NPR milkshake experiment labels YouTube screencap / NPR

Your feelings of hunger or fullness are not just based on what you eat, but on what you think you're eating, too, according to a recent study.

If you believe your meal is healthy, it may leave you feeling hungrier than when you've pigged out on food you think is unhealthy.

Psychologist Alia Crum figured this out in a clever experiment where she had participants drink milkshakes labeled either as an "indulgent" 620-calorie shake or a 140-calorie "sensible" shake (labeled as 104 calories above).

The secret? The beverages were actually identical, coming in at 380 calories. The surprise was that their actual caloric content had little effect on if the drinker felt full — it only mattered what the drinker thought the shake contained. The result wasn't just mental, either. Their bodies physically responded differently to the two shakes.NPR milkshake experiment - milkshakeYouTube screencap / NPR

When participants thought they had indulged in a high-calorie shake, the researchers saw a steep drop in the hormone that makes us feel hungry, ghrelin. In those who thought they were eating a healthy, low-calorie shake, their levels of ghrelin hardly budged, meaning they still felt hungry after finishing it.

If your body feels hungry, it has to prepare for the possibility that you won't find food any time soon. That means that those high levels of ghrelin do more than stimulate your appetite — they also slow your metabolism, Crum explained to NPR.

While we still need more research into this mysterious effect, it's really just a particularly striking example of a placebo: Your mind can really control your body — even, it seems, your appetite.

NPR milkshake experiment kale YouTube screencap / NPR

The research doesn't mean that higher-calorie meals are healthier than lower-calorie meals, only that our feelings about food may actually play a role in how we respond to them. If you can convince yourself that a healthy salad is actually a gut-busting indulgence, you might be able to leverage this finding to your advantage.

Here's NPR's full walkthrough of the experiment.