Studies Magnifier and Hat 
Solving the Diet-Cancer Mystery:
Scientific Studies Provide Clues

Controlled Trials: What Are They?

In controlled trials, scientists don’t just observe what happens to study subjects as they eat what they would usually eat. They make specific changes to the participants’ diets to see how those changes affect them. One group of study participants, the “intervention group”, consumes foods or nutrients scientists think may protect against cancer. Other participants, the “control group,” get a different “food prescription” – often just a placebo.

Test Group

Most controlled trials are randomized and double-blind. “Randomized” means that study subjects in the two groups don’t differ in any major way other than in what they’ve been asked to eat, so the results aren’t affected by unrelated things. “Double-blind” means that neither the scientists nor the people in the study know who’s in which group.

Strengths of Controlled Trials

Controlled trials avoid many of the types of bias that can be found in other studies. They also let scientists keep tight control over the enormous complexity of our daily menus.

Weaknesses of Controlled Trials

Controlled trials are often called the scientific “gold standard.” This can be true in many situations, but for unraveling the mystery of linkages between diet and cancer prevention, this investigation method may not be perfect.


It’s hard to “blind” people to dietary modifications. You know what you’re eating, and you’ll be aware if someone changes it. That’s why many controlled trials involving diet and cancer give the nutrients in the form of a supplement. But even if scientists show that an isolated supplement produces no anti-cancer effects, this wouldn’t tell us anything conclusive about how whole foods or diets made up of many different foods affect cancer risk.

And since cancer can take decades to develop, it’s hard to know if a study has lasted long enough. Nevertheless, a positive result from a controlled trial can provide strong evidence that a particular nutrient has a protective effect against cancer.

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