Diet advice changes by the minute. How are we supposed to figure out what to eat?

Even though their identities (and revenue streams) were wholly tied to veganism, dozens of these influencers have gone back to animal products, citing maladies like chronic acne or gastrointestinal distress or mental fog. Meagan Moon and Evan Rock, who are just as ethereally attractive as you’d expect a Hawaii-based holistic YouTube couple known as “Moon & Rock” to be, recorded their ex-vegan video with their infant daughter strapped to Rock’s chest. Admitting they knew their flip would be a “shock to a lot of our viewers,” Rock pleaded, “be gentle with us.”

No such luck. The spate of defections like theirs prompted a fierce backlash in the form of vegan “response” videos. My favorite was the F-bomb-drenched rebuttal from first-wave vegan YouTuber “Freelee the Banana Girl,” who ranted: “I’m sick to [expletive] death of hearing these [expletive] excuses to end animals’ lives again. From brain fog and pimples and gas to ‘My baby took all my protein!’ Get the [expletive] out of here!”

Shortly after discovering this YouTube drama, I had lunch with a vegan friend and did a double take when he asked for the Philly cheesesteak egg rolls. He was too young, and life was too short, he told me, to accept the joylessness he was feeling about food under such a restrictive diet.

I’m not here to pick on vegans. (Well, maybe just the super-preachy ones.) The fact is that adherence is a problem for every healthy diet out there. And given how imperiled our planet is, we should be applauding anyone who is helping in any way to reduce the unsustainable demands the cattle industry (and our national burger addiction) imposes on our land. Even setting aside the health implications, the ethical and environmental rationales for eating a plant-based diet are strong. Still, if once-disciplined vegans are now scarfing down meat and posting videos of themselves doing bacon taste tests with the reckless abandon of an Amish teen on rumspringa, what does that tell us about the hope for long-term compliance on even less restrictive diets?

All of this got me thinking. It’s been six years since my last deep dive in this magazine into the state of nutrition research. Against the constant hum of the low-fat-vs.-low-carbohydrate diet wars, I wondered, What do we really know now about what we should and shouldn’t be eating?

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It’s fair to ask why nutritionists can’t seem to make up their minds. Are eggs good or bad for us? Will coffee kill us or make us live longer? Should we believe the people pushing a low-carb, high-fat diet, or those extolling the exact opposite? Is it better to join Team Ketogenic or Team Mediterranean?

In fairness, nutrition is a relatively new — and not very well funded — field, so there are substantial built-in limitations. Most of our scientific nutritional advice comes from two types of research. Observational epidemiological studies track the general eating and health habits of hundreds of thousands of people over many years, to see who lives and who dies. (That’s largely how science demonstrated the strong association between smoking and lung cancer.) Randomized control trials attempt to isolate the role of one particular food by comparing small groups of people eating it against those in a placebo/control group.

The most sobering insight I gained during my reporting six years ago came from Stanford University nutrition professor Christopher Gardner, who specializes in randomized trials and confessed to me that he was suffering a professional midlife crisis. The overarching problem is that observational studies are essentially too big and randomized trials are too small. It’s impossible for an observational study to prove that the people enrolled in it lived longer or died sooner because of, say, their coffee intake, versus any number of other dietary, health, or genetic factors. But Gardner told me that when he or other scientists try to isolate the effect of a particular food in a randomized trial, they find “one little thing at a time never makes a difference.”

It would be great if scientists had the funds and stamina to run a host of massive, long-term randomized trials trying to isolate and test every possible dietary combination — and the public had the patience to wait for the definitive findings. Since that’s not going to happen, we’re doomed to more servings of contradictory advice.

If the foundational job of medicine is “first, do no harm,” nutritionists and public policy leaders have often fallen short over the years. A recent paper in the journal Science, written by Harvard researchers Walter Willett and David Ludwig and two colleagues, offers a catalog of mistakes in the history of nutritional advice in this country, going back to a boomlet of fear about diet-related disease following President Eisenhower’s heart attack in 1955. The Surgeon General’s Report on Nutrition and Health in 1988 identified cutting fat from the American diet as the “primary priority for dietary change.”

