In the 1960s and 1970s, obesity researchers referred to this supposed effect of taste on food intake and weight as the palatability hypothesis. But these researchers defined palatability on the basis of how much their experimental animals ate. If their rats or mice ate more of one food than another, the researchers assumed that they did so because they liked it better. The problem is that this concept of palatability “arises mainly from human experience; its existence in animals is an inference,” as the physiological psychologist Mark Friedman explained in 1989. In other words, the animals’ preference for certain foods could have been explained by other factors.

In fact, our perception of what tastes good depends very much on circumstances. Le Magnen made this observation early in his career, and it’s one reason why the subject of his own research evolved from olfactory stimuli to food intake. Le Magnen first noted that our assessment of odor changes with food consumption. The smell of a cinnamon bun baking in the oven will be considerably more enticing when we’re hungry than after we’ve eaten. Our subjective interpretation of taste changes as well. With the possible exception of inordinately expensive meals at fashionable restaurants, the memorable meals of our lives are likely to be those we ate when we were particularly hungry—after a day of hard work or a particularly strenuous workout. “It is often said and not without reason,” as Pavlov wrote in the 1890s, “that ‘hunger is the best sauce.’”

Le Magnen established that an animal’s response to a particular food correlates with how depleted the animal happens to be at the time, with the caloric value of the food, and with how rapidly it fulfills the animal’s nutritional requirements. Rats given the choice between caloric sugar solutions and zero-calorie but equally sweet saccharine solutions initially drink similar amounts of both, Le Magnen reported. They both taste good. But the rats will drink more of the sugar solution with each passing day—drinking three times as much on day five as on day one—while rejecting the saccharine solution after three or four days, having apparently concluded, metabolically, that it offers no nutritive value. If the rats drinking the saccharine solution, however, are simultaneously infused with calorie-bearing glucose directly into their stomachs, they will continue to drink the saccharine solution as long as they get the calories along with it. The taste hasn’t changed, but their post-absorption metabolic responses have. Foods that supply calories and other nutritional requirements quickly and efficiently will come to be perceived as tasting good, and so we learn to prefer them over others.