Last summer, I was walking down Harvard Street in Brookline, near Boston’s major hospitals, and was arrested by the sight of a figure sitting backward on a park bench, stripped to the waist, his legs and arms protruding from the rails. He was speaking, though not audibly to passersby, and gesticulating. On slightly closer examination I saw that it was David Ludwig, a professor at the Harvard Medical School whose work on obesity at Children’s Hospital, including with diabetic adolescents, I knew and respected. He was practicing a talk, Ludwig told me, on his new diet book, Always Hungry?, for his publisher’s sales team. He recapped some of the good news: limitless portions of nuts and full-fat yogurt, a permanent goodbye to counting calories, pleasant satiety at all times. I, of course, liked the sound of it. If I didn’t need to keep track of calories and could have unlimited, well, anything other than fruits and vegetables, which I already considered fair game, I’d be happy—though I will never give up hot fudge.

Whatever he was practicing worked. In January, the month of resolutions, Always Hungry? made its debut in the third position of The New York Times best-seller list’s how-to category, and it stayed ahead of weight-loss books including The Shred Power Cleanse and celebrity chef Rocco DiSpirito’s Negative Calorie Diet.

The truth is it’s not so hard to write a best-selling diet book: Give people a colorful, or doable, or sexy way to cut calories and they’ll lose weight. You could write yourself a diet right now, and if you stuck to it for two weeks you’d almost certainly lose weight. The key, of course, is sticking to any diet after the weight drops. Diet books begin to disappear from best-seller lists around the time when most New Year’s resolvers fall off the wagon. Diets are annoying to stick to, particularly ones that restrict calories. You get hungry. No matter how permissive a diet is about seemingly sinful foods (today, usually meat), if the deprivation of forbidden foods (pasta, bread) doesn’t get to you, the monotony will. Thus the need for more and more diets, like the tiresome, ridiculous, not-founded-in-historical-data (cavemen ate carbs!) Paleo.

Ludwig is promising an end to hunger, the kind of promise you have to make to sell diet books. What makes his plan different is that Ludwig isn’t a celebrity chef; he’s a pediatric endocrinologist who has been at the forefront of diabetes research and the current anti-sugar, anti-carbohydrate wave.

The questions of what matters in dieting, cutting carbs or counting calories, and whether or not fat needs to be restricted, are not settled. If they were, soda companies wouldn’t be on the ropes; government dietary guidelines wouldn’t need to change every five years and provoke arguments upon release; and publishers would be even poorer than they are now.

Illustration by Kuba Kołodziejak.

Ludwig is part of a school that includes his homonymic colleague, Robert Lustig, at the University of California, San Francisco, and the science journalist Gary Taubes, author of Good Calories, Bad Calories and The Case Against Sugar, to be published this year. Their view is that refined carbohydrates directly cause fat to accumulate—a step farther than the long-standing sugar rush, sugar drop phenomenon measured in foods high on the “glycemic index,” familiar for demonizing white potatoes. This school vehemently objects to simply counting calories as a way to diet effectively. It’s not just the number of calories that cause weight gain, its members say—the problem is hormonal, provoked by the kind of calories you eat. If the calories come from refined carbohydrates—not just sugar but grains without the bran and fiber in the kernel—they will wreak far more damage than the simple number of calories would suggest. White rice and white bread, dolefully even your favorite artisanal baguette, are the same as the dollop of apricot jam on top (though there’s good news about the schmear of butter or cream cheese). And it gets worse: Most “whole wheat” bread includes wheat berries so pulverized that they count as refined grains. All of this causes the overproduction of insulin, which directs fat cells to hoard calories, depriving the body of energy that unimprisoned calories could provide. This makes you both fat and hungry. Obesity, Ludwig says, is a “state of starvation.”

Conversely, the relatively high calories of full-fat foods will even themselves out; they will promote satiety (though the 20 or so minutes it takes for the brain to register satiety can be a dangerous window) and put a brake on the amount you consume. More important, your body will be able to process and use the calories as energy, rather than misdirecting fat cells to hoard them because of an excess of insulin.

