Oprah just invested millions in Weight Watchers. But does the program even work?

This week, Oprah Winfrey announced that not only is she a Weight Watchers member, but she likes the company's weight loss program so much she'd decided to buy a 10 percent stake, sending the firm's stock prices soaring.

It seems right to be skeptical at first about Oprah and a diet. For years, the media mogul turned yo-yo dieting into a televised sport, even abstaining from solid foods during one liquid dieting spell and  wheeling out a wagon of fat to demonstrate all the weight she lost. But now, according to her statement, things have changed.

"Weight Watchers has given me the tools to begin to make the lasting shift that I and so many of us who are struggling with weight have longed for," she said.

Weight Watchers has been around for more than 50 years, and unlike many other programs, it actually gets a lot about weight loss right. Though the company's stock prices have taken a hit lately, with the advent of so many free weight loss apps and online networks, the program remains remarkably relevant. Here's what you need to know about the company, its culture, and how Weight Watchers stacks up against the medical evidence.

Group meetings form the core of Weight Watchers

Weight Watchers has a cute origin story: It was started by a Queens, New York, housewife who struggled with her weight. Jean Nidetch, who died this year at the age of 91, just couldn’t slim down no matter how hard she tried. Fad diets, hypnosis — nothing worked. Legend has it that her Achilles heel was chocolate-covered marshmallow cookies and that she took to gorging on them in secret at night.

In 1961, after being mistaken for a pregnant woman, Nidetch decided to gather some friends who were also struggling to stay slim and form a support group. According to the New York Times, the meeting looked something like this:

[Nidetch] invited six friends, all overweight women, to her home for what turned into a group confessional, an exorcism of caloric demons that was the informal beginning of Weight Watchers. They all went on a diet, pledging mutual help through the abysses of anxiety, doubt and gnawing hunger. It worked. They soon brought more overweight friends to the meetings. Within two months, 40 women were attending.

By 1962, Nidetch had lost more than 70 pounds, which she kept off for the rest of her life. Believing in the power of group weight loss, in 1963 she founded the company with her husband and two friends.

In the early days, Weight Watchers members would pay $3 to attend weekly weigh-ins and support meetings. At the gatherings, people would talk about their triumphs and struggles. Group leaders — mostly women who had lost weight on the program — would guide the meetings, offering inspiring tips, tricks, and insights. Members would be advised to keep track of everything they ate in journals, and stick to a diet of lean meat, fish, vegetables, and fruits and avoid sugary and fatty foods, as well as alcohol. It was all so sensible in a diet world that favored liquid and grapefruit diets.

Weight Watchers has since expanded to more than 30 countries, employs around 25,000 people, and has annual revenues of $1.5 billion. Its two biggest shareholders are now Winfrey, who owns 10 percent of the company, and the Artal Group, which owns 51 percent.

Weight Watchers members count "points" to lose weight

Today, the format hasn't changed much — weekly meetings and tracking what you eat remain the program's staples. But there are a few new bells and whistles.

First off, people can now follow the program in new ways, such as joining as online members, where they participate through a Weight Watchers app and have access to "expert chat" support when needed.

In 1997, Weight Watchers introduced the "points" system, which is still used today. It’s basically an easy-to-use calorie counting system that’s a little more sophisticated, since it also takes into account foods' quality. Every food is assigned a value, calculated by an algorithm that accounts for protein, carbohydrates, fat, and fiber. Most fruits and vegetables have zero points, and exercising can earn bonus points that people can swap out for food and drink.

Every member gets daily and weekly points budgets, based on body mass index and weight loss goal. If users stick to their allotted points budget, they lose weight. The hallmark of the system is flexibility: Eat what you want, so long as you stay within your budget.

The reason Weight Watchers has endured: It's not a crash diet

While other weight loss programs have come in and out of fashion, Weight Watchers has endured pretty steadily, and I suspect that has to do with the holistic approach it takes.

The company slogan is, "Stop dieting, start living." Instead of focusing on starvation diets or fad foods, the program is designed to be flexible and adaptable to every user. Keeping a food diary is key, something science has demonstrated is associated with losing more weight in the long term. (One study showed that food diarists lost twice as much weight as those who didn't track.)

The program also emphasizes gradual weight loss (up to 2 pounds a week) and long-term lifestyle and behavior changes. That's where those meetings come in: People can check in, talk about their struggles, and learn from others.

So does Weight Watchers work?

Weight Watchers bills itself as being science-based, and the program actually doesn't conflict with any medical advice. It's sensible, and generally follows what a smart doctor would tell you about losing weight, which is probably why it's been endorsed in a number of studies as being one of the higher-quality branded diets out there.

But that doesn't mean it'll work miracles. This 2009 study compared dieters on a bunch of programs — including Weight Watchers, Atkins, and SlimFast — and found no marked differences between them after six months.

In a 2015 review of studies on Weight Watchers (and other diets), researchers found people on the program lost 2.6 percent more weight after a year than those assigned to control groups, which for many people would amount to a couple of pounds. (Dieters on Jenny Craig actually had a little more success, losing about 5 percent more than control groups.)

In 2014, researchers looked at the best available studies on the Atkins, Weight Watchers, and Zone diets and found that study participants on all programs lost a comparable amount of weight — about 5 to 7 pounds over the course of a year — but then tended to regain some in the long term.

"Despite millions of dollars spent on popular commercial diets, data are conflicting and insufficient to identify one popular diet as being more beneficial than the others," the researchers wrote.

So the problem with Weight Watchers isn't really a problem with the diet; it's the fact that weight loss is really difficult no matter what you do.

In many ways, the program actually reflects what researchers think is the best way to lose weight: cutting calories in a way that you like and can sustain, tracking what you eat, and focusing on eating more healthfully. If you're the type of person who would find group support and the accountability of weekly meetings helpful, Weight Watchers might be a good fit for you.

Just manage your expectations and be prepared to fork out about $400 for a year of membership at a time when there are many other calorie-tracking services similar to those by Weight Watchers — except online and for free.

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