Why Do We Overeat?: Homeostatic vs. Non-homeostatic Eating
Researcher Stephan Guyenet has spent a long-time pondering why it is that some people overeat consistently enough to become obese, while others don’t seem to fall victim to this fate.
Science has on several occasions pointed out that old sterotypical views of obesity being down to “laziness”, “greediness” or conciously stuffing our faces, are misguided, innaccurate and ignorant of the actual under pinnings of obesity.
Obesity is an extremely complex issue. And one of the areas that has been central to obesity research is that of how our food environment and neurobiology affects our liklihood of becoming obese.
This is where the work of the aforementioned Guyenet comes in. Guyenet has looked at obesity through the lens of food reward, palatability and neurobiology.
One core concept that is useful in understanding how obese folks can overeat to such an extent is that of homeostatic vs. non-homeostatic eating. From here, I’m going to let Dr. Guyenet take over to explain this important idea. This is taken from his discussion with me on the Sigma Nutrition Radio podcast…
“So essentially, as with many things in the body, your food intake is regulated. So your body has kind of like an optimal window of quantity of food that it wants you to get within the range of each day just to fill all your regular energy needs.
So you know just like our thermostat will regulate your air-conditioning and heating to try to keep your room temperature in the right zone, the brain tries to regulate your energy intake and your energy expenditure to keep your energy stores inside your body in the right zone.
There is kind of an optimum zone; too little and your at the risk of starvation and your body processes start to shutdown. Too much and you’re going to lose your ability to forage or escape from predators or fight if you need to or whatever. It’s going to reduce your fitness on either end, your reproductive fitness.
So homeostatic eating is eating in response to a perceived energy need by the brain. So the brain thinks you need energy and it makes you hungry, it makes you more interested in food.
So that can occur due to the activation of systems in the body. One of them is a short-term system, the satiety system. And then there is a long-term system called the energy homeostasis system that regulates body fatness specifically.
So ultimately food intake regulation occurs at the level of the meal. It’s like how much food are you going to eat at this meal. And you have communication that happens as you’re eating food and it goes down into your gut. You have sensors in your gut that very specifically detects protein, carbohydrate and fat. If your stomach detects stomach distension, all that information goes back up to your brain (mostly by the vagus nerve) and then that tells your brain stem that there’s a bunch food in your gut and that you don’t need to eat anymore because you have enough.
And then that short-term process is tweaked by this long-term system which measures the amount of body fat that you have. So that is communication between your fat mass and the hypothalamus.
So the hypothalamus is a part of your brain that’s at the bottom of your brain, right above the pituitary. The more fat that you have, the more of this hormone leptin you produce. The hypothalamus “hears” that message and it says “okay we’ve either got enough fat” or “we don’t have enough fat”.
It talks to your brain stem and it’s going to take you more food to feel full at this meal. So you might have to eat if your body fat stores were low because you haven’t been eating enough lately or you’ve been exercising a lot. Your hypothalamus says well you’re going to need to eat 50% more at each meal for the next week until you know to feel like your hunger has been satisfied. So that’s homeostatic eating and that’s your body just trying to maintain energy stores and it makes a lot of sense. But one thing I’ll mention as a little asterisk on this is, if you’ll note I said that the brain is regulated based on perceived energy need.
So it’s not necessarily that your body needs energy, it’s that your brain thinks you need energy.
You know obese people get hungry just like anyone else even though they have several times more fat than a lean person. They don’t need additional fat; they don’t even need to maintain their fat stores but their brain believes that they need to maintain fat stores and it will make them hungry if they don’t eat. That’s a process of dysregulation of that defended level of fat mass and that’s what we studied in our lab.
Non-homeostatic eating is very simply is eating for reasons other than energy needs. So whereas with homeostatic eating it’s basically like “okay I’m hungry I’m going to eat”, non-homeostatic eating is like “hey I’m at this party and there is a table with crackers and cheese in front of me and I just had dinner but everyone is eating and I just kind of want to eat it or it tastes really good or I’m drinking a beer”. I’m not hungry, I’m not thirsty but I like this alcohol, it’s making me feel relaxed, I like alcohol, I’m going to drink this 150 calories of beer”.
There are many, many reasons. You can eat because you’re stressed, you can eat out of habit. There’s many, many reasons why you would eat that don’t have anything to do with energy need and I think in many cases that’s where we get into trouble. Because I mean energy homeostasis tries to balance the energy level in your body, but if you’re constantly eating for reasons other than energy need your going to be pushing more energy into your body than it’s really asking for.”
You can listen to the full interview with Stephan Guyenet, PhD here:
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