Three years later, the US Department of Health and Human Services called on the food industry to produce thousands of new “processed food products” that would be low in fat. The industry was only too eager to comply, flooding supermarket shelves with SnackWell’s and other too-good-to-be-true processed foods that typically replaced the fat with heaping amounts of sugar. Our concept of a healthy diet has been continually clouded by industry and advocate profiteering, which has led to boxes of sugary Froot Loops sporting the American Heart Association’s seal of approval and buckets of greasy KFC chicken carrying the imprimatur of the Susan G. Komen breast cancer foundation.

On one level, our war on fat was successful. In the 1970s, the proportion of fat in the average American diet was 42 percent of total calories. Today, it is 34 percent. In a larger sense, it has been an abject failure. While fat consumption has gone down, the percentage of calories from carbs has seen a huge jump. Over the same period, probably not coincidentally, the rates of obesity and diabetes have exploded. As the Science paper points out, that spike has contributed to the first nationwide decrease in life expectancy since the flu pandemic 100 years ago. There’s a lot of harm going around.

What made that Science paper especially interesting is that it was jointly written by experts from different nutrition camps. Dr. Walter Willett, a nutrition professor at the Harvard T.H. Chan School of Public Health and the world’s most cited nutritionist, has overseen the Nurses’ Health Studies since 1980. Tracking the eating and health habits of a quarter-million nurses, it is the granddaddy of observational studies. Willett was furious during the 1980s and ’90s when people demonized good fat — the kind found in avocados, nuts, and olive oil — along with the bad fat in Big Macs and pepperoni pizza. He became an evangelist for the moderate Mediterranean diet: low in saturated fats (but high in healthy fats) and high in whole grain carbs, vegetables, and fruits, with a good amount of protein.

During a recent conversation, Willett assures me he is still very much on Team Mediterranean, even as others who started in his camp have moved away from animal products entirely. Although he agrees that a plant-based diet is best for our environment, he says, “I don’t advise that everybody go on a vegan diet. Veganism is very, very hard to sustain.” Instead, he advises a “flexitarian” diet, which is mostly plant based but allows for some limited animal products. It ends up looking a lot like the Mediterranean diet, just with a few more restrictions.

Dr. David Ludwig, his coauthor and fellow Harvard professor of nutrition, comes at things differently. As the co-director of the New Balance Foundation Obesity Prevention Center at Boston Children’s Hospital and an endocrinologist who has seen tens of thousands of patients, Ludwig says, “You become practical. You basically want something that works.” Doctors “have been telling people to eat less and move more for decades,” he says, yet here we are in the midst of a full-blown obesity crisis.

Although he ate a conventionally healthy diet — low fat, whole grains, fruits and vegetables — during his 20s and 30s, Ludwig found that he was gaining a pound or two every year. By 1995, he was a 37-year-old who had reached the cutoff for being classified as overweight. “As an obesity researcher, it was a little bit of an identity crisis,” he says. After coming across some older studies on the negative effects of carbohydrates, he cut way back on processed carbs and quickly dropped 20 pounds.

Now 61, Ludwig has seen similarly positive results restricting carbs with many of his patients. “We’ve come a long way in the last 20 years to realize that processed carbohydrates seem to have a negative metabolic impact,” he says. Even some unprocessed carbs are traps. When you eat a white russet potato, which he points out digests incredibly fast and has been hybridized to make the perfect french fry, your blood sugar shoots up — almost as if you had just downed a giant Slurpee.

Ludwig now alternates between a moderately low-carb diet and the incredibly low-carb, high-fat ketogenic diet. When carbs are digested into the bloodstream, they turn into a sugar called glucose. Ketosis happens when the brain is deprived of glucose and gets its energy by burning fat stores instead. Famously embraced by techies in Silicon Valley, keto is today’s hottest diet (displacing gluten-free). There are different versions of keto, but all ban carb-rich foods — even whole grain breads, legumes, starchy vegetables, and many fruits — and emphasize high-fat foods, like fatty cuts of meat, avocados, and vegetable oil. Ludwig concedes that when he’s on the keto diet, his meals are less enjoyable. But because the diet gives him greater mental clarity and hunger control, he finds more pleasure during the long stretches between meals.

