In the mid-1990s, when the low-fat diet was king, a young obesity researcher began to have his doubts.

Why, Dr. David Ludwig wondered, did his patients almost invariably gain weight on state-of-the-art diets? Why was he himself gaining weight? Ludwig started spending his nights and weekends in the library, digging through long-forgotten studies and formulating a theory that some of the very foods that diet experts were touting — bread, pasta, cereal — might prime the body for blood sugar swings, hunger pangs and fat storage.

Then, he tried out his theories on his first human test subject: himself.

“I started eating way more fat, cutting back on processed carbs. I ate a little bit more protein but really made no attempt to lose weight,” he said.

“The first thing I felt was a surge of energy. I used to feel in the middle of the afternoon this desperate hunger, but now I would feel satiated after eating and just a sense of energy and well-being. It was wonderful. Three months later, I'd lost about 20 pounds.”

That experience put Ludwig, now a researcher at Boston Children's Hospital and a professor at Harvard Medical School, on the long path to writing his new book, “Always Hungry? Conquer Cravings, Retrain your Fat Cells and Lose Weight Permanently,” an Amazon and Publisher's Weekly best-seller.

“The mantra is forget calories, focus on food quality and let your body do the rest,” he said.

There are no calorie restrictions in his eating plan, which includes fats such as nuts, full-fat dairy and olive oil, and dishes such as steak salad with blue cheese and coconut curry shrimp, as well as vegetarian alternatives. In the first stage of eating, you get about 50 percent of your calories from fat. Later that drops to 40 percent.

The plan initially eliminates processed carbohydrates such as bread and pasta, then phases them in cautiously.

“Our fat cells are triggered by [a] low-fat, highly processed carbohydrate diet, largely through raised insulin levels,” Ludwig said in a phone interview.

“I call insulin the Miracle-Gro for your fat cells. Insulin programs your fat cells to suck up and store too many calories, so there are actually just too few [calories] for the body, and the brain senses starvation.”

When that happens, the brain puts out a call to eat something — preferably one of those highly processed starchy foods that will raise your blood sugar fast — and if you obey, the cycle starts again. Add to that the calorie restriction that slows down your metabolism and makes you hungry, and, Ludwig said, you start to see why so many diets eventually fail.

Ludwig marshals an impressive array of studies to back his claims, including a high-profile 2012Journal of the American Medical Association study he co-authored, in which dieters on a low-carb diet burned about 325 more calories a day than dieters on a low-fat diet.

He also points to the PREDIMED study, published in 2013 in the New England Journal of Medicine. About 7,500 adults with heart disease risk factors were assigned to three diets: Mediterranean with lots of olive oil, Mediterranean with lots of nuts, or low-fat.

“They had to stop the study early because heart disease rates in the higher fat groups dropped so fast it would have been unethical to keep the control group eating the low-fat diet,” Ludwig said.

A leader in the weight-loss field who has published dozens of studies, Ludwig has seen a lot of changes since the mid-1990s, when he couldn't get government funding for his then-radical research.

Even Weight Watchers, that venerable pillar of American calorie-counting, has done away with formal limits on fresh fruit. Still, debate continues, with some prominent diet researchers saying that restricting calories remains very important.

“It's not case closed,” Ludwig said. “We just need to look at the latest USDA dietary guidelines. The first recommendation is balance your calories in and calories out.”

But the guidelines also give Ludwig's allies a boost, lifting a formal limit on cholesterol intake that's been in place for decades and placing a new limit on sugar.

The “Always Hungry?” eating plan begins with a two-week “boot camp” in which you refrain from eating added sugar, grains and refined carbohydrates. The theory is that by decreasing insulin levels and chronic inflammation, you can reprogram your fat cells, which will then release excess calories.

“Hunger diminishes, cravings subside, metabolism speeds up, and you lose weight naturally,” Ludwig writes.

There's a detailed meal plan for the “boot camp” stage, with a typical day including a breakfast of black bean tofu hash, a cold-cut lettuce boat snack with lemon tahini sauce, a steak salad with blue cheese at lunch, a trail-mix snack, and a dinner of broiled fish with creamy dill sauce, salad, sauteed kale and poached fruit with chocolate sauce.

After first trying this kind of eating when he experimented on himself in the mid-1990s, Ludwig decided to move from animal research to human research.

“I decided it was my life's mission to study this and not in the basic laboratory with mice, but with humans, doing clinical research and translating this in the clinic, with patients,” he said.

Today, he said, there's rising interest in his approach to weight loss among researchers and — perhaps to a greater extent — among members of the general public.

“Sometimes the public leads and then the leaders follow,” he said. “If people find that they can control their cravings and hunger better this way, that they feel better, that they start to lose weight and their cardiovascular risk factors decline, that's going to change the food industry. We vote with our ballots, but we also vote with our forks. The food industry takes note if we stop buying processed carbs and start asking for more whole foods, including the rich, higher-fat foods.”

nschoenberg@tribpub.com

Twitter @nschoenberg