The National Weight Control Registry's Shaky Science

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I was just tagged into a Facebook conversation where someone was claiming that the findings of the National Weight Control Registry (NWCR) “prove that the claim that weight loss fails about 95% of the time is absolutely false,” and “show how to lose weight and keep it off”. I’ve been hearing this for a lot of years, and I was sad to find that it’s still going around, so let’s take a closer look at the NWCR and their findings.

They call themselves “the largest prospective investigation of long-term successful weight loss maintenance” so let’s examine that:

In order to be a “success” by the NWCR’s definition, one must lose 30 pounds (which they consider a “significant amount of weight” regardless of the starting weight of the participant) and maintain the weight loss for one year. It’s worth noting that most people gain their weight back in years 2-5, so, following in the footsteps of many studies funded by the diet industry, the NWCR has given themselves a four-year efficacy cushion for initial registration. Also, all of the information is self-reported.

So, how many “successes” do they have? They started in 1994 and currently, their home page says they are following “over 10,000 individuals.” (Interestingly, I originally wrote about this in December of 2012 and at that time they were following… “over 10,000 individuals.”) So after 28 years they have “over 10,000” successes (and, remember, they are “the largest prospective investigation of long-term successful weight loss maintenance.”)

The first question is, do those 10,000 successes disprove the idea that weight loss attempts fail 95% of the time?

Thinking back to what my freshman Research Methods professor would have prompted me to ask, my first thought is to get a sense of proportion… “over 10,000” successes out of how many attempts? Finding the exact number of dieting attempts between 1994 and now is pretty difficult, but we can make some estimates just to get a sense of the situation. In researching how many weight loss attempts have been made since 1994, I found estimates ranging from 45 million to 80 million attempts per year. 

To be extra fair to the NWCR, let’s take the lowest number and go with 45 million weight loss attempts per year, and let’s do the calculation excluding both 1994 and 2022. So for 1995-2021, that would be 1,215,000,000 weight loss attempts. By this estimate, in order for the NWCR to mathematically disprove the idea that weight loss fails about 95% of the time, they would need somewhere in the neighborhood of 60,750,000 successes. As a reminder, they are tracking “over 10,000.”

Now, this is complicated by the fact that we don’t know how many of those dieting attempts year after year are the same people making multiple attempts. So let’s give the NWCR the absolute benefit of the doubt and say that there have only been 45 million total weight loss attempts since 1994. Even if that were the case, in order to show a 5% success rate mathematically, they would still need 2,250,000. Again, they are tracking “over 10,000.”

All of this to say, I don’t think that this comes close to disproving the claim that weight loss fails about 95% of the time.

But what about the claim that the NWCR findings “show how to lose weight and keep it off” Here, if it’s possible, the science actually gets worse. First of all, their “research findings” are based on the aforementioned tiny percentage of dieters they are following.

Here are their top-level findings:

• “98% of Registry participants report that they modified their food intake in some way to lose weight.

• 94% increased their physical activity, with the most frequently reported form of activity being walking.

Those who maintain weight loss:

• 78% eat breakfast every day.

• 75% weigh themselves at least once a week.

• 62% watch less than 10 hours of TV per week.

• 90% exercise, on average, about 1 hour per day”

I want to point out that these are the exact same numbers they were posting when I originally wrote about this in 2012, which means that either their sample is remarkably consistent, or this hasn’t been updated in at least a decade. Also “modified their food intake in some ways” and “increased their physical activity” is not exactly penetrating specificity in terms of a replicable intervention.

Moving past that and returning to the hallowed halls of Research Methods 101, the question my professor would have insisted I ask first is, again, about comparison. They have only collected information from 10,000ish people, which means they collected no information from the vast majority of dieters. How many of the millions of dieters who failed at long-term, significant weight loss since 1994 engaged in these exact same behaviors?

For example, they found that 78% of their “successes” eat breakfast every day. But we have no idea if that is a significant finding, and if eating breakfast actually had anything to do with their weight loss maintenance. For all we know, 88% of those who failed to maintain weight loss ate breakfast every day. Without information from the vast majority of people, the information they have from a relative handful is not particularly useful. Looking at their list, it’s not surprising that a group of people trying to lose weight would have high rates of these behaviors, since these behaviors are what people are commonly told to do in order to lose weight. That doesn’t mean they actually work. The NWCR is just collecting commonalities in a non-random sample of (what a large body of research suggests are) outliers.

In summary, to suggest that the National Weight Control Registry’s findings disprove the statement that weight loss attempts fail about 95% of the time, or that they show how to achieve significant long-term weight loss is an unsupportable conclusion based on basic math and science.

I think the research suggests that weight-neutral interventions provide more benefits with less risk.

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*Note on language: I use “fat” as a neutral descriptor as used by the fat activist community, I use “ob*se” and “overw*ight” to acknowledge that these are terms that were created to medicalize and pathologize fat bodies, with roots in racism and specifically anti-Blackness. Please read Sabrina Strings: Fearing the Black Body – the Racial Origins of Fat Phobia and Da’Shaun Harrisons Belly of the Beast: The Politics of Anti-Fatness as Anti-Blackness for more on this.