When Janet Holwell first joined Weight Watchers seven years ago, she lost 43 pounds in one year and considered the popular commercial weight-loss plan “miraculous.”

“I felt like I had found the magic key, the secret that eluded me all of these years,” said Ms. Holwell, who has maintained most of her weight loss by continuing to adhere to the program.

But the magic disappeared when Weight Watchers overhauled its weight-loss plan little over a year ago. Under the new system, called Points Plus, Ms. Holwell, has not been able to lose the five pounds she recently gained.

“It just doesn’t work for me,” said Ms. Holwell, 61, a research consultant who attends weekly Weight Watchers meetings in Middle Village and Glendale, Queens.

Millions of people around the world belong to Weight Watchers International, ranked best commercial diet plan by U.S. News & World Report last year, and even nonmembers look to it for guidance and recommendations. It is best known for its points system, which assigns specific values to different foods and permits each member a daily allotment. At its weekly group meetings, healthy eating and exercise are emphasized over rapid-fire results.

The latest iteration of the weight-loss plan, called Points Plus, was intended to steer people toward more healthy food choices, encouraging people to eat more fresh fruits by giving them zero points, as most vegetables already were. But many longtime members who were familiar with the earlier plan, like Ms. Holwell, have been grumbling about slow weight loss under the revised plan.

“I have been doing Points Plus for about a month and keep gaining and losing the same few pounds,” a commenter at one weight-loss Web site complained shortly after the new plan was introduced. Others chimed in to reassure her she was not alone.

In December, in a move that seemed to acknowledge the difficulty many dieters were having with the new system, Weight Watchers recommended that all members consider reducing their daily food intake, or points allotment, by 10 percent, not counting fruits and vegetables. (For those who’ve missed a few meetings, that means most women might cut their daily Points Plus allotment to 26 per day, down from 29.)

Although Weight Watchers officials say the change in points allotments was optional — that members could adjust their daily points up or down — and insist that it was not a response to members’ failure to lose weight, many longtime members unhappy with the newer plan say they feel vindicated. “I think they miscalculated,” Ms. Holwell said.

Many members said they were not given a choice. “One day we came in and they said there were changes, and suddenly I had 26 points,” said a member in New York City who asked that her name not be published to avoid alienating those in her group.

Company officials insist that the only reason Weight Watchers modified the plan was because they had become convinced members were getting more than adequate nourishment under the new plan and would not be harmed by eating less.

“We chose to be conservative when we introduced the plan, because we wanted to make sure that the things we stand for, nutritional health and well-being, weren’t going to be compromised,” said Karen Miller-Kovach, a registered dietitian who is chief scientific officer of Weight Watchers.

Still, she said the company had been following the progress of members who use online tracking tools and had found that dieters have been gaming the new system. “People were having to circumvent the system in order to lose weight at a healthy rate,” she said.

Judy Weinstein, a Manhattan opera director in her 50s, has attended Weight Watchers meetings regularly for nearly eight years and is very committed to the program. But while she found it enormously helpful when she first joined, losing 33 pounds, she has had less success with Points Plus.

So six months ago, she committed what was once the ultimate Weight Watchers no-no and cut her own points allotment. That Weight Watchers has now suggested this for all members, but as an option instead of providing clear guidance, disturbs her.

“That’s not really helpful,” Ms. Weinstein said. “People wouldn’t be here if they could do it on their own.”

Fruit has been a particular conundrum for dieters on the new plan. As fresh fruit “costs” zero points, dieters can have as much as they’d like, “within reason,” Ms. Miller-Kovach said. Many members dislike the vagueness of this recommendation, since they tend to overeat when left to their own devices. But people who are overweight did not become fat because they binged on fresh fruit, said Elizabeth Josefsberg, who leads meetings in New York City.

“You know how it is with a cookie — you want six cookies,” she said. “When you finish a banana, you don’t say, ‘Gosh, I want another banana.’ ”

Other experts are less sanguine. “No single dietitian I know would count fruit as a ‘free’ food if someone is on a diet and trying to lose weight. You have to account for it,” said Marjorie Nolan, a New York City dietitian who speaks on behalf of the Academy of Nutrition and Dietetics. She expressed surprise that even bananas (which used to cost two points under the previous Weight Watchers plan) are zero points.

“That just doesn’t make sense,” she said. “They’re a denser fruit.”

But Dr. Jeffrey Mechanick, vice president of the American Association of Clinical Endocrinologists, said there was no evidence that indulging in fruit impedes weight loss.

The reason fruit and most vegetables are zero points is that the formula actually “prepays” those points, Ms. Miller-Kovach said; the daily points allocation already includes an allowance for what the plan has determined is a typical daily consumption of fruits and vegetables. Ms. Miller-Kovach said she could not divulge the number of fruits and vegetables used in the calculation because the information is proprietary and not revealed even to participants.

But since average consumption of fruit is low in this country, usually not reaching the five to nine servings of fruit and vegetables a day recommended by government health experts, the prepayment may underestimate the effect of more liberal fruit consumption on waistlines.

Results of randomized clinical trials of the new Points Plus program have not been published in peer-reviewed journals. But two brief reports have been presented at scientific meetings on obesity, and the authors found no difference in weight loss between the old and new points plans.

In one of those studies, participants lost an average of 8.2 pounds over 12 weeks and saw significant improvements in their total cholesterol and triglyceride measures. But only 111 overweight adults completed the 12-week trial, and only 55 people were following the new Points Plus program.

Weight Watchers officials said the number of participants was sufficient to provide statistical proof that the new diet system works.

The new Points Plus plan also was evaluated in an earlier unpublished trial, Ms. Miller-Kovach said. And Weight Watchers has been following more than 12,000 members in Germany since the introduction of the new Points Plus program there. So far no differences in weight loss have been found between users of the new and old programs, she said.

The transition to a new system seems to have been traumatic for many members. Ms. Holwell is optimistic that the plan will work again for her but wonders now if it will need further revision.

“The jury is still out on the 26 points,” she said.

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This post has been revised to reflect the following correction:

Correction: February 8, 2012

The Consumer report on Tuesday, about criticism of changes to the Weight Watchers weight-loss program, described the government’s recommendation for daily produce consumption incorrectly. The five to nine servings that people are advised to eat daily involves fruit and vegetables, not just fruit.