We surveyed Americans and a panel of nutrition experts about which foods they thought were good or bad for you.

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 enough question, and a person who wishes to eat more healthily 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, we 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, like kale, apples and oatmeal, are considered “healthy” by nearly everyone. And some, like 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 Dariush Mozaffarian, the dean of the Tufts Friedman School of Nutrition Science and Policy. “And now we know about 40 or 50 percent.”

Here’s what we found.

Of the 52 common foods that we 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 it as healthy, but less 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, a SlimFast shake andgranola bars, 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. (You’d be surprised how many foods contain added sugar.) It’s possible nutritionists know this, but the public still does not.

On the other end of the spectrum, several foods received a seal of approval from our expert panel but left nonexperts uncertain. Most surprising to us was the reaction to quinoa, a “superfood” grain so often praised as healthful that it has become the subject of satire. (At the moment, The New York Times cooking site offers 167 recipes for quinoa, roughly a third of which are explicitly tagged “healthy.”)

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. Our colleague Neil Irwin measured mentions of trendy foods in Times coverage over the years, and found that quinoa had only recently picked up steam. Others may reflect mixed messages in press 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 health benefits, more consumption can obviously have real health costs.

We weren’t surprised to find areas in which both ordinary Americans and experts disagreed.

We expect researchers to be better informed about current research, and everyday consumers to be more susceptible to the health claims of food marketers, even if the claims are somewhat dubious.

But some of the foods in our survey split both the public and our panel of experts.

Four of the foods listed above – steak, cheddar cheese, whole milkand 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 your 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. (If you’re a steak lover and you find this news discouraging, our colleague Aaron Carroll has written that red meat is probably fine in moderation.)

It’s clear that many shoppers do want to eat healthful foods but are unsure what to choose. To gain some perspective on this, we asked Google which foods were most commonly part of a simple search: “Is [blank] healthy?” We used these results to generate some of our survey questions. The food people were likeliest to ask about was also one nutritionists generally approve of: sushi.

There are some areas of nutritional consensus. Nearly everyone agreed that oranges, apples, oatmeal and chicken could safely be described as healthy, and also agreed that chocolate chip cookies,bacon, white bread and soda could not.

Where does this leave a well-meaning but occasionally confused shopper? Reassured, perhaps: Nutrition science is sometimes murky even to experts.

Your overall diet probably matters a lot more than whether you follow rigid rules or eat just one “good” or “bad” food. Our colleague Aaron Carroll has published a list of common-sense rules for healthful eating, which represents a good start.

We also asked our experts whether they considered their own diet healthful, and how they described it. Ninety-nine percent of nutritionists said their diet was very or somewhat healthy. The most popular special diet type was “Mediteranean”; 25 percent of our nutritionists picked it. But the most common answer, even for experts, was “no special rules or restrictions.”