You cut out sugar for a week and feel great. Does that mean sugar was the problem?

Trying something and paying attention to how you feel is a completely reasonable way to learn about yourself, and it's how most people actually engage with their own health. They start trying something new and watch what happens next. Researchers can study this more formally through what’s called an N-of-1 study: a structured experiment designed to learn how something affects one person, with their biology, in their life.

This approach also has limitations we should be aware of.

The core difficulty is separating the effect of the one thing you changed from everything else happening in your life at the same time — your sleep, your stress, the season, a dozen other small habits shifting at once. When you stopped eating sugar, did you also increase your fiber intake or reduce how frequently you were eating? Simply trying something and noticing how you feel can only tell you that two things happened around the same time, not that one caused the other.

And even the most rigorous N-of-1 study can only reveal so much. It can tell you what happened for one person under the conditions that were tested, but not necessarily what would happen in a different situation or for somebody else.
That same problem shows up far beyond health — one case is often not enough to separate a real pattern from circumstance. An investor can't tell skill from luck off one good year. A teacher can't tell a great new method from a great class of students off one good semester.

Fortunately, studies with many people can do something N-of-1 studies can’t: look for patterns across people with different bodies, habits, and lives. That kind of data helps generate evidence that is more likely to hold up beyond any one person’s circumstances, which is vital because we can’t always test things for ourselves first. Sometimes doing so would be impractical, unsafe, or impossible. In a medical emergency, when there is no time for personal trial and error, doctors rely on evidence built across many studies and many participants to make informed decisions.

Both N-of-1 studies and larger studies can be valuable, but they answer different kinds of questions. One can help us understand what may work for a particular person; the other can help us see which patterns hold across many people. Together, they add to the evidence we use to make better decisions about health.

Pick something you've personally tried — cutting sugar, a new sleep schedule, a supplement, whatever comes to mind. Then ask: