https://www.theatlantic.com/magazine/archive/2023/05/ice-cream-bad-for-you-health-study/673487/?

NUTRITION SCIENCE’S MOST PREPOSTEROUS RESULT

Studies show a mysterious health benefit to ice cream. Scientists don’t want to talk about it.By David Merritt Johns

APRIL 13, 2023, 7 AM ETSHARE

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Last summer, I got a tip about a curious scientific finding. “I’m sorry, it cracks me up every time I think about this,” my tipster said.

Back in 2018, a Harvard doctoral student named Andres Ardisson Korat was presenting his research on the relationship between dairy foods and chronic disease to his thesis committee. One of his studies had led him to an unusual conclusion: Among diabetics, eating half a cup of ice cream a day was associated with a lower risk of heart problems. Needless to say, the idea that a dessert loaded with saturated fat and sugar might actually be good for you raised some eyebrows at the nation’s most influential department of nutrition.

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Earlier, the department chair, Frank Hu, had instructed Ardisson Korat to do some further digging: Could his research have been led astray by an artifact of chance, or a hidden source of bias, or a computational error? As Ardisson Korat spelled out on the day of his defense, his debunking efforts had been largely futile. The ice-cream signal was robust.

It was robust, and kind of hilarious. “I do sort of remember the vibe being like, Hahaha, this ice-cream thing won’t go away; that’s pretty funny,” recalled my tipster, who’d attended the presentation. This was obviously not what a budding nutrition expert or his super-credentialed committee members were hoping to discover. “He and his committee had done, like, every type of analysis—they had thrown every possible test at this finding to try to make it go away. And there was nothing they could do to make it go away.”

Spurious effects pop up all the time in science, especially in fields like nutritional epidemiology, where the health concerns and dietary habits of hundreds of thousands of people are tracked over years and years. Still, the abject silliness of “healthy ice cream” intrigued me. As a public-health historian, I’ve studied how teams of researchers process data, mingle them with theory, and then package the results as “what the science says.” I wanted to know what happens when consensus makers are confronted with a finding that seems to contradict everything they’ve ever said before. (Harvard’s Nutrition Source website calls ice cream an “indulgent” dairy food that is considered an “every-so-often” treat.)

“There are few plausible biological explanations for these results,” Ardisson Korat wrote in the brief discussion of his “unexpected” finding in his thesis. Something else grabbed my attention, though: The dissertation explained that he’d hardly been the first to observe the shimmer of a health halo around ice cream. Several prior studies, he suggested, had come across a similar effect. Eager to learn more, I reached out to Ardisson Korat for an interview—I emailed him four times—but never heard back. When I contacted Tufts University, where he now works as a scientist, a press aide told me he was “not available for this.” Inevitably, my curiosity took on a different shade: Why wouldn’t a young scientist want to talk with me about his research? Just how much deeper could this bizarre ice-cream thing go?

“istill to this day don’t have an answer for it,” Mark A. Pereira, an epidemiologist at the University of Minnesota, told me, speaking of the association he’d stumbled upon more than 20 years earlier. “We analyzed the hell out of the data.”

Just that morning, I’d been reading one of Pereira’s early papers, on the health effects of eating dairy, because it seemed to have inspired other research that was cited in Ardisson Korat’s dissertation. But when I scrolled to the bottom of Pereira’s article, down past the headline-making conclusions, I saw in Table 5 a set of numbers that made me gasp.

Back then, Pereira was a young assistant professor at Harvard Medical School. Hoping to address the newly labeled epidemics of obesity and diabetes, he initially focused his research on physical activity, but soon turned to the unsettled science of healthy eating. The status of dairy, in particular, was bogged down in simplistic and competing assumptions. “We just thought, Oh, you know, calcium and bones: It’s good for kids. But, oh, the saturated fat! Don’t eat too much dairy! ”

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Pereira and his co-authors tested these old ideas using data from a study, begun in 1985, that tracked the emergence of heart-disease risk factors in more than 5,000 young adults. After seeing the results, “we knew it was going to be very high-profile and controversial,” Pereira recalled. Pretty much across the board—low-fat, high-fat, milk, cheese—dairy foods appeared to help prevent overweight people from developing insulin-resistance syndrome, a precursor to diabetes. “I’ll tell you, this study surprised the heck out of me,” said one CNN correspondent, as Pereira’s study spiraled through the press.The Harvard group didn’t like the ice-cream finding: It seemed wrong.

