KEEPING FIT WASN'T EAXY IN 1837, EITHER
LEAD: HO-HUM, let's rehearse once more the predictable answers to the 1987 Health and Fitness Quiz. What are they saying about coffee and tea? ''Potentially harmful stimulants.'' Salt and sugar? ''Detrimental if overused.'' Fried and fatty foods? ''Unhealthy in the extreme.'' Dietary fiber and bran flour?