The Age of Obama: Timelapse of President Barack Obama over four years in office Why presidents age quickly Presidents face unabated, unfathomable stress. "You see it over a term," said Ronan Factora, a physician specializing in geriatric medicine at the Cleveland Clinic. It's a good study of chronic stress on a person's overall health. Obama and his national security team monitor the hit on Osama bin Laden. Pete Souza/White House; Photo altered to obscure a classified document. Changes in skin or hair are gradual, he said. If you do have a stressful event, nothing is going to happen right away. Nothing visible anyway. Inside the body, the pituitary gland jolts the adrenal gland, just above the kidneys. Hormones start coursing. Adrenaline cranks up heart rates and blood pressure. Cortisol, another hormone from the same gland, causes inflammation and preps the body for converting sugars into energy. "It's not intended that people would be chronically exposed to these levels," said Sherita Golden, a physician at the Johns Hopkins Medical Bloomberg School of Public Health. Cortisol strains the circulatory system, battering artery walls. The hormone also thins the skin, makes muscles waste and bones lose mass. The immune system weakens, and viruses that cause colds and cold sores take hold. Sleep turns fitful. "Your cognition slows, you may feel more depressed, your ability to concentrate goes down," Factora said. And it just builds on itself - a real cascade. The only known cure There is one known treatment: exercise. "It is the best benefit a physician can recommend," Factora said. There is no drug that can present as many benefits as exercise can. Obama plays basketball during his 2008 campaign. AP Photo/Jae C. Hong Obama is a fiend for exercise. In hour-long workouts, he has been known to hit treadmills hard, weight train with arms and legs, build quickness through "plyos" or plyometrics - exercises that involve explosive movements. He also throws footballs, shoots basketballs and thwacks at golf balls. His predecessors exercised, too, some of them fiercely. George W. Bush ran till his aging knees made cycling a better option. Presidents Carter and Bill Clinton jogged, while Ronald Reagan rode horses and split logs with such vigor, he once cut his thigh. President Gerald Ford performed a daily exercise regimen while still in his robe and PJs. Good exercise leads to better thinking, brain-mapping has shown. "Exercise actually brings more blood flow," explained Linda Fried, an epidemiologist and geriatrician at Columbia University. Parts of the brain are activated and they're associated with complex thinking and problem-solving. Workouts also force a president to - truly, finally, deeply - rest. Only then can the relaxed brain start to make creative associations. Infirmity and vice The job has compounded certain human frailties. Most famous perhaps is the lethal case of pneumonia that 68-year-old William Henry Harrison caught at his inauguration. Woodrow Wilson's stroke certainly limited his leadership of the country, and Franklin D. Roosevelt worked around the problems related to his polio more ably than might have been expected. But daily habits also affect presidential well-being in lesser-known ways. Dwight D. Eisenhower was so dedicated to his form of exercise that he played 800 rounds of 18 holes over eight years as president, according to Evan Thomas, the author of "Ike's Bluff: President Eisenhower's Secret Battle to Save the World." Then, in 1955, Eisenhower had a heart attack, and two years later, a stroke. Intestinal surgery came in between, all as he was staving off nuclear war and realigning Southeast Asia. "Toward the end," Thomas said, "he was taking an extra sleeping pill at night" - the powerful, old-school kind, with barbiturates. And that was on top of a nightly scotch, never more than five ounces, except when it was, Thomas said. "A couple of times he says to his doctor, "Let's get drunk." " To the best of the public's knowledge, recent presidents have not exacerbated their stress through bad behaviors such as drinking. Obama, however, confirmed that he had to kick a cigarette habit of unknown intensity at some midpoint in his first term. The side effects of smoking might show up as those lines in his face, the doctors said. While sun exposure can also make a face look withered, Obama's darker skin has melanin to alleviate UV ray damage. That same coloring, however, can make his white hair look more pronounced. A special lot Obama had a fitness test on Jan. 12, and the White House said the results would be released by February. His previous physical was in October 2011; it showed that he had added one pound since his