But the international media coverage didn’t mention what I’d seen in Table 5. According to the numbers, tucking into a “dairy-based dessert”—a category that included foods such as pudding but consisted, according to Pereira, mainly of ice cream—was associated for overweight people with dramatically reduced odds of developing insulin-resistance syndrome. It was by far the biggest effect seen in the study, 2.5 times the size of what they’d found for milk. “It was pretty astounding,” Pereira told me. “We thought a lot about it, because we thought, Could this actually be the case? ”

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There were reasons to be wary: The data set wasn’t huge, in epidemiological terms, and participants hadn’t reported eating that many dairy-based desserts, so the margin of error was wide. And given that the study’s overall message was sure to attract criticism—Pereira recalled getting “skewered” by antidairy activists—he had little desire to make a fuss about ice cream.

Pretty soon, Pereira’s peers found themselves in the same predicament. Building on the 2002 study and the growing interest in dairy, researchers at the Harvard School of Public Health decided to break out some of their most powerful tools. Since the 1980s, Harvard’s scientists have been collecting “food-frequency questionnaires” and medical data from many thousands of nurses, dentists, and other health-care workers. These world-famous studies have fueled a stream of influential findings, including some of the data that sparked the removal of trans fats from the food supply.

The results of Harvard’s first observational study of dairy and type 2 diabetes came out in 2005. Based on data collected from just one of their three cohorts, following men between 1986 and 1998, the authors reported that higher dairy intake, and higher low-fat-dairy intake in particular, was associated with a lower risk of diabetes. “The risk reduction was almost exclusively associated with low-fat or non-fat dairy foods,” a Harvard news bulletin explained. An article on Fox News’s website underscored the low-fat message: “There was no decrease in men who drank whole milk,” the story said.

Perhaps not whole milk, but what about butter pecan? Near the end of the Harvard paper, where the authors had arrayed the diabetes risks associated with various dairy foods, was a finding that was barely mentioned in the “almost exclusively” low-fat narrative given to reporters. Yes, according to that table, men who consumed two or more servings of skim or low-fat milk a day had a 22 percent lower risk of diabetes. But so did men who ate two or more servings of ice cream every week. Once again, the data suggested that ice cream might be the strongest diabetes prophylactic in the dairy aisle. Yet no one seemed to want to talk about it.

In the years that followed, research summaries generally agreed that high dairy intake overall was associated with a slightly reduced risk of diabetes, but called for more investigation of which specific dairy foods might have the greatest benefits. In 2014, Harvard’s nutrition team brought another dozen years of diet-tracking data to bear on this question. In this new study, total dairy consumption now seemed to have no effect, but the ice-cream signal was impossible to miss. Visible across hundreds of thousands of subjects, it all but screamed for more attention.

Following a pattern of incredulousness that was by then more than a decade old, Frank Hu, the study’s senior author and the future chair of Harvard’s nutrition department, asked the graduate student who’d led the project, Mu Chen, to double-check the data. “We were very skeptical,” Hu told me. Chen, who is no longer in academia, did not respond to interview requests, but Hu recalled that no errors in the data could be found.

the harvard researchers didn’t like the ice-cream finding: It seemed wrong. But the same paper had given them another result that they liked much better. The team was going all in on yogurt. With a growing reputation as a boon for microbiomes, yogurt was the anti-ice-cream—the healthy person’s dairy treat.

“Higher intake of yogurt is associated with a reduced risk” of type 2 diabetes, “whereas other dairy foods and consumption of total dairy are not,” the 2014 paper said. “The conclusions weren’t exactly accurately written,” acknowledged Dariush Mozaffarian, the dean of policy at Tufts’s nutrition school and a co-author of the paper, when he revisited the data with me in an interview. “Saying no foods were associated—ice cream was associated.”One scientist said that the ice-cream effect was “similar” in magnitude to, or “slightly stronger” than, the one for yogurt.

