Study at Tufts is taking a close look at test subjects and what they eat to unlock the mysteries of food

The thrill, he said, lay in the purpose. For three two-week stints, Hamdorf had agreed to become a study subject in a project seeking to answer questions that have troubled nearly every person who brings fork to mouth: What will happen in my body after I eat this food? Will it make me fat, raise my cholesterol, mess up my gut microbiome — or perhaps extend my life?

The Nutrition for Precision Health Study, a $170 million national research project seeking to enroll 10,000 people nationwide, aims to develop a way to pinpoint the optimal diet for every person. The study is being carried out by six clinical centers across the country, including the New England Clinical Center, run by Tufts University and Massachusetts General Hospital.

Current nutrition advice is based on averages, on what seems to work best for most people. But in many studies, people eating the exact same mix of nutrients have differing responses in measures like blood glucose or blood pressure, said Holly Nicastro, the national study’s program director.

The study will try to find out exactly why, so that someday — it is hoped — people will be able to consume a diet tailored to their unique biology.

The study is also seeking to overcome the challenges that have long dogged nutrition research. Typically, nutrition data relies on asking people what they remember eating. But memories are notoriously inaccurate, and often biased by, say, embarrassment over having eaten that entire sleeve of Oreos. Nutrition for Precision Health is testing other methods of documenting what people consume, including with an app, a questionnaire, or a tiny camera mounted on eyeglasses that is activated by chewing.

“We have the technology to actually get away from the reliance on memory,” said Sarah L. Booth, director of the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University. “This is one of the exciting aspects of this study. We may be completely revolutionizing how we study nutrition.”

Participants first complete a “module” in which they follow their normal diet for 10 days and report what they ate. If they want to continue, they choose either to pick up prepackaged meals to eat at home or to enroll in the “live-in diet” module, the tightly controlled sojourn that Hamdorf selected. By keeping people inside and under watch, researchers can track — with precision and certainty — what they eat, and what effects those foods have throughout the body.

Hamdorf was among an inaugural group of four participants in the live-in diet phase in Boston. All had previously signed up for the All of Us Research Program of the National Institutes of Health, which aims to enroll 1 million or more participants to contribute their health data, including genetics, to build a database that can inform thousands of studies. Only existing All of Us members are invited to take part in the nutrition study.

On the first day, the participants gave blood and saliva samples, had their bones scanned, their metabolic rates measured, and their fat-to-muscle ratio documented. They were also equipped with blood-glucose monitors attached at the waist to continuously track fluctuations in blood sugar, and wrist bands that tracked activity and sleep.

The next day, the daily routines kicked in: 6:30 a.m. weight and vital signs taken; 8 a.m. breakfast; 12 p.m. lunch; 3 p.m. snack; 6 p.m. dinner; 11 p.m. lights out.

The strict schedule proved more liberating than confining, Hamdorf said. “It feels very good to be structured like this,” he said. “You just show up for the meals, and you don’t have to go shopping or anything.”

Between meals and tests, participants were free to use the exercise equipment on the 13th floor, stretch on the mats in the yoga room, gather in the game room, or relax in their spacious private rooms with picture windows. They loved interacting with many of the 150 people working in offices, laboratories, and the kitchen at the Human Nutrition Research Center on Aging.

Occasionally, they took a stroll outside, but only when a staff member could accompany them, lest anyone get an urge for an ice cream cone; the researchers must be able to attest that the participants ate only what they were served inside.

And what were they served?

Precisely at noon near the end of their first two-week stint, the four participants trooped to the 11th-floor dining room, accompanied by Paul J. Fuss, clinical research manager, who stayed to keep an eye on them as they ate.

The kitchen staff brought them plates wrapped in plastic and labeled with their names. Each meal is tailored to the individual’s calorie needs; participants are not supposed to gain or lose weight.

Each also has a tiny rubber spatula to scoop up every crumb. “We’re pretty much asking you to lick the plate without licking the plate,” said Kayla Airaghi, a dietitian. If they can’t finish, the staff will weigh the leftovers, tracking consumption down to a 10th of a gram.

On this day, Lori Mattheiss, 60, of Andover got a hamburger, peaches, and potato chips; her husband, Tim Carter, 63, was served two hamburger sliders. Jane Cashell, 75, of Clinton, ate broccoli, chicken nuggets, and mac and cheese. Hamdorf got Spanish rice, chicken with taco seasoning, and cheddar cheese.

