THE first symptom of heart disease is sometimes sudden death. Never was that fact made clearer than in the ironic death last week of James Fixx, whose best-selling book ''The Complete Book of Running'' led tens of thousands to take up jogging and made him a guru of the running world. Mr. Fixx, whose transition from a heavy young man who smoked two packs of cigarettes a day into a trimmer, middle-aged nonsmoking athlete seemed to insure a healthy life, died at the age of 52 while jogging in Vermont.

Friends described him as being in fine physical condition and said he had not complained of any symptoms while running 10 miles a day and pursuing other vigorous physical activity. He had trounced his sister, Kitty Fixx Bower, in a tennis match on Cape Cod the day before his death.

His former wife, Alice Kasman Fixx, said, ''He never had any warning.''

''If he did,'' she said, ''he ignored it.''

Reports immediately after his death suggested that Mr. Fixx did not have a regular physician and had not gone for a routine checkup as his sister had urged him to do, even though his father had his first heart attack at the age of 35 and died of another one at 43.

News accounts of Mr. Fixx's death have led many to assume that such checkups would have detected the disease, brought about drug treatment or coronary bypass surgery, and saved his life. It might have.

But the insidious thing about heart disease, which is the nation's leading cause of death, is that it is often so secret and veiled that doctors cannot always detect severe cases such as Mr. Fixx's from routine tests. All individuals with heart disease are not candidates for bypass surgery, nor do all who have it benefit from it. It usually takes decades for arteriosclerosis to clog the arteries, thereby narrowing the stream of blood and reducing nourishment of the heart muscle. Further, as the painless, insidious process progresses, the body usually adapts to it by forming collateral pathways for blood to flow. Presumably, in Mr. Fixx's case, they were inadequate to protect against the heart rhythm abnormality that apparently killed him - an abnormality that may or may not have been triggered by a heart attack.

Dr. Eleanor N. McQuillen, Vermont's chief medical examiner who performed an autopsy on Mr. Fixx, said in an interview that all three of his coronary arteries were damaged by arteriosclerosis, the underlying cause of heart attacks.

Mr. Fixx's left circumflex coronary artery was almost totally blocked; only trickles of blood could flow through the pinholes that were left of the inside of that artery. About 80 percent of the blood flow in the right coronary artery was blocked. The chief nourishment to Mr. Fixx's heart came from blood flowing through the third artery, the left anterior descending, which was less severely affected. Nevertheless, half that artery was blocked in places.

There was additional arteriosclerotic damage to a portion of Mr. Fixx's aorta and the arteries in his legs, but no blockage. The disease spared the arteries that fed his brain.

Mr. Fixx's case ''will be a big question raiser'' because of the irony of his death while jogging and the debate about the health benefits of exercise, said Dr. Robert S. Ascheim, a cardiologist who practices at 435 East 57th Street and teaches at New York Hospital-Cornell Medical Center.

''Does running benefit you? Nobody really has a clear answer,'' Dr. Ascheim said.

Nevertheless, many people believe exercise prevents death from premature heart disease. Dr. Ascheim said that the severity of Mr. Fixx's heart disease would not have made bypass surgery imperative but that national studies have shown that medications would have been advised.

As the debates go on, optimists will say that exercise prolonged Mr. Fixx's life, and pessimists will contend that it shortened it.

Because of the uncertainties, one of the thorniest questions physicians face is how many tests and which ones to advise to screen for heart disease in middle-aged males who have no symptoms. Many physicians would not have been particularly suspicious of Mr. Fixx's having heart disease because he was in such fine physical condition and because of the apparent absence of symptoms even when exercising strenuously.