But yogurt made so much more sense. In a way, it was confirmation of something that everyone already knew. From the start of yogurt’s entrée into the American diet, it had been perceived as an exotic food from a faraway land, quivering with vague health-giving properties. Even after being spiked with sugar in the ’70s and ’80s to better suit the U.S. market, yogurt still retained its image as an elixir.

Furthermore, a growing body of literature suggested that yogurt’s health benefits might be real. Harvard had found, a few years earlier, that eating yogurt was associated with reduced weight gain; researchers at the university were interested in its possible effects on gut bacteria as well. Other studies—including those that first revealed the ice-cream signal—had also sketched the slender outlines of a yogurt effect. When Chen and Hu pooled together findings from this research, added in their latest data, and performed a meta-analysis, they concluded that yogurt was indeed associated with a reduced risk of diabetes—a potential benefit, they wrote, that warranted further study.

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Regarding ice cream’s potential benefits, they had much less to say. I asked other experts to compare the 2014 yogurt and ice-cream findings. Kevin Klatt, a nutrition scientist at UC Berkeley, said the ice-cream effect was “more consistent” than yogurt’s across the studied cohorts. Deirdre Tobias, an epidemiologist at Harvard, the academic editor of The American Journal of Clinical Nutrition, and a member of the advisory committee for the 2025 update to the U.S. dietary guidelines, agreed with that assessment. Even Dagfinn Aune, an epidemiologist at Imperial College London and a peer reviewer of the Chen and Hu paper, said that the ice-cream effect was “similar” in magnitude to, or “slightly stronger” than, the one for yogurt.

So how did the Harvard team explain away the ice-cream finding? The theory went like this: Maybe some of the people in the study had developed health problems, such as high blood pressure or elevated cholesterol, and began avoiding ice cream on doctors’ orders (or of their own volition). Meanwhile, people who didn’t have those health problems would have had less reason to give up their cookies and cream. In that scenario, it wouldn’t be that ice cream prevented diabetes, but that being at risk of developing diabetes caused people to not eat ice cream. Epidemiologists call that “reverse causation.”

To test this idea, Hu and his co-authors set aside dietary data collected after people received these sorts of diagnoses, and then redid their calculations. The ice-cream effect shrank by half, though it was still statistically significant, and still bigger than the low-fat-dairy effect that Harvard had publicized in 2005. In any event, if people who received adverse diagnoses cut back on their ice cream, you might expect that they’d also cut back on, say, cake and doughnuts. So shouldn’t there be mysterious protective “effects” for cake and doughnuts too? “There should be,” Mozaffarian said. “That’s why the finding for ice cream is intriguing.”

The new analysis was hardly a slam dunk. On paper, the yogurt and ice-cream effects still looked pretty similar. “Within the realm of statistical uncertainty, they’re identical,” Mozaffarian told me. But in the 2014 paper, he and the other authors had argued that “reverse causation may explain the findings” for ice cream. And as academia’s public-relations machinery came to life, nuance went out the window.

“Does a yogurt a day keep diabetes away?” asked the press release that went out on publication day. “Other dairy foods and consumption of total dairy did not show this association,” said Hu, the senior author, in an ice-cream-free appraisal included in the release and echoed in Harvard’s own press bulletin. “Yogurt has approached wonder-food status in recent years,” a Forbes article on the paper noted. “In the new study, other forms of dairy like milk and cheese, did not offer the same kind of protection as yogurt.”

Hu says today that the Harvard researchers felt confident in their conclusions about yogurt largely on account of their meta-analysis, and the fact that prior clinical studies and basic science research supported the idea that probiotics improve metabolic outcomes. “For ice cream, of course, there is no prior literature,” he said. Given that the ice-cream effect was diminished when they tested their reverse-causation theory, he called it “much more plausible” that yogurt would help prevent diabetes than ice cream.

After his paper was published, it didn’t take long for the Harvard group’s good news about yogurt to take hold as a dominant scientific narrative. Two years later, when a team of researchers based in the Netherlands and at Harvard analyzed all the evidence it could find on dairy and diabetes, the yogurt effect popped out. A featured graph from the team’s 2016 paper in The American Journal of Clinical Nutrition summarizes data from about a dozen studies: As someone’s yogurt intake mounts to roughly one-third of a cup a day, their risk of getting diabetes shrinks by 14 percent.

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