“It’s not the food we normally eat,” Mattheiss said. “I’m eating things I haven’t eaten since I was a kid.” On other days she’s had a Yodel, Kool-Aid, canned fruit cocktail, and Fritos.

In this two-week session, participants consumed what they called the traditional American diet. The researchers, however, resist labeling it; officially it’s described only by its contents, high in refined grains and sugar-sweetened drinks and low in fruits, vegetables, whole grains, and fish.

In the second and third two-week visits — which were spaced apart with at least two weeks in between — the participants ate high-fat and high-protein fare, and, finally, a diet replete with fruits, vegetables, beans, nuts, whole grains, and fish (the four liked this one best).

The three diets were chosen not because they’re recommended but because they are the most common diet patterns in the United States, said Sai Krupa Das, a senior scientist at the center and principal investigator for the New England branch of the study. “They are reflective of what we consume as a nation,” she said.

And the goal is not to determine which diet is best, but rather to measure individuals’ varying responses to the different components.

The researchers acknowledge it will be difficult to attract a diverse population to this phase of the project. How many people can put their lives on hold for two weeks at a time? Carter and Mattheiss, who trade stocks online, worked during their stays, and other remote workers might be able to do the same. But bus drivers? Restaurant owners? Parents of young children? Even the $6,200 stipend for completing all three two-week sessions may not be enough to compensate for lost work.

Despite the challenges, the NIH’s Nicastro said, “We do have ambitious diversity goals,” which they expect to achieve at least with “Module 1,” in which participants eat what they normally do and record it. But even for the more demanding modules — the prepackaged meals or the live-in diet — research sites are working with churches, barbershops, community centers, and other places to sign people up for All of Us, and then recruit them into the nutrition study, Nicastro said.

Rafael Pérez-Escamilla, a professor at the Yale School of Public Health, who is not involved in the study, believes the Nutrition for Precision Health will probably answer some fascinating scientific questions — but, he said, it won’t do much to improve people’s health or reach those suffering the worst effects of a poor diet.

“We live in a country where about 70 percent of adults are either overweight or obese, and 70 percent or more of their calories are coming from ultraprocessed or junk foods, including sugar-sweetened beverages,” he said. “The problem is heavily concentrated among the poor.”

Pérez-Escamilla would much prefer to see a similar investment in efforts to increase access to healthy foods, such as “produce prescription programs” that supply debit cards to purchase fresh fruits and vegetables.

Christopher Gardner, a Stanford professor who has studied the health benefits of dietary components but is not involved with the Nutrition for Precision Health study, called it an “incredibly ambitious” project. He predicted that the researchers would identify the key good and bad bacteria in the gut, and which foods promote them.

Gardner serves on the scientific advisory board for a private company, Zoe, that is offering personalized nutrition advice based on biological information. People take at-home tests and get instructions on what to eat based on the results.

This is exactly what the NIH study eventually hopes to offer, but Das, the principal investigator in Boston, said the results will have stronger scientific backing. “The market’s always ahead of the science,” she said, when asked about Zoe.

At the end of their two-week stay, the participants face two full days of testing, every aspect of their biology measured. On the last day, they drink two cups of vanilla Ensure and then sit in a chair from 8 a.m. to 1 p.m., giving blood samples at set intervals to measure how nutrients are metabolized.

The “live-in diet” sessions are expected to wrap up in mid-2026. Then, with a trove of data from each individual, multiplied across thousands of participants, the project will deploy artificial intelligence to come up with proposed algorithms for determining who should eat what. But that’s not the end: A series of studies will have to be conducted to validate those algorithms.

Nicastro is eager to learn what factors are driving individual responses. “It could be genetics, it could be microbiome, it could be something about the environment, and or probably a lot of these things mixed together,” she said. The ultimate goal is to enable doctors or dietitians to test for certain factors and then produce a personalized eating plan.

Meanwhile, the first four participants, settled back at home with their lime-green souvenir water bottles, are already finding themselves making changes in their diet. Cashell loved the third diet so much she’s trying to re-create the recipes in her kitchen. Hamdorf noticed that during the experiment he never felt hungry, yet looked forward to each meal — and realized he needed to eat more, and drink more water. Matthiess and Carter are trying to eat less in the evening and include more fruits and nuts.

All were gratified to have been part of the study.

“You don’t have many opportunities to contribute to important scientific research,” Carter said. “And I feel lucky that I am able to do that.”

Felice J. Freyer can be reached at felice.freyer@globe.com. Follow her @felicejfreyer.