Some of the most important experiments for understanding the role of food palatability/reward in body fatness were performed by Dr. Michel Cabanac and collaborators in the 1970s (hat tip to Dr. Seth Roberts for the references). In my recent food reward series (1), I referenced but did not discuss Dr. Cabanac's work because I felt it would have taken too long to describe. However, I included two of his studies in my Ancestral Health Symposium talk, and I think they're worth discussing in more detail here.
Dr. Cabanac was initially interested in exploring the possibility that human body fatness is biologically defended around a "setpoint". He proposed the existence of a system called a "ponderostat" (pondero = weight; stat = the same) that was responsible for this effect. Personally, I prefer the term adipostat (adipo = fat), as fat mass rather than body weight is what is regulated by this system. His first experiments on the subject were published in the journal Nature in 1971 (2).
To search for an adipostat, he took advantage of an effect called alliesthesia. Alliesthesia is the phenomenon whereby the pleasantness of an external stimulus depends on the internal state of the organism. It sounds more complicated than it is. One example is that entering a hot tub feels better when you're cold than when you're already hot. The pleasantness of the stimulus (hot water) depends on your internal state (temperature of the extremities). Another example is that food tastes better when you're hungry than when you've just eaten a large meal ("hunger is the best sauce"). Again, the pleasantness of the stimulus (food) depends on your internal state (hunger).
To address the possibility that there is a setpoint for body weight regulation, Dr. Cabanac and his colleagues designed a protocol to measure changes in alliesthesia in response to food. Volunteers were first asked to taste sweet solutions* at several concentrations, and asked to rate their pleasantness. Second, they were asked to drink an unflavored drink containing 50 grams (~200 calories) of glucose. Third, they were offered the palatable sweet solutions again and asked to rate their pleasantness. Here's what the graph looked like. Pleasantness is on the vertical axis, and the horizontal axis represents increasing concentrations of the sweet solution:
The investigators found what one might expect, that the pleasantness rating of the sweet solutions was lower after the glucose load than before, suggesting some degree of satiety.
Next, they had the same volunteers restrict their calorie intake by eating less of their typical diet, until they had lost 8-10 percent of their body weight. The investigators then repeated the same experiment as above. The result was quite different this time:
There was no change in the pleasantness ratings before vs. after the glucose load, suggesting that the glucose was not able to induce the same degree of satiety as it was before weight loss. Basically, they had a hunger that could no longer be satisfied by a normal amount of calories, implying that 1) the body has a way of measuring its energy stores, 2) deciding if they are appropriate, and 3) adjusting appetite accordingly. We now know that leptin signaling plays an important role in this process.
After the volunteers were allowed to return to their normal diet, they regained the lost weight, and their alliesthesia returned to normal. The investigators remarked:
The results therefore support the working hypothesis, and it seems possible to conclude that there exists in the body weight control system a biological "ponderostat"...Part II: What Resets the Ponderostat?
Obesity could be a re-setting of the ponderostat at a higher value.
Dr. Cabanac and colleagues attempted to answer this question in a subsequent study, performed in 1976 (3). They used the same alliesthesia technique, and tested the subjects before and after weight loss just like in the previous study. However, this time they used a different method of weight loss. They had volunteers obtain all of their calories from a "bland liquid diet" (Renutryl), ad libitum, in other words calories were not deliberately restricted at all. All subjects spontaneously reduced their calorie intake and lost weight (3.1 kg over ~17 days), after which the investigators used their alliesthesia protocol. Here's what they found:
Their experiment showed no change in alliesthesia before vs. after weight loss using the bland liquid diet, suggesting that their "setpoint" had decreased in parallel with their body weight, rather than staying the same as it had in the comparison group that lost weight by eating fewer calories of a normal diet. They concluded:
...palatability of diet influences the setpoint of the ponderostat.Another interesting anecdote from the paper that strengthens the finding:
In the present experiment, the subjects reduced their intake voluntarily and were always in good spirits, while in the previous experiment, the subjects had to continually fight off their hunger and would spend the night dreaming of food.That, ladies and gents, is the difference between someone who is at his setpoint and someone who is not.
There are a few caveats that we have to keep in mind when interpreting these studies:
- The number of participants was small
- It was not possible to isolate palatability/reward from other factors, i.e., palatability/reward were not the only factors that differed between normal diets and the bland diet (FYI, the bland diet was high in refined carbohydrate including sugar)
- The investigators measured body weight, an indirect proxy for body fatness
- These studies do not imply that other factors besides palatability/reward (such as physical activity) are not important
* Made with sucrose (table sugar).