Research Review: Food reward and overeating

By Helen Kollias

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Imagine having no desire to eat food.

Imagine that you are never hungry, and that when you eat, you don’t enjoy the taste, texture, nor act of eating. Eating would feel like doing your taxes or cleaning out the cat litter box multiple times a day. You’d probably procrastinate about doing it, and eventually die of starvation.

We need food to survive. Luckily, we’ve evolved strong mechanisms that make sure we get food. Our bodies are wired to want to eat food and get a sense of satisfaction (reward) from eating.

The problem occurs when wanting to eat food becomes WANTING to eat food — particularly food that isn’t good for us.

I’m sure you have a Homer donutesque response to something… or many things. And for many of you, it may feel at times as though it’s hard to control those responses. Now you’re getting into food addiction.

Homer-Doughnut

In fact, there is a whole area of research looking at food addiction that tries to figure out if people with weight problems get too much satisfaction from food… or not enough satisfaction.

Goldielocks of reward: Does too much or too little food reward cause obesity?

Researchers looking at brain function and overeating have come up with two opposite potential explanations why people overeat.

  1. Hyper-response: food triggers a much bigger pleasure (aka reward) response in overeaters compared to normal eaters, which eventually causes obesity. Basically, food gives a bigger high in overeaters. Many of you will agree that chocolate cake or poutine (French fries with cheese curds and gravy) has drug-like effects.
  2. Hypo-response: food triggers a much smaller pleasure (aka reward) response in overeaters compared to normal eaters. Overeaters eat more to compensate for lack of food reward response. Basically, food gives less of a high to overeaters, causing them to eat more to get the same high. If this is you, you have to eat the whole cake to get the same level of happiness that  someone else gets with a small piece of said cake.

Great — the damn researchers sit in their labs hypothesizing and come up two exact opposite explanations for why we overeat. Curse the damn researchers! Wait… I’m a researcher. Okay, I’m going to defend my fellow researchers, because it looks like both ideas might be true.

First thing to understand is that you need to split eating food into two parts:

  1. Response to getting the food and anticipating eating it. I call this food anticipation — the dreaming of how good x, y or z food will taste.
  2. Response to tasting and ingesting the food. I call this the food response — the parts of your brain that light up when you have food x, y or z in your mouth.

You might get a lot of reward from food anticipation but not a lot from food response, for instance, which means that the idea of food is much more appealing than the actual experience of eating, which leaves you unsatisfied… so you crave more food… and so on.

Next, it is possible to start out hyper-responsive to food, but become hypo-responsive — to go from more to less reward. Let’s look at drug addicts as a comparison.

Initially, researchers think that people more likely to get addicted are more sensitive to a given drug. They get a bigger high, at least initially, but through chronic use they need more and more of the drug to get the same response. They become desensitized.

It’s like the difference between new parents and parents with five kids. With the first new baby, parents hover and panic over every small burp and sniffle; they crank the baby monitor up and every time the baby makes a squeak or snort they freak out. By the time the fifth kid rolls around and the overall noise level of the household has increased to something approximating a jet engine, the kid has to scream for a solid hour at 120 decibels before the tuned-out grownups notice.

In the same way, people start out over-sensitive, but may become less sensitive as time goes on and the stimulation increases.

Researchers are still working on the details, but that explains how overeating can involve both hyper- and hypo- responses to food.

Hyper-response to food

Research suggests that if you prefer high-fat, high sugar foods, you risk weight gain and higher risk of obesity.

Generally, obese individuals say that eating food is more rewarding. And children whose parents are obese are more likely to become obese, and have a preference for high-fat food (it’s not clear yet whether this is from mom being overweight during pregnancy, or the foods and behaviours in the home environment; it’s probably a bit of both).

Yes, I know saying the people who like to eat more, eat more isn’t earth shattering, but remember this is all self-reported preferences — not brain activity.

Hypo-response to food

Instead of asking people if they like food more, what if you got rid of the middleman (you) and went straight to the source of pleasure: your brain.

Since your brain isn’t much of a talker (what with the not having a mouth problem) the only way to tell if the brain is getting more or less pleasure from eating something is to do a functional MRI (fMRI) scan that looks at brain activity.

When you taste something that you like, certain parts of the brain light up (are activated), and an fMRI will show this. This includes parts of your brain you didn’t know you had. For instance, your right lateral orbitofrontal cortex, frontal operculum, dorsal striatum, and insula are activated when you taste yummy food (1).

The dorsal striatum is really interesting, because it’s sensitive to dopamine, a neurotransmitter, released by another part of the brain (substantia nigra) in response to food (among other things). Dopamine is important for a lot of things including voluntary movement, motivation, and reward.

In one study, for instance, researchers found that rats that binged on sugar end up with more dopamine in their brain (2). More dopamine equals more of a high (reward) to food.

