Mind over matter?

Recently one focus of obesity research has been into the way the body registers hunger and satiety, or fullness. We've discovered an amino acid (these types of amino acids are known as peptides) called ghrelin which appears to help signal hunger as well as fullness. When the stomach is empty or immediate stores of energy are low, ghrelin is secreted from the stomach and passed through the bloodstream to the brain, where it connects with certain receptors in the brain - somewhat like a key in a lock - to help produce the physical feeling of hunger.

When the person eats, ghrelin is no longer secreted by the stomach, and the receptors in the brain stop signaling hunger and start signaling satiety. (Please note that this explanation of the process is somewhat of an oversimplification.) Short version: high levels of ghrelin = hunger; low levels of ghrelin = no hunger.

In theory, the ghrelin response would rise and fall in proportion to the amount of calories a person ate and would help keep the body at a stable weight. In practice, however, it's not that simple: the communications system involved is far more complex than this description, and abnormalities in other gut hormones can profoundly affect the process. People who are clinically obese, for example, often have the post-eating ghrelin response (when ghrelin levels should fall) reduced or even absent, keeping the body feeling hunger even though it has consumed fuel enough to create feelings of fullness in those of normal weight.

Further complicating this picture is the fact that what we think and believe can have a profound effect on our experience of food: Dr. Wansink's experiment involving labels on wine comes to mind. Other studies have shown that those who believe they have eaten more calories feel fuller than those who believe they've eaten fewer calories - even when the number of calories eaten are the same. Further, what is known as "the placebo effect" can cause people to feel that they've taken a medication, such as a pain reliever, when in fact they've taken no functioning medication at all (a "sugar pill").

With this in mind, researchers at Yale and Arizona State University recruited 46 adult men and women for a "Shake Tasting Study" (Health Psych 2011;30(4):424-429). On two separate occasions one week apart, the participants taste-tested one of two vanilla milkshakes. They were told that the study was to evaluate whether the two shakes "tasted similar" and to measure their body's response to the nutrients in the shake by taking regular blood tests over the course of several hours.

The two shakes were actually identical, but on each tasting occasion the milkshake was presented with a different label: one label described the shake as "an indulgence - decadence you deserve" with 620 calories, while the other was labeled "Sensi-Shake: guilt free satisfaction" wtih 140 calories. Neither label was correct with respect to the actual number of calories in the milkshake.

The blood tests were taken just before and at regular intervals after the participants drank the milkshake and were used to measure ghrelin levels in the blood. In theory each participant's ghrelin levels should have responded in generally the same way regardless of which label the shake had - they were actually consuming the same beverage, after all. Instead, their bodies seemed to react as though they had actually consumed the higher- or lower-calorie food. Oddly enough, the hunger the participants actually reported feeling was about the same regardless of which shake they believed they were drinking.

What this means for you

It's clear that the hunger-to-satiety process is far more complex than the simple math of the number of calories you eat, but it's not "mind over matter," either. Eating less (and better) and moving more is still the best advice we have for weight loss. Research into this and other facets of metabolism will continue - we're making new discoveries all the time.

First posted: January 21, 2015