Going meatless is nothing new.

Think back to geometry class. Remember the Greek philosopher Pythagoras? His theorem about a right triangle? You know, "the hypotenuse of a right triangle equals the sum of ..." Don't worry about finishing the sentence. The point is that historians know him for more than his mathematical genius. They also consider Pythagoras, born around 580 B.C., the "father of vegetarianism."

For nearly 2,500 years both Europeans and Americans called those who followed his dietary practices Pythagoreans. The term vegetarian wasn't used generally until the founding of the British Vegetarian Society in 1847.

Pythagoras' argument in favor of a fleshless diet was three-pronged (like a triangle): religious worship, physical health, and ecological responsibility. And those reasons continue to be cited today among people who prefer to go meatless.

While there have always been vegetarians among earth's population, many chose this way of life because of necessity rather than preference. The medieval world regarded grains and vegetables as animal food. Only poverty compelled humans to replace meat with such fare.

Meat was a symbol of class status. The more frequently you ate flesh, the more elevated your position in society. Samuel Johnson's 1775 English dictionary displays this cultural bias in his definition of oats: Oats: a grain which in England is generally given to horses, but in Scotland supports the people."

Vegetarianism and religion. Vegetarianism is often linked to religion, and the strength of the tie seems to relate directly to the age of the religion. For example, Islam, at a relatively young 1,300 years, has no strong vegetarian heritage. Mohammed did preach kindness to animals and once saved insects from suffering by making his men put out a fire in an anthill. But most Muslims today would not consider vegetarianism a religious requirement.

Buddhists, on the other hand, following Guatama's example of nonviolence and noninjury to living creatures, have practiced vegetarianism for 2,500 years. Hinduism can trace vegetarian principles back some 5,000 years to the Vedic scriptures that eulogize the abstinence from meat and plead with the faithful to eschew flesh.

Jews cite the first chapter of Genesis as prescribing man's original diet:

And God said, Behold, I have given you every herb bearing seed, which is upon the face of all the earth, and every tree, in the which is the fruit of a tree yielding seed; to you it shall be for meat" (Genesis 1:29).

Despite this heritage, the Jews came to view the absence of meat as a deprivation, often abstaining as a symbol of grief and mourning. The first-century Jewish philosopher Philo maintained that God had forbidden the use of pork and shellfish because these were the most tasty of all meats. He said it was God's method of curbing desire and pleasure.

Early Christianity, with its roots planted in Jewish tradition, saw vegetarianism in a similar vein-as a modified fast to purify the body. Tertullian (A.D. 155-255), Clement of Alexandria (A.D. 150-215), and John Chrysostom (A.D. 347-407) taught that avoiding meat was a way to increase the discipline and willpower necessary to resist temptation. This made dietary restrictions, such as vegetarianism, rather common in early Christian behavior. And the belief has been handed down through the years in some form or another-for example, the Roman Catholic prohibition of meat (except fish) on Fridays and during Lent.

The religious tie to vegetarianism became popular in fifteenth-century England, which is probably the best beginning point for modern vegetarianism.

Thomas Tryon (1634-1703) a self-taught philosopher and student of the Protestant mystic Jakob Behmen declared in 1657, "The Voice of Wisdom ... called upon me for separation and self-denial.... I betook myself to water only for drink, and forbore eating any kind of flesh or fish, confining myself to an abstemious self-denying life." His book, The Way to Health, published in 1691, advocated a vegetarian diet and was widely read by the thinkers of the day. It even influenced the young Benjamin Franklin (17061790) to become a "Tryonist" for a time.

Tryon's message also influenced Dr. George Cheyne (1671-1743), a prominent London physician. Years of indulgence had left him a gross 440 pounds; so he decided to go on the vegetarian diet described by Tryon. Cheyne's success prompted his 1724 publication of "Essay of Health and Long Life" recommending a meatless diet.

Methodism's founder, John Wesley, was both Cheyne's patient and a convert to vegetarianism. Cheyne counted other such luminaries as Pope, Swift, and Hume as friends and as a result many of the literary lights of England began praising if not always practicing a vegetarian diet. Cheyne's essay spawned quite a bit of controversy among fellow members of the Royal Society of London, where it was hotly debated.

Apparently Cheyne's side of the argument convinced Dr. Antonio Cocchi of Florence. His first visit to the Society, as a guest of its president, Sir Isaac Newton, occurred during one of these discussions. Some years later in August of 1745, following his return to Florence, Cocchi delivered a lecture on the virtues of the "Pythagorean" diet, producing a revival of italian vegetarianism. His lecture, no doubt, echoed the sentiments of fellow countryman Leonardo da Vinci (1452-1519), who many years earlier had claimed, "I have from an early age forsworn the use of meat and the time will come when men will look upon the slaughter of animals as they now look upon the slaughter of men."

