First doctor: “This morning for breakfast he requested something called ‘wheat germ, organic honey and tiger’s milk.’”

Second doctor: “Oh, yes. Those are the charmed substances that some years ago were thought to contain life-preserving properties.”

Fans of Woody Allen may recognize that snippet of dialogue from his 1973 comedy, “Sleeper.” The main character, a health-food store owner somehow frozen in 1973, has been thawed out 200 years later. He awakens to a world he can barely fathom, down to the kinds of food now said to constitute a sound diet. Everything that nutrition specialists once said was good for you, or really bad, turned out to be wrong.

First doctor: “You mean there was no deep fat? No steak or cream pies or hot fudge?”

Second doctor: “Those were thought to be unhealthy — precisely the opposite of what we now know to be true.”

Moviegoers laughed. They recognized how they were whipsawed by contradictory expert pronouncements about what they should or should not eat to stay healthy. On this score, not much has changed. How many times have Americans read about a study damning this or that food, only to then hear the revisionist opposite? Avoid eggs, we were told; they clog your arteries. Wait, we then heard, eggs have nutritional value. Coffee can give you cancer. Hold on, coffee can improve brain function. Butter is terrible. Well, not really. Again and again, yesterday’s verity becomes today’s punch line.

The vagaries of nutrition claims infuse the latest episode of Retro Report, video documentaries exploring major news developments of the past and how they still resound. This installment harks back to the 1970s, when many health authorities asserted, with unshakable confidence, that a diet low in fat and cholesterol was essential for a healthful life (wheat germ and tiger’s milk presumably optional).

“Fat-free” became a mantra, not to mention a marketing tool to sell breakfast cereals and high-caloric snacks. If anyone qualified as a heretic back then, it was Dr. Robert C. Atkins, a cardiologist who died in 2003. The Atkins diet encouraged loading up on fat-laden foods like steaks and omelets, and steering clear of pasta, bread and other carbohydrates.

But conventional wisdom held that fat was bad, period, with relatively few Americans distinguishing between saturated fats (meat, eggs, dairy products) and healthier unsaturated fats (fish, vegetable oils, nuts). Typically, people turned to breads, cereals and potatoes — and to sugary soft drinks — for the calories they no longer got from protein-rich foods.

“Diet is a trade-off,” Gary Taubes, a science journalist and the author of “Why We Get Fat,” told Retro Report. “If we reduce the amount of fat, you have to replace it with something.” That something tended to be carbohydrates. The result? Carbo-loading Americans grew fatter. “We put the whole country on a low-fat diet,” Mr. Taubes said, “and, lo and behold, we have an obesity epidemic.”

Obesity has proved a stubborn plague, one that the Centers for Disease Control and Prevention says affects 35 percent of American adults — about 79 million people. Tens of millions more, while not technically obese, are overweight. Not coincidentally, diabetes is a big national headache, even if the C.D.C. reported last month that new cases of the disease had begun to decline.

As for that low-fat diet, a major federal study concluded in 2006 that its health benefits were greatly overrated. Such a diet, researchers found, had no effect on the risk of heart disease or cancer, the two biggest killers in the United States

This Retro Report episode comes as the federal government is again rethinking its Dietary Guidelines for Americans. They form a nutrition template that Washington issues every five years and is a bible for millions of the diet-conscious, whose numbers undoubtedly include many who overindulged during the holidays and entered the new year with a pledge to shed pounds.

The new guidelines are expected to be issued this month by the Departments of Agriculture and of Health and Human Services, which tend to follow the recommendations of an advisory committee. One likely eye-catcher is a new assessment of cholesterol, long an archvillain. It seems destined for rehabilitation to some degree. Months ago, the advisory committee concluded that the dietary intake of cholesterol (the body produces this waxy, artery-obstructing matter on its own) had no real effect on blood levels of LDL, the so-called bad cholesterol. “Cholesterol,” the committee said, “is not a nutrient of concern for overconsumption.”

There is a conspicuous American tendency to cling to a favored diet as the gateway to good health, keeping weight down, staving off cancers and banishing heart attacks. A consequence is an abundance of regimens — vegan, gluten-free, Paleolithic, fruitarian and many more — each promoted by its adherents as the one true path.

But nutrition experts, including those in this Retro Report, caution that life is complex, and that we are more than what we eat. Among them is Dr. Barbara V. Howard, who was a principal investigator in the 2006 federal study of low-fat diets.

“We are not going to reverse any of the chronic diseases in this country by changing the composition of the diet,” Dr. Howard said when that report was issued. “People are always thinking it’s what they ate. They are not looking at how much they ate, or that they smoke or that they are sedentary.”

Other explanations for why one person gains weight and someone else does not may include sleep patterns, genetic predispositions and the compositions of individual microbiomes — the trillions of microbes residing inside the human body. Some health researchers even question the significance of exercise in keeping pounds off, regardless of its other benefits. Among other things, one has to move around quite vigorously to hold the weight down. A Big Mac, for instance, has 540 calories. To burn it off, a person would need to jog or to swim laps for about 45 minutes. Not every Big Mac eater exercises that strenuously.

Politics, too, can enter the picture. An example is the experience of Michael R. Bloomberg, a forceful public-health advocate when he was New York’s mayor. Despite resistance from restaurant owners and their political allies, Mr. Bloomberg pushed through a ban on trans fats (almost universally deemed a health hazard), and required fast-food outlets to post calorie counts.

But he hit a political and a juridical wall when he also sought to restrict the sale of sodas and other sugary drinks of dubious nutritional value. State courts shot down his plan on procedural grounds. Mr. Bloomberg found himself widely denounced for having taken government nannyism too far.

Then again, no one ever said figuring out the elements of a healthy diet would be easy. Dr. Dariush Mozaffarian, a nutrition specialist at Tufts University, suggested to Retro Report that accepted wisdom was not necessarily wise. Much of what has shaped dietary guidelines, he said, are “basically best guesses.” Uncertainty abounds. “I think now we know about 50 percent of what we need to know,” Dr. Mozaffarian said.

So is it possible that steaks, cream pies and hot fudge will someday be called the cornerstones of a healthy diet? Maybe if you can stick around for 200 years, you’ll find out.