“You may be able to change your metabolism with your mind,” explained a report on National Public Radio and a YouTube video in April 2014.
The story featured a study that gave people a 300-calorie milkshake labeled as either a “Sensi-Shake” with only 140 calories or an “Indulgence” shake with 620 calories. Meanwhile, researchers monitored levels of ghrelin, which is often called the “hunger hormone,” in the participants’ blood.
Rises in ghrelin signal hunger and slow metabolism, explained the NPR reporter, while a big meal causes ghrelin levels to drop, which starts “revving up metabolism so we can burn these calories we just ingested.”
The study’s surprising finding: after the participants consumed what they thought was the “indulgent” shake, “ghrelin levels dropped about three times more” than after they drank what they thought was the “sensible” shake.
“So in theory, if you want to lose weight, you could try eating healthy food with an indulgent mindset,” says the YouTube video, which was produced by NPR. “You would feel fuller and your metabolism would increase.”
First of all, the study never measured ghrelin’s effect on metabolism (or even how much food the participants ate at their next meal). Nor have others.
“If you give animals ghrelin injections either subcutaneously or directly into the brain, they increase their food intake, increase their body weight, and burn less fat,” says Jenny Tong, an associate professor of endocrinology and a ghrelin expert at the University of Cincinnati who was not involved in the milkshake study.
But giving ghrelin to cancer patients who are losing weight doesn’t help much, she says.
“In normal humans, we only have very short-term studies measuring changes before or after meals. To say that ghrelin has such a profound effect on metabolism, the evidence in humans is lacking.”
Among the uncertainties:
Ghrelin boost. Ghrelin dropped more when people drank the ‘indulgent’ shake in part because ghrelin had climbed more during the 40 minutes they spent looking at the labels before drinking. Odds are, that’s because the high-calorie label depicted an ice cream sundae.
In the world outside the lab, might higher ghrelin levels—spurred by the sight of indulgent foods—lead people to eat more?
Past a half hour? The study measured ghrelin levels for only half an hour after people drank the milkshakes.
“Looking for a longer time might be interesting,” says Tong. For example, in a recent study, “carbohydrate—that is, glucose—was the most potent ghrelin suppressor, but after two or three hours, there was a rebound. Protein suppressed ghrelin less potently, but it lasted longer.”
Other research? In a similar study, ghrelin levels were no lower after people ate a yogurt that was labeled “high-calorie” than after they ate an identical yogurt that was labeled “low-calorie.”
“The findings of the milkshake study are intriguing,” says Tong. “But it’s not as simple as just saying that ghrelin increases or decreases after a meal and that explains how our body regulates metabolism. That’s a bit of a stretch.”
Sources: Health Psychol. 30: 424, 2011; J. Clin. Endocrinol. Metab. 93: 1971, 2008; Obesity 21: 1548, 2013.