Cheyne's book remained popular and was still in print 46 years after its initial publication. Among those influenced by his recommendations was Dr. William Lambe (1765-1847), who said that "the use of the flesh of animals is a deviation from the laws of his [man's] nature, and is universally a cause of disease and premature death."

Arguments such as these captured the attention of William Cowherd (1763-1816), a dissident cleric of the Church of England. In 1800, breaking with the church, he established his own offshoot sect in Manchester. Cowherd taught that the diet commanded in Genesis 1:29 was a religious responsibility. He felt so strongly about this that the only qualifications for membership in his Bible Christian Church was adherence to a vegetarian diet for at least six months.

William Medcalfe (1788-1862) joined the group in 1809 and in 1817 led 40 other church members on the arduous 11-week journey to America. Shipboard life was not conducive to vegetarianism, and only 11 of the adults and seven of the children landed in Philadelphia without backsliding.

Medcalfe was an active evangelist for the fleshless diet and in 1823 his Abstinence From Animal Flesh became the first book on vegetarianism published in the United States.

One convert to the dietary plan, if not the religious principles, of Medcalfe's group was Sylvester Graham (1794-1851) of the Graham cracker fame. After visiting the church in 1829, he began to recommend a vegetarian diet to the thousands who attended his lectures on health.

American notables such as Boston physician William Alcott, his cousin, Bronson Alcott (1779-1888) Louisa May Alcott's father), and Dr. Russell Trall (1812-1877), "water cure" enthusiast, were among Graham's supporters. In 1850 they, along with Medcalfe and others, founded the short-lived American Vegetarian Society.

Vegetarianism in America. American vegetarianism was almost a casualty of the preoccupation with the Civil War. During that time the fledgling Seventh-day Adventist Church kept the ideas alive. Ellen White (1827-1915), one of the denomination's founders, began to advocate a vegetarian diet in 1863.

In 1866 the denomination opened a health institute in Battle Creek, Michigan, but soon realized that its survival depended on properly trained medical personnel. With the support of the church, young John Harvey Kellogg, a convert to Adventism and the vegetarian diet, entered and completed medical school. During his schooling he lived, he said, on a diet of Graham bread and apples.

In 1876 Kellogg became director of this institute, calling it Battle Creek Sanitarium, and under his guidance it soon became a world-renowned health resort and breakfast cereal manufacturing center.

Kellogg was an eloquent spokesman for the vegetarian cause, and when the Vegetarian Society of America reorganized in 1886 he was honored as the keynote speaker. His Natural Diet of Man (1923) was typical of books on vegetarianism published during the latter part of the eighteenth and the early part of the nineteenth centuries.

These publications served to move vegetarianism from a "cultish" practice to one based more on science. Yet they still contained much anecdotal evidence and self-serving rhetoric.

Nutritionists of the 1920s and 1930s were often unwilling either to condemn or promote vegetarianism, feeling that good evidence for either position was lacking. Vegetarians were more likely to be pitied than praised, and both the lay public and medical community expressed concern for their well-being.

Current trends. The explosion of scientific research after World War II largely removed the stigma of a meatless diet. The recently conducted California Adventist Health Study has shown that male Seventh-day Adventist vegetarians outlive the general population by 8.9 years. When subgroups within the study were compared, vegetarian Adventists had a 3.7-year advantage over their meat-eating counterparts.

Eighty percent of Americans now believe that a vegetarian diet is more healthful than meat eating, and some 50 million have markedly reduced or eliminated red meat from their diets.

It's estimated that about 4 percent (8 million) of the American adult population are vegetarian. According to a 1990 Vegetarian Times poll, these individuals come from all walks of life and have practiced their diet for an average of 6.8 years. The major reason given for adopting a vegetarian lifestyle was a desire to eliminate the suffering caused to animals. Health was the second most cited reason, followed by concern for the environment.

Fittingly, the British are still ahead. The British Vegetarian Society recently reported that almost one third of all British young people are vegetarian. Of the 149 colleges and universities in Britain, all but two provide daily vegetarian menus.

Today a bronze likeness of Pythagoras overlooks the ancient harbor of his namesake village, Pythagorean, on the Greek island of Samos. More than 2,500 years have passed since he sat on these shores and contemplated mathematics and the meaning of life.

Ironically, it is next to impossible to find a vegetarian meal in any of the dockside eateries that surround the harbor today.

Pythagoras would still be gratified to know how widely his vegetarian doctrine has spread, but perhaps a little disappointed that it is for his mathematical theorem that he is best remembered.

Glen Blix, Dr.P.H., is assistant professor in the Department of Health Promotion and Education in the School of Public Health at Loma Linda University, Loma Linda, California.