According to FDR's grandson Curtis:
"The careless habits developed during the demanding and stressful war years—no regular rest, inattention to his diet, on top of continuing to smoke two packs of Camels a day—were catching up with him."
Dr. Howard G. Bruenn, a cardiologist who was consulted on FDR's condition and examined FDR shortly before his death, published an article in 1970 entitled "Clinical Notes on the Illness and Death of President Franklin D. Roosevelt" in the Annals of Internal Medicine. In the article, he detailed Roosevelt's condition based on his surviving medical notes and his memory. Bruenn's after-the-fact diagnosis is recounted by Barron H. Lerner in his carticle "Crafting Medical History: Revisiting the 'Definitive' Account of Franklin D. Roosevelt's Terminal Illness" in the Bulletin of the History of Medicine:
"As has been well documented, Bruenn’s article was a refutation of the claims of Ross T. McIntire, an ear, nose, and throat specialist and vice admiral in the U.S. Navy who was Roosevelt’s primary physician. Throughout the president’s tenure in office and after his death on 12 April 1945, McIntire insisted that Roosevelt had always been a healthy man. Roosevelt’s cough, shortness of breath, and fatigue, which Bruenn had diagnosed as being due to severe hypertension and congestive heart failure, were dismissed by McIntire as colds, the flu, or psychological strain. McIntire had initially rejected Bruenn’s conclusions, but he later relented and allowed the cardiologist to essentially take over the case. Because the press largely took McIntire at his word, Bruenn’s findings, and his role in Roosevelt’s care, were kept quiet."
Lerner's article is a re-examination of Bruenn's article, and concludes that, while Bruenn's article is reasonably reliable, it is problematic for two reasons. One, because Bruenn broke doctor-patient confidentiality by writing the article. And two, it may have been written to downplay the severity of FDR's condition, and dismiss accusations that he was not mentally fit enough by the time of Yalta Conference in February 1945 to lead the nation.
To that point, another physician has since said FDR's medical condition was likely more severe than Bruenn's 1970 article let on. According to the book When Illness Strikes the Leader: The Dilemma of the Captive King, by Jerrold M. Post and Robert S. Robins:
"Dr. Bert Park, a neurosurgeon who has painstakingly researched the medical histories of several important twentieth-century leaders has observed the illnesses from which Roosevelt was suffering—hypertension, anemia, chronic obstructive pulmonary disease, and congestive heart failure—in concert almost certainly would have severely impaired his mental functioning. In particular, Park believes that Roosevelt was suffering from 'sub-acute chronic diffuse hypoxia of the brain'. This is a very reasonable explanation for the drowsiness, problems in concentrating, and episodes of semi-stupor that affected Roosevelt during his last two years of life. Yet despite the obviously increasing impairment of Roosevelt's leadership capacity, Dr. McIntire continued to reassure the president, his family, and the public that Roosevelt was in good health for a man his age."
The authors there are paraphrasing Dr. Bert Park's own book Impact of Illness, wherein Park accuses FDR's physician Ross T. McIntire to have been incompetent, even for the era.
Regardless, FDR's medical issues were severe. General "stress" was not the cause of his death, as McIntire had alluded to at the time. A two-pack-a-day smoker, who had suffered from a paralytic disease (thought at the time to be polio, though more recent medical analysis has claimed it was more likely Guillain-Barré syndrome), who had a poor diet, and also drank regularly if rarely to excess..."Stress" was just icing on the cake. He died of a cerebral hemorrhage, at a time when anti-clotting medication was virtually non-existent outside of aspirin, and blood pressure medication was also not very advanced. War or not, his lifestyle, medical history, and treatments available to him in 1945 were not going to give him a rosy prognosis. He died at age 63. Even today, the same type of lifestyle kills people at a similar age. The stress of managing a nation at war surely didn't help, but there's no reason to think peace would have allowed him to live substantially longer, had he lived the same sort of lifestyle in 1941-45.