NEW YORK — Is popcorn good for you? What about pizza, orange juice, or sushi? Or frozen yogurt, pork chops, or quinoa?

Which foods are healthy? In principle, it’s a simple question, and a person who wishes to eat more healthfully should reasonably expect to know which foods to choose at the supermarket and which to avoid. Unfortunately, the answer is anything but simple.

The Food and Drug Administration recently agreed to review its standards for what foods can be called “healthy,’’ a move that highlights how much of our nutritional knowledge has changed in recent years — and how much remains unknown.

With the Morning Consult, a media and polling firm, The New York Times surveyed hundreds of nutritionists — members of the American Society for Nutrition — asking them whether they thought certain food items (about 50) were healthy. The Morning Consult also surveyed a representative sample of the American electorate, asking the same thing.

The results suggest a surprising diversity of opinion, even among experts. Yes, some foods, such as kale, apples, and oatmeal, are considered “healthy’’ by nearly everyone. And some, such as soda, french fries, and chocolate chip cookies, are not. But in between, some foods appear to benefit from a positive public perception, while others befuddle the public and experts alike. (We’re looking at you, butter.)

“Twenty years ago, I think we knew about 10 percent of what we need to know’’ about nutrition, said Dr. Dariush Mozaffarian, the dean of the Friedman School of Nutrition Science and Policy at Tufts University. “And now we know about 40 or 50 percent.’’

Of the 52 common foods that the Times asked experts and the public to rate, none had a wider gap than granola bars. More than 70 percent of ordinary Americans we surveyed described them as healthy, but fewer than a third of nutritional experts did. A similar gap existed for granola, which less than half of nutritionists we surveyed described as healthy.

Several of the foods considered more healthful by everyday Americans than by experts — including frozen yogurt and SlimFast shakes — have something in common: They can contain a lot of added sugar. In May, the Food and Drug Administration announced a new template for nutrition labels, and one priority was to clearly distinguish between sugars that naturally occur in food and sugars that are added later to heighten flavors.

On the other end of the spectrum, several foods received a seal of approval from the expert panel but left nonexperts uncertain. Most surprising was the reaction to quinoa, a “superfood’’ grain so often praised as healthful that it has become the subject of satire.

In addition, tofu, sushi, hummus, wine, and shrimp were all rated as significantly more healthful by nutritionists than by the public. Why? One reason may be that many of them are new foods in the mainstream American diet.

Others may reflect mixed messages in news coverage of the healthfulness of foods. Shrimp was long maligned for its high rate of dietary cholesterol, though recent guidelines have changed. And public messages about the healthfulness of alcohol are conflicting: While moderate drinking appears to have some benefits, more consumption can obviously have real health costs.

Several of the most controversial foods — including steak, cheddar, whole milk and pork chops — tend to have a lot of fat. And fat is a topic few experts can agree on.

Years ago, the nutritional consensus was that fat, and particularly the saturated fat found in dairy and red meat, was bad for the heart. Newer studies are less clear, and many of the fights among nutritionists tend to be about the right amount of protein and fat in a healthy diet.

The uncertainty about these foods, as expressed both by experts and ordinary Americans, reflects the haziness of the nutritional evidence about them.