It often feels as though certain flavours and textures have a “magic hold” on us, making some foods irresistible. But is it only your palate that controls your food choices?
The answer is no. The mouth is just the first checkpoint – deciding whether the food should be accepted or rejected. Once inside, the post-ingestive phase begins. Now, it’s the turn of the digestive system to ’taste’ the food and talk to the brain about your meal choice.
It is known that post-ingestive processes can lead animals and humans to prefer one food over another, but could they also lead individuals to actively seek out certain foods? And if so, what are the neural mechanisms involved?
To investigate these questions, the team devised a task in which the palatable aspects of food were taken out of the picture. In this task, mice would freely press levers to receive a direct injection of food into the stomach. In one of the experiments, one lever triggered the injection of a high-calorie solution and the other of a low-calorie solution.
Remarkably, even though the mice were not able to taste the food, they ended up consistently spending their efforts on the “high-calorie” lever. This, among other results, established a new form of post-ingestive learning; one that instructs animals to actively seek out foods that are more nutritious.
Next, the team proceeded to investigate the neural mechanisms involved.
Question #1: How does information about the nutritional value of food pass from the digestive system to the brain? The team speculated that this happens via the Vagus Nerve: a long nerve that connects the brain and multiple internal organs. In particular, they focussed on the branch that carries signals from the liver, which is well positioned to broadly sense the nutritional value of the food.
When the researchers lesioned that branch, the ability to acquire this new type of learning was reduced. This exciting finding raised question #2: where in the brain were the post-ingestive signals being sent to?
The team began with the immediate suspect – dopamine – a molecule involved in various physiological and cognitive processes, including feeding. However, direct links between post-ingestive signals and the activity of dopamine neurons had not been previously shown.
With further experiments, the team demonstrated that these neurons were very much a part of the post-ingestive learning processes. Not only did they respond to post-ingestive signals, their response was significantly reduced if the hepatic branch of the vagus nerve was cut. To complete the picture, the researchers also showed that the activity of the dopamine neurons was necessary for the post-ingestive learning to occur.
Together, the results of the study reveal a novel learning process – orchestrated between the digestive system and the brain – that compels animals to seek out food that they never actually tasted. This testifies to the potency of the subconscious processes that control behaviour.
The team is building on these findings to investigate food choice in humans. Future work will include simultaneous measurements of behaviour and brain activity to address relevant problems for human health and medicine.