Nov 20 2023

Hunger Circuitry

One of the organizing principles that govern living organisms is homeostasis. This is a key feature of being alive – maintaining homeostatic equilibrium both internally and externally. Homeostatic systems usually involve multiple feedback loops that maintain some physiological parameter within an acceptable range. For example, our bodies maintain a very narrow temperature range, our blood has a very narrow range for pH, salt content, CO2 concentration, oxygen levels, and many other parameters. Each cell maintains specific concentrations of many electrolytes across their membranes. Organisms maintain the proper amount of total fluid – too much and their tissue becomes edematous and the heart is overworked, too little and they cannot maintain blood pressure or tissue function.

Some of these homeostatic feedback loops are purely physiological, happening at a cellular, tissue, or organ level. You don’t have to think about the pH of your blood. It can manage itself. But many involve behavior as part of the homeostatic system. If you are dehydrated you get thirsty and seek out water. If you are cold you bundle up and seek warmth. If your body requires nutrients you get hungry and eat.

That last one, hunger and eating, turns out to be very tricky. First, this is mostly a behaviorally driven homeostatic system – how much do we eat, what do we eat, when do we eat, and how physically active are we (combined with many metabolic factors). Second, this behavioral homeostatic system is very context dependent. The system anticipates future needs and future conditions, not just our body’s needs in the moment. It also appears that behavior driven by a homeostatic imperative can be extremely powerful. Think about how motivated you get to find water when you are extremely thirsty.

We like to think that we have complete free will but, without getting into the ultimate philosophical question of whether or not free will even theoretically exists, the concept of “free” is complex. It’s more of a continuum. If we take a neuroscientific view, the brain certainly has circuitry (executive function) that allows for top-level planning and execution. We can think strategically about our behavior and countermand our behavioral imperatives. I love the scene from Dune where Paul has his hand in the pain box. Every instinct in his brain is telling him to remove his hand from the box, but if he does the Reverend Mother will kill him. He has to prove he is “human” by overriding his instincts. We can conceptualize his struggle as different circuits in his brain competing for dominance, and he better hope that his executive function wins out (spoiler – it does).

But there are at least two significant limitations on our executive function circuitry’s ability to countermand deeper more primitive behavior circuits. The first is that executive function is very high energy, it requires a lot of brain power. Meanwhile, the more basic behavioral circuitry is optimized for efficiency and is more singular in purpose. So we may be able to override our motivational circuitry for a short period of time, but not indefinitely.

Perhaps even more significantly, that deep motivational circuitry hacks into our decision-making circuitry. This gives us the illusion that we are making top-level decisions, when in fact more primitive circuitry has already made the decision for us (metaphorically speaking, but you get the idea). We then rationalize the decision so that we can comfort ourselves with the illusion (the lie, really) that we made the decision for perfectly rational reasons. Or at least we relieve ourselves of any guilt by giving ourselves permission (I will exercise twice as hard tomorrow, or I worked hard today so I deserve it). Or we convince ourselves that we were helpless in the face of our basic motivational imperatives (which is perhaps the rationalization closest to the truth).

In any case, it is therefore helpful to understand behavioral circuitry in the mammalian brain, since it determines a lot of our behavior. One example is the circuitry that regulates eating behavior. We need to eat enough to maintain our stores of energy and to have proper nutrients, without overeating which wastes resources that can be spent elsewhere any  can lead to degradations in health and function. A recent study illuminates one piece of the eating behavioral puzzle. The study is in mice, but very likely is similar to human circuitry.

You can read the study for the details, but the bottom line is that the hormone ghrelin, which is released in the gut, acts on a specific population of cells in the ventral hippocampus. This is part of the brain involved in memory and these cells are thought to involve how our memory of past events influences our current behavior. It seems that hunger is our baseline state, while these ventral hippocampus cells project to the nucleus acumbens, and their activity inhibits feeding behavior. Ghrelin inhibits the activity of these neurons, restoring hunger and feeding behavior. So ghrelin is a hunger hormone, released by the gut when it senses it is empty and needs more food. It’s important to note that this involves decision-making circuitry in the brain.

It’s interesting to think about why we would have evolved so that the more primitive behavioral circuitry seems to have the upper hand against higher-level executive function circuitry. This suggests that our basic needs – food, water, warmth, protection, sex – is generally more important to our survival than any plans we might have for ourselves.

As individuals, what do we do with this information, other than just marvel at the complexity of our brains? That’s a good question, and not one I can thoroughly explore today. But I will say it is important to recognize, and not to engage as a strategy a behavioral plan that involves constantly overriding our primitive circuitry for a very long time. Don’t count on swimming against the current. Rather, think of ways to make it easy on yourself, to engage in strategies that leverage the motivational circuitry rather than working against it with brute force will power. Don’t surround yourself with temptations that you then have to constantly resist. Make optimal behavior the default, and make it easy. Leverage other circuitry, like social pressure, to tip the balance towards the behavior you want.

In the end it is perhaps far more accurate to portray our behavior not as a result of rational decision-making, but mostly as a result of a competition among various circuits in our brains. The rational circuits are just one among many, and they are not the most powerful. They are easily deceived and will even trick themselves with motivated reasoning and rationalization. So leverage all your circuits in the direction your executive function wants to go, and utilize outside feedback (things like science, facts, and the opinions of others) as a check against your own self-deception.

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