When it comes to weaponizing the anti-sugar, don’t-count-calories school of thought, shadings vary. Lustig says that eating the vaguely defined “real food,” meaning processed as little as possible (he’s in the Bay Area, where these terms carry currency), and cutting out added sugar, will solve everything. He points to evidence that the saturated fat in dairy might protect against diabetes and heart disease, but stops short of blanket recommendations of dairy or red meat (“I’m not a diet guru or a drug pusher”), focusing instead on battling sugar. In a study of which he was lead author in the journal Obesity last fall, obese children’s insulin, cholesterol, and blood pressure improved in just ten days when the added sugar in their diets was replaced by starch (bagels, not doughnuts—“not good” or high-fiber food, Lustig told me, but without the sugar). Taubes doesn’t buy evidence against red meats and says he has yet to be convinced against the original Atkins approach, with its permissiveness about red meat; Ludwig is more cautious and hedges his bets by favoring full-fat dairy over red meat. (His Harvard School of Public Health colleague, Walter Willett, a voice in the wilderness promoting 40-percent-fat Mediterranean diets in the height of the low-fat storm, is still adamantly against red meat.) Everyone seems to like, in fact love, dark chocolate; Ludwig eats low-sugar dark chocolate every day, because “it’s a great way to increase fat intake,” and, I suspect, because he wants to.

Dariush Mozaffarian, long an associate professor at the Harvard School of Public Health and now dean of the Gerald J. and Dorothy R. Friedman School of Nutrition at Tufts, agrees with his former colleague Ludwig that “it’s totally correct not to count calories.” He adds two more considerations to Ludwig’s, neither of which concern sugar and insulin. One is altering intestinal bacteria in the now-fashionable microbiome, which translates to eating the probiotics in yogurt, which he says helps rats maintain weight even without reducing calories. (Although he believes the long-term studies showing that people don’t and can’t stick to low-fat diets, he says he hasn’t seen enough evidence that whole-fat diets are widely beneficial to health.) Another is the idea that the brain’s pleasure centers can be retrained with time and consistent diets, as his Tufts colleague Susan B. Roberts showed over six months of comparing fMRIs in a small number of obese patients who stuck to a more-healthful diet against others who did not. The foods like french fries and fried chicken that lit up the pleasure centers of both groups at the start dimmed over time for the dieters, and the foods like whole-wheat pasta and other foods on their diet burned brighter.

Consistency is likely the key in all of this, says William H. Dietz, one of the country’s authorities on obesity, for 15 years the director of the obesity program at the Centers for Disease Control and now director of the Sumner M. Redstone Global Center for Prevention and Wellness at the Milken Institute School of Public Health at George Washington University. Dietz is a longtime believer in the importance of “energy balance,” meaning that unless you put out more energy than you take in you won’t lose weight—the simple physics of calories in, calories out. But he’s also a pragmatist who says the problem is less the “provocative hypothesis” needed to sell diet books than the practicality of sticking to a diet. For him, variety may be the spice of the life but it’s also the enemy of successful weight maintenance: When the foods in front of you are unfamiliar, he says, you lose your cues of what and how much will sate you. You also buy more, of course, which accounts for food manufacturers’ constant brand extensions. In this view, monotony is the dieter’s friend, though he doesn’t say it quite that way—he prefers consistency. His own diet involves sticking to the same cereal and fruit at breakfast, sandwich and pretzel chips at lunch, and protein, salad, and sometimes bread at dinner.

That’s where I’ll land, and likely you will too. As Dietz points out and as Ludwig’s diet (not to mention behavioral psychology) is premised on, once something becomes routine you crave it less—including food that was once forbidden. The cookie jar in our house is now filled with the nuts on which Ludwig declares open season. I’m trying every full-fat grass-fed straight-from-the-farm yogurt I find. And did I mention cheese? If I follow Always Hungry’s directions, I’ll pair it with a tablespoon of butter, oil, or mayonnaise, one or two tablespoons of nuts, or a quarter avocado at lunch. I’m even allowed ice cream. As for the hot fudge—I plan to apply to Susan Roberts for brain retraining.