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There was a time, in the distant past, when studying nutrition was a relatively simple science.
In 1747, a Scottish doctor named James Lind wanted to figure out why so many sailors got scurvy, a disease that leaves sufferers exhausted and anemic, with bloody gums and missing teeth. So Lind took 12 scurvy patients and ran the first modern clinical trial.
The sailors were divided into six groups, each given a different treatment. The men who ate oranges and lemons eventually recovered — a striking result that pointed to vitamin C deficiency as the culprit.
This sort of nutritional puzzle solving was common in the pre-industrial era. Many of troubling diseases of the day, such as scurvy, pellagra, anemia, and goiter, were due to some sort of deficiency in the diet. Doctors could develop hypotheses and run experiments until they figured out what was missing in people's foods. Puzzle solved.
Unfortunately, studying nutrition is no longer that simple. By the 20th century, medicine had mostly fixed scurvy and goiter and other diseases of deficiency. In developed countries, these scourges are no longer an issue for most people.
Today, our greatest health problems today relate to overeating. People are consuming too many calories and too much low-quality food, bringing on chronic diseases like cancer, obesity, diabetes, and heart disease.
Unlike scurvy, these illnesses are much harder to get a handle on. They don't appear overnight; they develop over a lifetime. And fixing them isn't just a question of adding an occasional orange to someone's diet. It involves looking holistically at diets and other lifestyle behaviors, trying to tease out the risk factors that lead to illness.
Today's nutrition science has to be a lot more imprecise. It's filled with contradictory studies that are each rife with flaws and limitations. The messiness of this field is a big reason why nutrition advice can be confusing.
It's also part of why researchers can't seem to agree on whether tomatoes cause or protect against cancer, or whether alcohol is good for you or not, and so on, and why journalists so badly muck up reporting on food and health.
To get a sense for how difficult it is to study nutrition, I spoke to eight health researchers over the past several months. Here's what they told me.
In many areas of medicine, the randomized controlled trial is considered the gold standard for evidence. Researchers will take test subjects and randomly assign them to one of two groups. One group gets a treatment; the other gets a placebo.
The idea is that because people were randomly assigned, the only real difference between the two groups (on average) was the treatment. So if there's a difference in outcomes, it's fair to say that the treatment was the cause. (This was how James Lind figured out that citrus fruits seemed to have an effect on scurvy.)
The problem is that it's just not practical to run these sorts of rigorous trials for most important nutrition questions. It's too difficult to randomly assign different diets to different groups of people and have them stick with those diets for enough time to find clues about whether certain foods caused certain diseases.
"In an ideal world," said the British physician and epidemiologist Ben Goldacre, "I would take the next 1,000 children born in Oxford Hospital, randomize them into two different groups, and have half of them eat nothing but fresh fruit and vegetables for the rest of their lives, and half eat nothing but bacon and fried chicken. Then I'd measure who gets the most cancer, heart disease, who dies the soonest, who has the worst wrinkles, who's the most clever, and so on."
But, Goldacre adds, "I would have to imprison them all, because there’s no way I would be able to force 500 people to eat fruits and vegetables for a life.’"
It's undeniably a good thing that scientists can't imprison people and force them to stick to a particular diet. But it means that real-world clinical trials on diet tend to be messy and not so clear-cut.
Take the Women's Health Initiative, which featured one of the biggest and most expensive nutrition studies ever done. As part of the study, women were randomly assigned to two groups: One was told to eat a regular diet and the other a low-fat diet. They were then supposed to follow the diet for years.
The problem? When researchers collected their data, it was clear that no one did what they were told. The two groups basically had followed similar diets.
"They spent billions of dollars," says Walter Willett, a Harvard physician and nutrition researcher, "and they never tested their hypothesis."
Conversely, it is possible to conduct rigorous randomized control trials for very short-term questions. Some "feeding studies" keep people in a lab for a period of days or weeks and control everything they eat, for example.
So instead of randomized trials, nutrition researchers have to rely on observational studies. These studies run for years and track very large numbers of people who are already eating a certain way, periodically checking in to see, for example, who develops heart disease or cancer.
This study design can be very valuable — it's how scientists learned about the dangers of smoking and the benefits of exercise. But because these studies aren't controlled like experiments, they're a lot less precise and noisy.
An example: Say you wanted to compare people who eat a lot of red meat with fish eaters over many decades. One hitch here is that these two groups might have other differences as well. (After all, they weren't randomly assigned.) Maybe fish eaters tend to be higher-income or better-educated or more health-conscious, on average — and that's what's leading to the differences in health outcomes. Maybe red meat eaters are more likely to eat lots of fatty foods or smoke.
Many observational studies — and other nutritional research — rely on surveys. After all, the scientists can't hover over every single person and watch what they eat for decades. So they have subjects report on their diets.
This poses an obvious challenge. Do you remember what you ate for lunch yesterday? Did you sprinkle nuts or dressing on your salad? Did you snack afterward? Exactly how many potato chips did you eat?
Chances are you probably can't answer these questions with any certainty. And yet, a lot of nutrition research today rests on just that kind of information: people's self-reporting from memory of what they ate.
When researchers examined these "memory-based dietary assessment methods," for a paper in the
