Marilyn Diamond, Who Wrote a Blockbuster Diet Book, Dies at 81

Marilyn Diamond, who with her husband at the time, Harvey Diamond, wrote a blockbuster 1985 diet book, “Fit For Life,” which attracted millions of adherents to their fruit-and-vegetable-based regimen but which also drew sharp criticism from the medical establishment, died on Sunday in Roanoke, Va. She was 81.

Her daughter, Lisa Lusk, said the death, at a care facility, was caused by complications of dementia.

“Fit for Life,” which spent some 35 weeks on the New York Times best-seller list, promoted a diet rich in fruits and vegetables with little or no meat, a regimen that nutritionists have long embraced. The book also presented ideas that were ahead of their time, like veganism, caloric restriction and drinking oat and nut milk to replace dairy.

Above all, the Diamonds emphasized the importance of diet to a person’s overall health; until then, dieting had primarily been seen as a means for weight loss.

The couple also promoted the notion that certain foods should not be eaten together — for example, proteins and carbohydrates — because, they wrote, they “cannot digest efficiently in the stomach at the same time.”

Among their most contentious ideas was “natural hygiene” — that with a proper diet the body, when afflicted by disease, could heal itself without the need for medical intervention.

The book offered “a dangerous, unbalanced, protein-deficient diet, based on pseudoscientific nutritional principles,” Theodore Berland, a diet expert and a former president of the American Medical Writers Association, told The Ottawa Citizen in 1986.

For better or worse, many of the book’s ideas, like drinking raw cow’s milk or avoiding cooked foods, have become popular tenets in some parts of today’s wellness movement. So has the couple’s anti-establishment stance: They insisted that their credentialed critics were beholden to big business and intent on keeping Americans improperly nourished.

“The Diamonds display the same hatred toward health professionals that Al Capone had for lawmen,” one of their many critics, the hematologist Victor Herbert, wrote in 1988.

Such criticism did not stop millions of Americans from following “Fit for Life.” The couple became frequent guests of daytime television talk shows, like Oprah Winfrey’s and Merv Griffin’s. Mr. Griffin said he lost 25 pounds on the diet.

The Diamonds wrote two more books, “Living Health” (1987), a cookbook, and “Fit For Life II” (1989). They were early proponents of healthy cooking demonstrations — now common — on live TV.

They also hit the seminar circuit, recruiting a young Tony Robbins to assist. (Mr. Robbins, who became one of America’s best-known self-help speakers, died in February.)

Mrs. Diamond was born Marilyn Martha Horecker on June 4, 1944, in Washington and raised in the Maryland suburbs and then Westchester County, N.Y.

Her father, Bernard Horecker, was a biochemist who became dean of the Weill Cornell Graduate School of Medical Sciences in Manhattan; her mother was Frances (Goldstein) Horecker.

Marilyn married Peter Neuwirth in 1965, while she was a student at New York University. She graduated with a degree in romance languages in 1968.

The couple lived on Long Island before moving to the Los Angeles area, divorcing in 1973.

She met Mr. Diamond while shopping at an organic food store. They married in 1976.

The Diamonds bonded over their shared views about traditional diets and the medical establishment: She said she had bad knees because of dairy products that had “leached calcium” from her bones, while he blamed doctors for his father’s death in a hospital.

At the time, Mr. Diamond had already been developing his career as a nutrition expert. He had a doctoral degree from the American College of Life Science, an unaccredited institution based in Austin, Texas.

In 1994, Mrs. Diamond married Donald Schnell, who had an interest in alternative medicine and taught people how to walk across hot coals.

Together they wrote “Fitonics for Life” (1996), which emphasized what they called “high-energy eating”; in contrast to Mrs. Diamond’s earlier advice, the new book endorsed eating dairy and meat within a balanced diet.

In 2013, she and Mr. Schnell published “Young for Life: The Easy No-Diet, No-Sweat Plan to Look and Feel 10 Years Younger.” It sold well, though not nearly as well as “Fit for Life.”

Mrs. Diamond and Mr. Schnell divorced in 2014.

Along with her daughter, from her first marriage, she is survived by her sons Greg Neuwirth and Beau Diamond; her sisters Linda Lally and Doris Colgate; and four grandchildren.