Researchers gave 223 people wrong DNA-test results — and they did it on purpose. Not out of spite, but rather to see if this can trigger a placebo effect and physically change the participants’ body. The effects show just how much our beliefs can affect our body, even at a molecular level.

The placebo effect (and its dark cousin, the nocebo) can be surprisingly powerful. Image credits: Patrick Nygren.

DNA testing has become increasingly popular in recent years, with approximately 1 in 25 Americans already testing their genetic background largely through commercial companies. The two most common reasons people do these tests is to check their genetic ancestry and to look for signs of genetic disorders or conditions. But the test results themselves could also have an important impact.

Previous studies have shown that when people read the results of these tests they can exhibit significant emotional and behavioral changes. Now, researchers want to take this one step further and see whether they can even have a physiological effect and actually change participants’ bodies. Brad Turnwald and Alia Crum, devised two clever (and sneaky) experiments.

They gave half of the participants wrong test results. The first group received false information regarding a gene variant they had CREB1 — which can cause increased body temperature when working out, lower aerobic exercise capacity, and reduced cardiovascular improvements when working out. Essentially, people were told they have a genetic predisposition to worse results of physical activity. All participants carried out a treadmill test before and after being given the results. Participants who were told they have this gene variant reported feeling more worried and less in control of their exercise — a significant psychological effect. But the effect went much deeper: they demonstrated statistically significant physiological changes that signaled decreased exercise capacity, such as a lower maximum capacity for CO₂:O₂ gas exchange. They also stopped running significantly sooner than in the initial test.

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Meanwhile, another group of people were told they had the opposite (protective) variant of this gene. For the second trial, they exhibited no significant physiological changes, but were able to run significantly longer before reporting that the exercise felt hard.

This is a clear indication of the placebo effect, as well as its gloomier cousin, the nocebo effect (which is like a placebo, but with negative effects). This isn’t really surprising, as the placebo effect was previously documented to have a physiological effect. For instance, when people believe they are being administered a painkiller, their brain can start to release natural painkillers known as endorphins.

To see whether the results are valid for other types of genetic tests, researchers also carried a secondary experiment, this time centered around FTO gene, one of the best-studied genetic risk factors for obesity. The high-risk form of the FTO gene is associated with lower self-reported and physiological satiety — people with it take longer to feel full. Participants consumed a 480-calorie meal after fasting overnight and reported how full they felt at various points before and after eating. A week later, they had to do the same thing, after being told whether they have the “good” or “bad” variant of the FTO gene.

Compared to the baseline session, participants who were told they had the protective genotype reported a 1.4-fold increase in how full they felt after eating. They also experienced a 2.5-fold increase in levels of GLP-1 — a peptide that signals satiety to the brain. So not only is their mind being put into a very specific mindset, but the body starts to act on it too.

“Mindset matters,” agrees Catharine Wang,  who studies community health at Boston University and was not involved in the study. “This study reminds us to think about how some types of genetic information might lead to unintended consequences. If we give genetic risk information on [certain] traits, it’s easy to see how a self-fulfilling prophecy would kick in.”

This doesn’t really get us closer to understanding what placebo really is, but it does show how strong of an effect it can have. Even in the case of a relatively new scenario such as a genetic test, its effects can be surprisingly severe and quite damaging — the self-fulfilling prophecy Wang mentions.

However, it’s not like we should stop people from getting genetic tests, researchers say. Instead, we need to pay more attention to how these results are presented. In a longer scheme of things,  we should pay great attention to the mindset we place ourselves in. It could very well change our life for the better — or for the worse.

“What we hypothesize could happen [over the long term], based on other work on mindset and placebo effects, is that there is a potential for these effects to grow and fester over time,” he says. He points to the exercise framework as a possible example: “If I learn that my capacity to exercise is lower, for instance, it may become easier for me to recall times in which I got tired faster than other people,” he notes. “The next time I exercise, it may feel more difficult, which may lead me to quit my exercise sooner or worry that there’s no point in exercising anyway.”

The studies were published in Nature Human behavior.