February 2010 physical (his 2011 weight: 181, very good for a man who was then 50 and 6-foot-1). Like all presidents since 1992, Obama has been under constant medical watch: a military physician is on hand wherever a president goes. Burton Lee,whom the first President Bush brought to the White House to monitor his health, agreed with Mariano that presidents are a special lot. They push their bodies and minds, and thus they develop a greater capacity to fight off infection. They shake enough hands to fell a lesser creature, he said. But the mental intrusions - the sense that someone needs something from the president every moment of every day - are as insidious as the germs. "It's just a phenomenally demanding job," he said. You never get one minute off. Despite the extraordinary stress levels, many recent presidents have lived well beyond normal life expectancy. Ronald Reagan and Gerald Ford died at 93; Jimmy Carter and the George H.W. Bush are 88. Doctors are coming to understand that stress may have an upside. "Human beings need some degree of stress to keep their systems tuned," Fried said. Some people enjoy the stimulation of it and the excitement and couldn't live without it. Plus, human minds literally seek reasons to live. "Many people, as they get older, deeply care about future generations and the world's survival," Fried said. If they have a chance to make a difference, that keeps people healthy. As Texas governor, George W. Bush was an avid runner, and sometimes invited journalists to pant alongside. But as president, Bush's knees literally got weak, and mountain biking became a better option, always on agent-protected terrain like his ranch in Crawford, Texas, or the Secret Service training site in Greenbelt. A teetotaler, the president went to bed early and exercised often, which helped stave off stress. Between meals, as a candidate, Bush famously chomped Cheetos, and as president, never lost his hankering for good Tex-Mex cooking. On many occasions, Bill Clinton's physician found it difficult to determine whether he was super-stressed or full-on infirm. They proved the two basic conditions of presidential life. "We were worried about Clinton when he was being impeached," she recalled. He looked like he had it all together, but we worried. All she and her colleagues could do was ask; all he would say was "I'm tired." Mariano could still clearly envision both the first President Bush and Clinton pushing on, with bags under their eyes, to the next campaign event. "One factor Bill Clinton had in his favor: he could power-nap," she said. When his helicopter went up, his eyelids would go down. Then upon landing, he announce, "OK, I'm ready! Showtime! After a bout of heart disease, cardiac surgery and a new commitment to vegan dieting, Clinton has regained strength - if not weight. Doctors were hesitant to attribute his ailment to all that cortisol damaging his circulatory system. "We don't have a genetic twin of President Clinton," said Factora, and that twin would have to rest for the eight years Clinton did not in order to qualify as an experimental control. The first President Bush didn't just work out, he worked out vigorously," said his physician Lee, citing four-mile runs and ball games with Marines at Camp David. Once he even played tennis with Pete Sampras. "He broke the Stairmaster at Camp David, he pounded it till it didn't work," Lee recalled. If I'm on it for five minutes, you have to take me out on a stretcher. Like appearance, endurance is highly individual; one person crumbles in circumstances where another thrives. Plus, what one person actively avoids, another embraces. "The one thing I noticed is that presidents have very unusual personalities," Lee said. Each is a different person, but for all, there is no easy day. While abstemious in an era when other statesmen chain-smoked cigars, Abraham Lincoln saw his legendary strength dwindle. At the start, the hale president was an able horseman who wrestled soldiers and challenged them to strength tests. One such test required a three-foot axe to be held out from his body with one hand, which is much harder than it sounds, said David Von Drehle, the author of "Rise to Greatness: Abraham Lincoln and America's Most Perilous Year.." By the time of his death, the president had been wracked by insomnia and lost interest in food. He arrived at the White House as a sinewy 6-foot-4, 180-pound strongman. In the course of four years, he dropped 30 pounds. "He was sunken-eyed and grizzled, nothing like that bright-eyed lawyer of Springfield," said Von Drehle. Lincoln sat for a famous series of portraits, and "by the last set of photographs, he looks 75 years old, but he's 56."