Lacey , 22 Jan 2016.
You started the diet with good intentions. Maybe you even shed a few pounds (yay!). And boom. It’s over. Oh well… today is a flop so I’ll start again tomorrow, you say. And the same problem, I’ll start again on Monday. You continue to start-stop diets this way. So infuriating! You were doing so well!
In the Western world, resisting the urge to overeat (or eating when we are not hungry) is a huge challenge. Through experience, habits and our amazing ability to integrate information about our environment and link it to food, we have been trained and drilled to overeat. We are constantly bombarded with stimuli that motivate us to look for food and to overeat. Huh? This doesn’t make sense! How can I be motivated to ruin the diet I just started?
Basically, you are constantly encouraged to reach for that second helping of mashed potato, buy a bar of chocolate when you were shopping for vegetables, or stop for some fast food. Together, your stomach, your brain, your environment, your TV… even your parents, have conditioned you to overeat.
Blame them all!
But, there is good news. Neuroscientists are starting to unravel the communication processes between our stomach and our brain that have trained us to expect food and motivate us to overeat. Maybe, armed with this information you can empower yourself against them? Good luck!
Don’t feed the lions
Here’s the thing: eating is necessary for survival. Our bodies have created an advantageous system for us to eat as much as possible in a short window of time… you just never know when food might be made unavailable or when you will be attacked by a lion, for instance.
But in the Western world, where for the most of us food is plentiful, and we generally only see lions at the zoo, our feeding mechanisms evolution can be harmful to our health.
Much of our eating habits in the Western world are reliant on learnt behaviors that have been taught to us by our caregivers as we grow up, and structured into our adult lives through schedules and social norms. Our parents insist we sit down and eat three meals a day and finish what is on our plates; society makes it necessary to socialize and make business deals with food; and our schools and workplaces tell us when to eat. Not only that, but many of the foods we freely have access to are tasty but often low in nutrition while high in fat and sugar.
Advertisers of junk food exploit this loophole in our modern day physiology. Billboards on the side of the road, quirky adverts and baking shows on TV, and constant social media feeds and websites that help you to compile all those recipes you have cooked or will never cook… they are all conspiring against our ability to resist the urge to eat and then overeat.
And they are all taking advantage of the connection between your stomach and your brain. How?
Your stomach is tricking you: hormones predicts scheduled meal times, not nutritional needs
You feel a twinge in your belly, you look at the clock and it’s noon: lunch break! A hormone in your stomach, often referred to as the hunger hormone, called ghrelin, signals it’s time to eat. The actions of ghrelin are not limited to your stomach. In fact ghrelin travels to your brain and communicates messages to encourage eating. Ghrelin is part of your conditioned biological response that occurs prior to food consumption.
The lab of Scott Kanoski at University of Southern California prefers to refer to this hormone as the meal anticipatory hormone due to the fact that it isn’t necessarily signaling your need to eat (nutritionally) but more your expectation that you should eat. It is your lunch break after all. Perhaps you smelled food from a nearby café or you saw your co-workers lunch.
Ghrelin levels increase in anticipation of a meal and also allows you to eat large quantities of food and override basic energy and nutritional needs. Rats that are injected with ghrelin consume more meals and large quantities of food.
A part of the brain called the hippocampus is densely populated with sensors, or receptors, for ghrelin. The hippocampus enhances and integrates reminders or cues in your internal and external environment to help you locate food e.g. visual, olfactory or auditory messages. When the hippocampus is activated by ghrelin, food-related cues are enhanced and become difficult to ignore - pushing you to search for food.
The hippocampus does this, scientists think, by taking advantage of memories of a previous experiences of eating (yum!), triggers from our bodies (hunger hormones, state of nutrition etc.) and also context-related external cues (sight of your fridge or a fast food advert). All of this information influences our decisions on what, where, when to eat.
In other words, the hippocampus integrates all our learned associations with food and promotes us to eat… and then overeat. You may have heard of Pavlov’s dog that was trained to associate the ringing of a bell with food… and eventually salivated in response to the bell even when there was no food. This process is called conditioning. Our brains are trained to recognize cues that mean food.
The Kanoski lab found that when ghrelin was injected directly into the hippocampus of rats it caused them to overeat. They were then able to prevent this overeating by using a drug that prevented ghrelin from acting in the hippocampus… but: the drug only worked in rats on a fixed meal schedule. The anti-ghrelin drug did not reduce the amount of food eaten by rats with constant access to food (even fatty/sugary food). This is good news! It suggests that the tricks cues, ghrelin and the hippocampus to promote eating could be modulated by re-evaluating spaced apart meal times.
