"So I've been trying to stay healthy. I've been getting workouts regularly. I've been eating reasonably well I've got to say... You know, I've been trying to stay fit."
Obama joked regularly on the campaign trail that he's "skinny but tough."
On a swing through Louisiana, Obama was offered more food by a woman who said that he needed to "put a little more meat on his bones." And at a Wisconsin fish fry restaurant, patrons said he needed to eat more.
Still, one psychologist said one shouldn't draw "pathological interpretations" from Obama's health regimen.
"Unlike the rest of us, he doesn't have to work at it at all," said Dr. Michael Lowe, professor of psychology at Drexel University, whose specialty is eating disorders and obesity. "And maybe he wouldn't gain much weight if he didn't watch his weight and exercise a lot.
"As paradoxical as it may seem, some of the healthiest people who exercise the most and watch their diet most carefully are, in fact, lean," he said. "They may be doing that purely for health reasons and not for weight reasons."
The American norm is "so unhealthy" that it's easy to suspect Obama might be over-doing the exercise and calorie counting, said Lowe. Still, he said that more than one hour of working out a day could signal a compulsive aspect.
Other signs of obsession would include mood changes when a workout is missed or self-perceptions about body image are not "on target," he said.
"It's hazardous to draw wide-ranging conclusions about someone's personality, but the number one excuse Americans who don't exercise give is that they are too busy," said Lowe.
"Who could be busier than the president? And yet he found the time," Lowe added. "I'm playing armchair psychologist here, but you can interpret that he is incredibly healthy or he's got a little bit of a problem."
But Nelson, who has watched Obama work out at his Philadelphia-area gym on five occasions, sees a healthy attitude in the next president.
"I sense from him that it's as therapeutic as it is healthy running on the treadmill," he said. "I think he collects his thoughts."
And, said Nelson, Obama sets a good example for other Americans.
"It's definitely a time when fitness has to be on the table," he said. "There are so many health issues now, and it's good to see a president involved in fitness and look so youthful in comparison to others."