“To make a change in how you look, you are talking about a significant period of training,” Dr. Kraemer said. “In our studies it takes six months to a year.” And, he added, that is with regular strength-training workouts, using the appropriate weights and with a carefully designed individualized program. “That is what the reality is,” he said.

And genetic differences among individuals mean some people respond much better to exercise than others, said Dr. Mark Tarnopolsky, an exercise researcher at McMaster University in Hamilton, Ontario. He added that although he does not think the before-and-after photos in ads are doctored, most people will not change so markedly no matter how hard or long they work. “I believe they are taking the top one or two people out of thousands,” Dr. Tarnopolsky said.

People who did change their bodies say six months is a bare minimum to see real change.

Schuyler Antane, 43, a research scientist, is one. He began in January 2006 with a diet, which meant, he said, “letting go of the foods that taste good, but are wicked evil. And no more beer.”

In three months, he had lost 10 pounds and was down to 190 pounds on his 5-foot-8-inch frame. Then he read a magazine article on 5-kilometer races and decided to try to run. He could run for only five minutes when he started, and it took two months to train for his first race. But he kept at it and improved. Within six months, he weighed about 150 pounds. Then he added bicycling and swimming, becoming a triathlete. That, he said, got him to his fighting weight of 140 to 145 pounds.

“My beer belly is long gone,” he said. “The only flab in my midsection is excess skin, but I am not vain enough to have an operation.”

Now, said Mr. Antane, who runs with a group in Princeton on Thursday nights, “everything changed  my outlook on life, who I hung out with, how I felt about myself.”

Jim Lisowski, 45, the owner and chief executive of SciTec, a research and development company in Montgomery, N.J., said he had let himself slip out of shape, going from 189 pounds to 225 pounds. He is 5-foot-10 1/2. Then his wife bought a joint membership at a gym within walking distance of his office. At first, he went sporadically, but he decided to get serious after about three years.