Blame your parents: scheduled meal times have conditioned you to overeat
You probably grew up eating three meals a day. We eat by a schedule defined by our family, school or place of work. This kind of habitual eating behavior is learned (babies certainly don’t follow a meal schedule), and is known in science as conditioned feeding.
Being obese or overweight is a huge problem in the Western world (mind the pun). And it is not mainly due to consumption of foods that are more calorific… the bigger driving force behind the obesity problem is portion sizes and the number of meals and snacks.
To look at how only having access to food during defined windows of time affects eating, the Kanoski lab, only allowed rats access to food for 4 hours a day. Usually research rats are able to access nutritious food any time they want (and generally are not overweight) but after a week of timed access the rats dramatically increased their intake of food in that time window (just like when we eat large quantities at our defined lunch break). By day four the rats had almost tripled their consumption of food in those four hours! The rats overate in order to compensate for longer periods without access for food.
Being trained to eat large meals in short periods of time in this way, akin to meals consumption in Western society, is called meal entrainment, and is triggered by those conditioned food cues discussed above. The ghrelin hormone signal from your stomach tricks you into thinking that you need to eat (anticipation), partly through noticing food cues (like the time), but it only does this if you have been eating on a fixed meal schedule!
We have been conditioned to overeat in short time windows.
Your fixed meal schedule motivates you to find food and overeat
Ok, so we get that hormone signaling from your stomach works in concert with parts of your brain important for spotting signs that make us to want to eat. But how does this actually lead us to OVEReat once we have met our nutritional demands?
The Kanoski lab discovered that the hippocampus does this through stimulation of orexin-containing cells in the hypothalamus, a part of the brain that is important for motivated food finding and intake (in fact “orexigenic” means “appetite stimulant”). It has been known since the 50s that you can control eating behavior through stimulation (causes eating) or lesion (stops eating) of the hypothalamus.
Through a series of experiments, the Kanoski lab found that the hypothalamus was not bothered by ghrelin’s trickery to eat, nor could the hippocampus enforce ghrelin’s will to eat without the hypothalamus (lesions to the hypothalamus prevented overeating) or orexin signals (blocking orexin receptors prior to injecting ghrelin into the hippocampus prevented overeating).
Thus, stomach hormones trick us into eating by enhancing the signals that make us want food, which, in turn, activates the circuits that motivates us to find food and eat.
Ghrelin, or orexin, in this circuit could be great targets for the treatment of disorders of eating, such as overeating.
Mind your calories
Globally around 39% of adults were overweight in 2014 (W.H.O). Researchers are beginning to understand how the mind-gut-mind connection, as well as how carrying too much weight, directly impacts your brain health.
Appetitive changes are found in many mental health conditions. We talked about three components of the mind-gut connection: 1) Ghrelin found in your stomach (levels increase in chronic stress, post-traumatic stress disorders, depression and others); 2) Hippocampus where memories for food are kept (sufferers of amnesia and Alzheimer’s disease associated with hippocampus cell death have trouble remembering that they ate and are less sensitive to internal cues on energy requirements); and 3) Hypothalamus (orexin) that motivates us to eat (narcoleptics (a sleeping disorder) have alterations in hypothalamus orexin signals and often have problems maintaining a healthy weight). Together they drive us to eat even if we do not need to and all three of these have links with brain health.
How many of us suffer from “emotional eating” or change our eating habits in response to stress? Understanding these mechanisms of overeating, especially in relation to cues and triggers in our environment, will help researchers develop strategies to treat brain disorders that have appetitive and poor maintenance of healthy weight as symptoms.
So… next time you start your diet… take a moment to evaluate what triggers you to eat. Rarely do you see a billboard or an advert encouraging you to buy fresh vegetables. Try grazing rather than eating three meals, remove triggers from your house/ office, or thwart the effects of ghrelin by keeping healthier snacks on hand in case you encounter a huge poster for a bar of chocolate. Maybe just the knowledge of how our minds throw us off track will be enough to motivate us to ignore the motivation to overeat? Let me know how it works for you.
To view the original paper discussed in this blog “Hippocampus ghrelin signaling mediates appetite through lateral hypothalamic orexin pathways” in eLife please click here.
The Blog was written by Carolyn Lacey, Scientific Outreach Manager at Neurexpert. To learn more about Carolyn and Neurexpert, please click here.
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