The past decade has toppled many of our received ideas about health and wellness. Recent studies have told us — after countless desserts ceded to SnackWell’s, and early morning hours on the treadmill — that fat doesn’t make us fat, and exercise doesn’t make us thin. And now, the naturalist journalist Michael Pollan tells us supplement-popping, enriched-soda-drinking Americans that the nutrition science we’ve come to rely on is doing us more harm than good.

In his new manifesto, “In Defense of Food” (Penguin Press, 256 pages, $21.95), Mr. Pollan writes that the latter part of the 20th century ushered in America’s “Age of Nutritionism,” in which whole foods such as those our ancestors ate have been largely supplanted by fortified, food-like products, prepared in factories and laboratories. Many of these processed edibles are low in fat and make all sorts of health claims; but they have ultimately turned us into an overfed but undernourished people prone to many chronic ailments. “What the Soviet Union was to the ideology of Marxism, the Low-Fat Campaign is to the ideology of nutritionism — its supreme test and, as now is coming clear, its most abject failure,” he writes.

This book is the prescriptive follow-up to Mr. Pollan’s 2006 best-seller, “The Omnivore’s Dilemma,” which traced the history, through ingredients, and the environmental and ethical fallout of four very different meals consumed by the author. While “The Omnivore’s Dilemma” examined what we might be eating, “In Defense of Food” purports to tell us — succinctly in the first paragraph, and in more detail in the last 54 pages — what we should be eating.

Mr. Pollan’s back-to-basics decree: Eat food. Not too much. Mostly Plants.

The first directive might sound intuitive. But Mr. Pollan asserts that so much of the “foods” we consume amount to little more than nutrient-rich science experiments.

Time was, bread was made with flour, yeast, water, and a pinch of salt, and without wheat gluten or cellulose or high fructose corn syrup; cheese didn’t contain sodium erythorbate — and certainly didn’t come in aerosol-like canisters. The imitations that did exist were easy to recognize. By the late 19th century, five states had passed statutes requiring margarine to be dyed pink so consumers wouldn’t mistake it for butter; after those laws were struck down, Congress went on to pass a law requiring imitation food products be labeled as such. The U.S. Food and Drug Administration, in a move backed by the food industry, repealed that imitation law in 1973, blurring the line between food and food-like.

Food producers have attempted to fortify their food-like products with nutritional add-ons, but there’s a problem with that approach, Mr. Pollan explains: Nutrients, such as beta-carotene and folic acid, don’t seem to have the same health benefits in supplements or enriched food products, as they do in, say, carrots and whole grains. He chalks it up to the notion that a whole food is more than its nutrient makeup — and to our incomplete understanding of what food delivers to us.

So in the final quarter of “In Defense of Food,” the dogmatic Mr. Pollan instructs us to work outside of this nutrient-centric system. He reminds us to “eat food,” giving pithy pointers for recognizing (and avoiding) highly processed foods. “Don’t eat anything your great grandmother wouldn’t recognize as food” and “Shop the peripheries of the supermarket, and stay out of the middle” are among his “eating algorithms.” He tells us to seek out foods that are both organic and local, even if it means spending more money — while acknowledging that many Americans cannot afford such a premium.

As for the “mostly plants” component, Mr. Pollan has no use for the traditional carbohydrate-heavy food pyramid, insisting that the largest portion of a healthy diet should be made up of vegetables and fruits, naturally rich in vitamins and antioxidants; animal proteins and grains, preferably unrefined, play mere supporting roles.

But it is, perhaps, the “not too much” part of the equation that will require the most significant paradigm shift. Because whatever Americans are eating, we’re eating a lot of it — often quickly and often alone, in cars, at our desks, or in front of the television. This multitasking approach to food has left us too distracted to sense our own fullness. He introduces us to more functional food cultures: In France, there are taboos against second helpings; in Okinawa, Japan, they practice a principle called [ITAL hara hachi bu], or eat until you are 80% full. “Americans typically eat not until they’re full (and certainly not until they’re 80% full) but rather until they receive some visual cue from their environment that it’s time to stop: the bowl or package is empty, the plate is clean, or the TV show is over,” he writes, citing studies from several American psychologists.

On this front, Mr. Pollan’s suggestions are of the sensible — if not wholly original — sort: Eat meals, not snacks, that you’ve cooked yourself … at a table … with a dining partner or two … and don’t feel the need to clean your plate.

Despite what he sees as a durable, if unhealthy, collusion of nutrition science, government, the food industry, and big pharmaceutical companies (benefiting from our chronic illnesses), Mr. Pollan is surprisingly, and convincingly, optimistic about the prospects for grassroots change in the way America eats dinner.

Here in New York, the past year has yielded a proliferation of popular greenmarket-themed eateries, with menus touting fare that was locally raised or grown. And the author writes of a resurgence of farmers’ markets and the rising demand for organic foods. “[F]or the first time in a generation, it is possible to leave behind the Western diet without having also to leave behind civilization,” he writes. “And the more eaters who vote with their forks for a different kind of food, the more commonplace and accessible such food will become.”

Ms. Birkner edits the Food & Drink section of The New York Sun.