Both Ludwig and Willett practice what they preach and have been able to maintain their slim frames for decades. So whose diet is healthier?

Dr. Safi Khan was the lead author of a meta-analysis published in the Annals of Internal Medicine this year. It reviewed nearly 300 randomized controlled trials to determine the effect certain diets, vitamins, and supplements had on the risk of heart attack, stroke, and death. What did it find? “Don’t bother with multivitamins,” Khan tells me. “You’re wasting your money.” As for a low-fat vs. Mediterranean diet, the meta-analysis suggested that neither has much of an impact. However, he acknowledges his confidence in that finding is lower than in the one about multivitamins, given the limited availability of good data specific to those diets.

For a true comparison, we’d need a respected researcher to do a rigorous, randomized head-to-head study comparing people eating a high-quality, low-carb diet and people eating a high-quality, low-fat diet. That’s exactly what Christopher Gardner did last year. The Stanford nutritionist — the same one who told me in 2013 that he was suffering a professional midlife crisis — worked with colleagues on a study closely tracking the eating habits of more than 600 people for a full year. Half of the participants were assigned to a low-fat diet, the other half to a low-carb diet. Regardless of their assigned group, all participants were prodded to eat only good stuff: lots of veggies (ideally from farmers markets) and as little added sugar and refined grains as possible. Gardner and his colleagues wanted to see which diet had better results, and to determine if they could predict which would be a better fit based on certain genetic or biological markers.

Here’s what they found: In the low-carb group, some participants lost 40 to 60 pounds while others gained 10 to 20. And in the low-fat group? Just about the same wide variation. Neither diet was better, and researchers had no success in predicting who would do better on one versus the other.

All of this suggests that your co-worker who won’t stop yapping about how keto will change your life might be completely right. Or he could be entirely wrong.

This much we know: No diet will work for everyone.

If this absence of clarity is enough to trigger your own midlife crisis, don’t despair. The good news is that, despite all the disagreements about whose diet is better, there does seem to be an emerging consensus about what the essential components are to any good diet.

Willett, Ludwig, and Gardner all agree on these three recommendations:

1. Dramatically reduce (or eliminate) added sugar.

2. Dramatically reduce (or eliminate) refined grains and processed carbs (and processed meats).

3. Fill your plate with as many vegetables as possible (though not white potatoes!), favoring the green, leafy stuff and other non-starchy varieties as well as those providing healthy fat, like avocados.

If you follow those principles for the bulk of your meals — while also getting regular exercise and good sleep, and of course not smoking — you should be most of the way there. At that point, if you want to join one of the big diet teams, or form your own, have at it. It probably won’t matter much.

In 2011, I read a new book, Why We Get Fat, by Gary Taubes. A prominent science journalist, Taubes has played a major role in spreading awareness about the negative metabolic effects of carbs. Reading the book inspired me to take the carb-free plunge. Although keto was not part of our popular vocabulary back then, I soon found myself in ketosis. People started complimenting me on looking thinner. And the weight continued to peel off. Before long, I had dropped 25 pounds — more than I had wanted. I felt as if I were on one of those alpine slides on the slopes of Vermont and had finally managed to get it cruising at a fast clip, only to somehow forget how to work the brake. At one point, a friend at work with a maternal instinct pulled me aside, and soothingly asked, “Is everything OK?”

I went to my doctor, who gave me two assurances. First, everything was fine. Second, at least some of those pounds would surely be back. He was right on both counts.

This past January — when, by law, we all have to start thinking again about our diets — I decided I wasn’t interested in a magic pound-melter. What I most wanted was something that would be sustainable for me. I cut way back on added sugar and upped my exercise, trying to get in a long, vigorous walk just about every day. I figured if I started walking in the frigid air of winter, I’d have no excuse in the warmth of spring and summer. In time, I lost enough weight so that my clothes felt looser.

