In a little more than three months, 36-year-old Francisco Lovera managed to do what he never thought he could: He lost more than 30 pounds off his formerly 210-pound frame.
While enrolled in a weight loss program at the Alpert Medical School at Brown University in Providence, R.I. , he learned how to make gradual lifestyle changes that led him to weight loss success.
"I learned all the benefits of having a balanced diet and more physical activity," he said. "I didn't starve myself or spend three hours a day at the gym."
In addition to exercising, he cut down on a number of foods, including meat, tortilla chips and sugary beverages, and started eating more fruits and vegetables.
The foods Lovera cut out of his diet are among those that have made the biggest change in his weight for years to come, according to a new study by researchers at the Harvard School of Public Health.
The researchers looked at three separate studies over a 20-year period that included more than 120,000 people to see how their lifestyle affected their weight change over every four years. Researchers identified the amount of weight gained based on the specific type of food the participants said they ate regularly.
"We know that when people gain weight, they've got a problem with energy balance -- they're eating too much or they're using too few calories in exercise," said Dr. Rena Wing, a professor of psychiatry at Brown's Alpert Medical School who was not involved in the study. "But this article went beyond the general statement to pinpoint specific eating behaviors that might be the culprits."
Relationships between weight and lifestyle factors were assessed every four years over a 20-year period. The study participants, who were not obese and had no chronic conditions, gained an average of 3.35 pounds within each four-year interval, and that weight gain was most closely associated with these specific foods and habits:
Potato chips (an average gain of 1.69 pounds)
Potatoes (an average gain of 1.28 pounds)
Sugar-sweetened drinks (an average gain of 1 pound)
Unprocessed red meats (an average gain of .95 pounds)
Processed meats (an average gain of .93 pounds)
Alcohol (an average gain of .41 pounds with one drink a day)
Smoking (recent quitters gained an average of 5.17 pounds, but over time, gained only .14 pounds)
Watching TV (an average weight gain of .31 pounds per hour per day)
Weight loss was associated with eating:
Vegetables (an average of .22 pounds lost)
Whole grains (an average of .37 pounds lost)
Fruits (an average of .49 pounds lost)
Nuts (an average of .57 pounds lost)
Yogurt (an average of .82 pounds lost)
Knowing what foods contribute to a larger increase in weight over time can help people understand how to maintain a healthy weight balance, the researchers said. Preventing excessive weight gain rather than focusing on weight loss could help stem the obesity epidemic, they said.
"The outcomes of the study are very much like those of the [federal government's] dietary guidelines -- increase plant foods, lower intake of solid fats and added sugars while boosting activity," said Connie Diekman, director of university nutrition at Washington University in St. Louis.
Many of the foods mentioned -- potato chips, sugary drinks, processed meats and alcohol -- are associated with a sedentary lifestyle, so experts aren't surprised the study linked them to weight gain.
"They are filled with empty calories and spike blood sugar and insulin levels, which lead to fat accumulation and weight gain," said Dr. Gerard Mullin, associate professor of medicine at the Johns Hopkins School of Medicine in Baltimore.
On the other hand, yogurt may have an impact on microorganisms in the intestinal tract that help regulate appetite and insulin resistance, Mullin added.
"People should not feel that avoiding these foods will guarantee protection from obesity," said Keith Ayoob, associate professor of pediatrics at the Albert Einstein School of Medicine in the Bronx, N.Y. "It's ultimately all about excess and avoiding it wherever it is in your diet."
Portion Control and Small Steps Keys to Success
Lovera said he definitely cut down his portion sizes, and he learned the value of gradually replacing some of the bad foods with healthier options.
He said he never ate a lot of potato chips. His weakness was tortilla chips. He loved indulging in a plate of nachos.
"Now, I buy oranges and apples," he said. "I have an apple or an orange in the middle of the afternoon."
He also said he never drank a lot of soda or sugary drinks, but enjoyed them when he ate out. Whatever he drank then he now replaces with sparkling water.
"The little changes really helped a lot," he said.
The bottom line is not to eat less, but to eat less unhealthy, said Dr. Beverly Green, a scientific investigator at the Group Health Research Institute.
"Buy real food, preferably foods that are environmentally sustainable," said Green. "Avoid restaurants, bags and boxes, unless you know the product is very healthy."
Many nutrition experts hope there will be many more people like Lovera who realize that little things really do mean a lot when it comes to weight loss.
"If people can slowly change their behaviors -- eating one spoonful less, adding veggies a few times a week -- they can improve health and change their body weight," said Diekman.
The research also assessed other lifestyle factors, like sleeping habits. The study found sleeping less than six hours a day or more than eight hours a day was also associated with weight gain.
Physical activity, on the other hand, was linked to weight loss. Experts have long stressed the importance of being active as part of a healthy lifestyle.
Lovera started walking for about 10 minutes a day and worked his way up to a 25-minute walk during lunch and another walk in the evenings. And, he said, he's proof that cutting out a lot of the foods associated with weight gain can lead to successful weight loss.
He only has seven more pounds to lose to reach his target weight of 170 pounds, a healthy weight for someone who is 5 feet 10 inches tall. He believes that with the changes he's made in his life, he can stay in that range. He also hopes his story can inspire others to take those same small steps.
"I feel like a person who's more in control of my life and my future," he said. "I feel as if I've gotten my life back."
Additional reporting by ABC News' Bojana Zupan and Jane Kurtzman.