Volumetrics... okay. So you take a rodent, feed it chow, the same chow. For months on end. What's happening? A specific volume of food equals a specific level of nutrition--calories, macros, micronutrients etc. You can make a near perfect prediction just from weighing or measuring. Stomach stretch from eating this homogenous food also makes a pretty accurate prediction--it's got a more perfect correlation to the nutrient content of the food than if the animal is running around eating random bugs and vegetables. Maybe a given volume of "food" really does become a satiety signal/predictor. Change the food--make it more calorie dense or less--and calorie intake may change, especially in the short-term--because, food intake is much briefer than food assimilation, a prediction about how much nutrition is in the food to be eaten is somehow made by the system. Over time--if the relation of nutrition to volume changes, the system is "disappointed," and learns to disregard or discount the predictive value of volume--maybe there's some subtle difference in mouth-feel or flavour.

Mice are cheap, so we get to see this, change their chow, for a week or two, calorie intake might indeed drop or increase if you add wood chips or some other heart-healthy fiber or fat to the mix. Appetite normalizes after that week or so--although whether the animals get fatter or not doesn't always match this normalization of appetite.

People are expensive, the studies in them are. So you do a one-meal study, feed them two different puddings, make them as indistinguishable from one another as possible. So, they eat more calories when it's more calorie dense, less when it's less calorie dense.

But it can't be a static thing, there has to be a learning process. Put a person on an all heavy cream diet, 1600 calories a cup, versus some percent fat milk that has 200 calories a cup--they don't simply eat 8 times as many calories and get that much fatter. We learn, like the mice, what we'll get from the food. Maybe appetite doesn't adjust perfectly, but it does adjust. We need room for those who get most of their calories from heavy cream but are the leanest they've been in their adult lives (at least without wanting to eat their neighbour's brains).

So back to that pudding. What if we eat low fat pizza all the time. We know we won't waste away, eating to appetite--we'll probably learn that it takes more pizza to hit a certain nutrient intake. Not talking explicit learning, more implicit--what our bodies are doing with the input. Maybe next time we eat the more standard, higher calorie/fat pizza--we're used to eating high volumes of food, because that's what volumetrics has done for us--made us feel that high amounts of stomach stretch are necessary for us to be nourished. Sometimes competitive eaters eat high amounts of cabbage to "stretch" their stomach. Stretch might be the wrong concept, our stomach capacity is generally fairly voluminous, ask a watermelon. But maybe they are learning to ignore stomach stretch as a satiety signal, by filling up with such low-grade nutrition.