At the start of summer, when my family decided they wanted a reset, I was in a good position to up my game. Instead of strict veganism, my wife, who is a professional chef, recommended we go with vegan plus seafood and egg whites for protein. Fresh seafood, which is so plentiful in New England in the summer, would provide a good source of omega-3 fatty acids. And we would pack our plates with lots of fresh veggies while also working in tofu, chickpeas, and other plant-based proteins (so we didn’t overdo it on seafood and run the risk of mercury poisoning).

My three daughters and I signed on, though each of us fine-tuned the diet to suit our particular needs and interests. Although we all reduced our intake of added sugar and processed carbs simply as a byproduct of eating healthier food, I have paid more attention to that than they have. In contrast, they’ve paid more attention to avoiding meat and dairy, while I swap in eggs for egg whites from time to time and occasionally eat meat. In general, though, we’re all trying to be more mindful of what we put into our mouths.

Despite my determination to reduce sugar, I’ll take the real stuff over any of those sugar substitutes that may someday evoke the same what-were-we-thinking response that a can of saccharin-dense Tab now does. (Fun fact: I lost a day of vacation and an absurd amount of money this summer sitting in a Cape Cod animal hospital after our dog got into a bag of full-calorie Godiva truffles made with the sugar substitute xylitol, which is toxic to dogs.)

What have I been eating? For breakfast, I generally have either an egg white omelet or a piece of toasted sprouted-grain bread with a banana and some Teddie chunky natural peanut butter. For lunch, I’ll have a big salad with some kind of protein or dust off my father’s recipe for a great Lebanese lentil dish called mujadara. For dinner, I’ve been motivated to experiment more in the kitchen, producing some tasty dishes like scallops with cilantro and grilled asparagus or clams with peppers and whole wheat linguini.

After a couple of months on this diet, we’ve all been pretty happy with the results. (I don’t weigh myself often, but the last time I did, I had dropped about a dozen pounds since the start of the year.) Most important, we all feel healthier.

What makes me most hopeful is that, for three reasons, it feels more sustainable than other diets. First, it offers enough variety to keep it from becoming boring and enough flexibility to allow each of us to tailor it as we see fit. Second, we’ve made changes to our kitchen and pantry to make it easier for us to make good decisions consistently. When you open our fridge, you now find a tower of Tupperware (which we continually replenish) containing a host of prepped fresh veggies, so we can quickly and easily make a tasty salad — and baby carrots and cut cukes to dip into hummus or salsa rather than using chips. When you open our cupboard, the boxes of crackers and sleeves of cookies have been pushed to the back, fronted by mason jars containing a variety of nuts. We try to keep plenty of avocados on hand — cut one in half, sprinkle some Everything Bagel seasoning over it, and you’ve got a delicious snack. Finally, this diet feels sustainable because we have deliberately set a low bar for special occasions. Is it a holiday or someone’s birthday? Are we on a trip or is a friend visiting from out of town? Or are we just out to eat at a favorite restaurant? If so, everyone is entitled to a hall pass — meats, sweets, whatever you choose. And so far those allowances haven’t derailed any of us from sticking with the program at home.

By embracing the concept of minimum deprivation for maximum benefit, and by building in levers that allow us to celebrate without guilt, I hope we’ll be able to turn this diet into a lifestyle. But the truth is, I have no idea if we’ll be able to stick with it — or if what worked for my family this summer will work for yours.

One of the problems with diets is that there are too many dry drunks around — people who found a way to drop some weight and now want to force everyone else to see the light. To anyone who lectures you with certitude, instead of humility, about what you should be eating, I would invoke the unforgettable words of Freelee the Banana Girl: “Get the [expletive] out of here.”

Neil Swidey is the Globe Magazine’s staff writer. E-mail him at swidey@globe.com. Follow him on Twitter @neilswidey.