Ask people what they like first for breakfast, and most will probably mention orange or grapefruit juice or citrus in some other form.

That's fine with Dr. Louis Tobian Jr., chief of the hypertension section at the University of Minnesota, who himself downs 12 ounces of grapefruit juice every morning. Tobian also consumes two bananas, a sliced potato sauteed in a little vegetable oil and a glass of skim milk.

Like citrus, all these foods are rich in potassium. While Tobian, 65, believes that science has correctly pointed the finger at high salt and fat levels in the modern diet as contributing to stroke, high blood pressure and heart attacks, he has come to think that low potassium levels are also partly to blame.

"Early man got most of his sustenance from gathering roots, leaves, seeds and fruits; his tools weren't good enough to permit him to bag much game for meat, and what meat he got was very lean," he said. "We know this both from the analysis of petrified human fecal matter found at prehistoric sites and from the eating patterns of the remaining pockets of Stone Age man around the world -- Amazonian Indians, the !Kung bushmen of the Kalihari desert, and Solomon Islanders, for example.

"Eating only natural foods, man took in low amounts of sodium and very high amounts of potassium during his first 4 million years," Tobian continued. "So primitive man was designed to run on a low-sodium, high-potassium diet. Although we now consume two-thirds less potassium than prehistoric peoples, there is no reason to think that our requirements for it are any less."

But in the 10,000 years since the invention of agriculture, hasn't evolution allowed us to adapt to a nutritionally different way of life? Tobian doubts it.

"Evolutionary changes come about in the interest of maintaining the ability to reproduce," he said. Stroke, heart disease and hypertensive kidney failure "typically strike after 45 or 50, when people are no longer of reproductive age. By that time, it's a matter of indifference to nature whether we live or die. She's had no incentive to alter our biology to run on less potassium."

Tobian has also been impressed by the fact that potassium intake among blacks in the Deep South and among the people of Scotland is only about 15 percent that of primitive hunter-gatherers -- roughly half what most of us consume. (With Scots this is because their soil is poorly suited to growing vegetables. With blacks in the Deep South it is because traditional cooking involves peeling and simmering vegetables for hours, thereby letting the potassium escape into the cooking water.)

"While other elements in their diet -- or sheer coincidence -- may be a part of the problem," said Tobian, "one has to consider that this may explain why both these groups have stroke and heart attack rates that are among the highest in the world. And you have to wonder, too, if low potassium intake isn't the reason that blacks in the Southeast have 18 times more kidney failure caused by high blood pressure than middle class whites."

All of which led Tobian to embark on a series of animal experiments designed to find out whether his hunch about the benefits of potassium was correct.

For one of the experiments, he chose an inbred strain of rats that becomes hypertensive and subject to progressive kidney damage when fed a diet laced with salt. Some of the animals were then given potassium supplements to bring their diet up to hunter-gatherer standards.

Tobian had expected that the blood pressure of the supplemented rats might fall and was rather surprised when it did not. However, these rats developed 50 percent less kidney damage than the others. And at autopsy they were found to have been largely spared the thickening of artery walls in the kidneys that typically occurs -- in susceptible people as well as rats -- when blood pressure is high.

For another experiment, Tobian and his colleagues bred a colony of hypertensive stroke-prone rats. Here, too, a "hunter-gatherer" level potassium supplement exerted a protective effect. Of 24 hypertensive rats not given the supplement, 20 were dead within four months. But there was only one fatality in four months among 50 rats of the same strain with equally high blood pressure that, except for extra potassium, were given identical fare.

In still another Tobian experiment with hypertensive stroke-prone rats, the surviving animals were killed at the end of two and a half months for careful study of their brains. Of the 10 survivors from the "normal" potassium group, four were found to have small spots of bleeding there, indicative of little strokes. Yet in none of the brains of the 34 supplemented survivor rats was there any evidence of hemorrhage, even though their blood pressure had been just as high.

What had apparently happened, said Tobian, is that the extra potassium had maintained the elasticity of the rats' artery walls, enabling them to withstand elevated blood pressure and resist stroke.

Tobian is among the first researchers to investigate the relationship between potassium intake and damage from hypertension.

He is the first to admit that it would be difficult for modern urbanites to take in 10 grams of potassium daily, the level typical of hunter-gatherer diets, past and present. But five grams (0.18 ounce), he believes, is feasible and still offers considerable protection to artery walls, especially if people are careful to go easy on animal fat and salt.

Among the foods Tobian highly recommends for the purpose -- besides citrus fruit, bananas, skim milk and potatoes -- are lean meat or fish, melons, mushrooms and virtually all vegetables. However, he is no fan of tomato juice; "it's loaded with salt." And he prefers vegetables boiled, baked, steamed or stir-fried in their skins to those that are peeled or deep fried. Peeling vegetables, he says, causes them to lose potassium when they are cooked (by whatever method), and the fat you have to use to deep fry them needlessly adds calories.