Of the roughly 46 million turkeys that Americans will consume this Thanksgiving, an estimated 85 percent will be purchased as blocks of solid ice. But as ubiquitous as frozen turkeys may be, their origin story isn’t one that’s particularly well known. In the middle of the 20th century, these birds made their way to grocery-store shelves only after a scientific study revolutionized the practice of turkey farming in the U.S.—and, in the process, laid the groundwork for one of the most important breakthroughs connecting diet and heart disease.

Until the 1940s, turkeys were only sold freshly slaughtered, creating a logistical nightmare for suppliers around the end of November and a feast-or-famine seasonality to sales. But during World War II, in an attempt to bring a taste of home to soldiers stationed around the globe, the military froze and shipped turkeys to camps in Europe, Africa, and the Pacific. It was a nice idea, but by the time the fowls were served to men in mess halls, they’d developed a fishy flavor.

To figure out how to make the birds more palatable, the army turned to Fred Kummerow, a young Kansas State University biochemist with an interest in nutrition. Kummerow had done his Ph.D. thesis at the University of Wisconsin-Madison on the role of linoleic acid in averting blood clotting, and had later conducted research at Clemson University on pellagra. The disease, then widespread in the South, can cause swollen tongues, rashes, erratic behavior, and, ultimately death. Kummerow’s team helped to identify lack of niacin (vitamin B3) as the cause, and eradicated it by convincing grits manufacturers to fortify their product.

For his turkey investigation, Kummerow bought a group of newly hatched birds and fed half of them a diet of linseed meal, the standard food used by turkey farmers. The other half received a diet of cornmeal. After six months, the birds were slaughtered and placed in cold storage. After several more months, the turkeys were retrieved, cooked, and eaten, so that researchers could compare the flavors of the two groups of birds.  

“In taste tests,” Kummerow recalled, “you could tell the difference.” The standard linseed-meal diet, he discovered, resulted in fat deposits that were more prone to oxidation—a degradation of lipids that imparts a rancid taste, even in cold storage. Changing the turkeys’ feed to cornmeal supplemented with vitamin D, an antioxidant, created meat that retained its mild, fresh taste. With this new knowledge, turkey farmers could now raise birds year-round and freeze them, reducing costs and spoilage and transforming their business model. (The National Turkey Federation was so pleased that in 1948, it even gave the scientist a $2,000 bonus, then enough money to buy a new car and groceries for a year.)

Kummerow—who at 101 is certainly one of the world’s oldest working scientists, if not the oldest—looks back on the discovery as the foundation for what would become his life’s calling. The pellagra project had already “showed me that one thing in the diet can make all the difference,” he said. So when he was able to trace turkey-fat oxidation levels to variations in their feed, he knew he was onto something with potentially wide-ranging implications. “And so I’ve worked on oxidations and fatty acids ever since. And I’m still working on that.”

In 1956, Congress greatly increased National Institutes of Health funding for heart-disease research following president Dwight D. Eisenhower’s heart attack the previous year. Kummerow, now at the University of Illinois, jumped at the change to continue exploring the connection between diet, lipid oxidation, and tissue health he’d identified a decade earlier.

Because feeding studies with humans are difficult, he did the next best thing: research on cadavers. Autopsying 24 bodies donated by a local hospital, he observed that heart tissue from subjects who’d died from atherosclerosis (fatty deposits in artery linings) contained abnormally high levels of trans fatty acids, as did the aortic tissue and arterial walls. In 1957, Kummerow published a short piece in the journal Science describing his discovery and calling for scientists to “determine what effect, if any, trans fatty acids have on the normal metabolic process.”

The warning went largely ignored for decades.

Trans fatty acids first appeared in the American diet in the early 20th century as the partially hydrogenated vegetable oils used in margarine and vegetable shortening. Because these fats were cheap, extended shelf life, and created an appealing texture, they became a standard ingredient in many snacks and baked items, as well as a common oil for frying fast food.

Like saturated fats, trans fats are solid at room temperature; unlike saturated fats, they contain a molecular structure that rarely occurs in nature (trans-fat molecules are created with heat and pressure). The synthetic lipids are metabolized normally, but once they become part of coronary endothelial cells, they appear to contribute to lipid oxidation and other factors that cause the chronic inflammation associated with atherosclerosis.

It took legal action to persuade public-health officials to accept the link between trans fatty acids and heart disease. In 2009, Kummerow submitted a petition to the Food and Drug Administration requesting a ban on trans fats; in 2013, fed up by the agency’s silence, he sought an order from a federal judge to compel the FDA to respond. Evidence and support had been mounting for decades, culminating with a 2012 review by the Harvard professor Walter Willett of the ways in which various types of fat affected heart health. Willett found a protective effect from a diet high in polyunsaturated fats, a “weak or nonsignificant” correlation between saturated fat or total fat intake and coronary heart disease, and a “clear adverse effect” from consumption of trans fats.

In June 2015, the FDA revoked trans fats’s status as a substance Generally Recognized as Safe (GRAS) and gave the food industry three years to halt its use. But there’s still little agreement about how the lipids damage the human body. Kummerow, who says he wants to “solve heart disease,” has been working on this issue for years—or, to measure it another way, through the consumption of billions of frozen turkeys.