and family began to express concern.
In Day 21, Spurlock has heart palpitations. Consultation with his concerned 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 deliberately drinks himself to death over a similar time period. Despite this warning, Spurlock decides to continue the experiment.
Spurlock makes it to day 30 and achieves his goal. In thirty days, he "Supersized" his meals nine times along the way (five of which were in Texas, three in New York City). All three doctors are surprised at the degree of deterioration in Spurlock's health. One of them states that the irreversible damage done to his heart could cause a heart attack even if he lost all the weight gained during the experiment. 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 8 years of eating it once a month).
Text at the conclusion of the movie states that it took Spurlock 5 months to lose 20 pounds (9 kg) and another 9.5 months to lose the last 4.5 pounds. His girlfriend Alexandra Jamieson, a vegan chef, began supervising his recovery with her "detox diet," which became the basis for her book, The Great American Detox Diet.
"The bottom line, they're a business, no matter what they say, and by selling you unhealthy food, they make millions, and no company wants to stop doing that."
The movie ends with a rhetorical question, "Who do you want to see go first, you or them?" with a cartoon tombstone for Ronald McDonald ("1954-2012") as a backdrop. The cartoon of the tombstone originated in The Economist where it appeared in an article addressing the ethics of marketing toward children.
In the DVD release of the movie, a short epilogue was added about 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. However, it is shown that the salads can contain even more calories than hamburgers, if the customer adds liberal amounts of cheese and dressing onto them prior to consumption. It is claimed by McDonald's that these changes had nothing to do with the film.
Another issue that Spurlock focuses on is the way McDonald's targets young children with ads before the kids themselves realize how harmful the food is. In the movie, Spurlock jokes that he will battle the socialization of his children by punching them in the face every time they pass a McDonald's so that the golden arches do not elicit happy memories