However, it’s not just about how much dopamine you have, but whether you can “hear” that dopamine. Not only do you need dopamine but also you need receptors for the dopamine to attach to in order to get that happy reward feeling. Fewer dopamine receptors means less reward — even if the overall level of dopamine is high.

Researchers found a specific type of dopamine receptor, D2, that is related to obesity. Scans show that obese people have fewer D2 receptors in a given area compared to lean people. This means that eating food triggers dopamine to swim around your brain, but if you don’t have as many receptors you aren’t as satisfied (rewarded), so you eat more. That leads to gaining weight and becoming obese.

dopamine addiction flowchart

And another, related question: Does this brain response change depending on how lean or fat you are? Well, read on.

Research question

This week’s review looks at how gaining weight changes your brain’s response to yummy food (chocolate milkshakes).

Stice E, Yokum S, Blum K, Bohon C. Weight gain is associated with reduced striatal response to palatable food J Neurosci. 2010 Sep 29;30(39):13105-9.

Methods

26 overweight and obese young women (average BMI=27.8, average age=21.0 years old) volunteered for this study.

Everybody had two brain scans: one when they were tasting a milkshake and one when they were tasting a taste-less solution.

Häagen-Dazs ice cream! Where do I sign up?

Since this study is trying to figure out how weight gain or lost affects how we respond to food, the researchers needed to find some yummy mouth-watering food to activate the reward part of the volunteers brain.

Not surprisingly, Häagen-Dazs ice cream was involved in lighting up the reward centres of the brain. The milkshake in this study had 4 scoops of Häagen-Dazs vanilla ice cream, 1.5 cups of 2% milk, and 2 tablespoons of Hershey’s chocolate syrup – basically a very decadent, high-fat/high-sugar (aka yummy) milkshake.

The only downside was the volunteers only got 0.5 mL (yes, 500 microlitres) of the milkshake… and they couldn’t swallow it. Researchers wanted the participants to taste the milkshake, but limit mouth movement and avoid swallowing, since those cause another part of the brain to light up.

The taste-less solution was a mix of a salt (KCl, 25mM) and baking soda (NaHCO3, 2.5mM) – basically saliva. Why not water? Water has a subtle taste that still triggers changes in your brain. Gee, somehow artificial saliva sounds yucky, even though it’s tasteless.

MRI scans

Each volunteer was shown a picture of a milkshake for 2 seconds, and then either given a taste of the milkshake or a taste of artificial saliva. Functional MRI scans were done and the blood oxygenation level-dependent (BOLD) response between the two tastes was compared.

Six months later

After 6 months all the volunteers went through the same functional MRI scans and tasting procedures. Why? I’ll explain that in the results section.

Results

The volunteers said they liked the milkshake better than the artificial saliva (r=68)! Wow!

MRI scans of their parahippocampal gyrus, the food reward centre of the brain, correlated with what the volunteers said they liked (r-0.84-0.91).

So far no earth shattering findings, people like milkshakes more than saliva and when they said the liked milkshakes more, their brain agreed.

Gaining and losing weight: Changes in brain food reward response

The point of the study was to see whether gaining or losing weight changed how peoples’ brain responded to food. By doing the MRI scans twice separated by 6 months with the same people, the researchers got people who

a) gained weight (average of 6.4 pounds);
b) lost weight (average of -6.8 pounds); and
c) stayed the same (average of 0.5 pounds).

The stable weight group had no change in their response to the yummy milkshake; their food reward centre was activated the same amount both times.

The weight loss group had a little increase in activation of their food reward centre, but not enough to be considered significant in the scientific sense. To truly be sure that this finding is true the researchers would need to do the experiment again with more people — otherwise this difference may just be a fluke.

The weight gain group had way less activation of their food reward centre (right caudate) when they tasted the milkshake 6 months later (after having gained 6.4 pounds of body weight). This means that gaining weight caused the brain to have less of a high when tasting a yummy shake than before the weight gain.

Conclusion

Gaining weight reduces the activation of the food reward centre of the brain (right caudate).

This is the first study using fMRI scans to show that gaining weight changes how your brain responds to food.

Your brain becomes less sensitive to the reward of eating food when you gain weight, increasing the chance you’ll eat more to get the same reward causing you to gain even more weight.

This explains a lot about why it’s so hard to lose weight by just eating less. It’s hard because you’re not getting the same reward as someone leaner than you, even if you eat the same amount. It also explains why weight gain can be cumulative.

Bottom line

If you’re overweight and you eat the same things as your skinny friend, chances are you’re not getting the same food high.

Should you throw your hands up in the air and say “See, I told you so”? Well, you can do that or you can be more conscious of eating and eat healthier food, accepting that you might not always love it. Eventually, you will lose weight and you will get more of a high.

Weight loss is just like drug detox: The beginning really sucks, and later it sucks less, until your new normal is being a lean person.

If you want to know more about food addiction here is a link to a presentation by Eric Stice (first author of this study).

And here is a link to all the presentations at the Obesity and Food Addiction Summit.

References

Click here to view the information sources referenced in this article.

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