And then the withdrawal symptoms begin.

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How bad are they? Oh, nothing much -- just precisely like weaning yourself off a crippling drug habit.

According to research on mice (whose neurochemical system is comparable to that of humans), you don't even need to be morbidly obese to experience these symptoms -- all you need is to stuff your face with high-fat foods for six weeks or so. Then replace all that junk food with a carefully maintained, healthy diet and you get to live through all the joy and glory of a painful, mentally excruciating rehab period. The test subjects had significantly increased levels of stress hormones and abnormally high amounts of CREB, a protein linked closely to the dopamine reward system (aka the thing in your brain that gets you high). What other things can manipulate these brain levels? Just harmless little substances like cocaine and methamphetamine.

Photos.com

And sure enough, the mice in the study became anxious and annoyed, avoided open spaces, and showed all the signs and symptoms of drug withdrawal just a few days after being deprived of their precious fatty foods.

Although this makes dieting infinitely harder than your average Weight Watchers ad makes it seem, it's not totally impossible. The trick here is to avoid yo-yo dieting, which sends you plummeting into an everlasting downward spiral of dopamined-up binge eating and withdrawal-laced, stressed-out dieting. If you want to quit heroin, you sure as fuck don't give yourself an occasional "cheat" day or week to treat yourself.

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There are few things more appetite-inducing than the smell of freshly baked bread. Conversely, hold your nose while you eat and you'll see just how bland even the most delicious dish can become. The smell of food is a massively important part of the eating experience, as your nose sports a direct line to the Food Control Center of your brain. This is why these days a whole lot of research is being devoted to figuring out why fat people's noses seem to work differently from everyone else's.

Multiple studies have found a link between hunger and sense of smell -- specifically, your nose works better when you're hungry. This would seem to make sense from an evolutionary point of view -- you need your nose when you're hunting or gathering to both find your food and navigate your environment (i.e., knowing when predators are coming). Once you've eaten, your sensitivity to smells in general goes down, but strangely, your sensitivity to food smells goes up.

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And in the obese? Their ability to smell food goes way up after they've eaten.

Scientists have no idea why either of these things happens. It's possible that fat and thin people both smell things better while eating because we need to be able to detect spoiled food and know whether the orange thing we're eating is delicious cheese or a disgusting carrot. But when you crank this ability up to 11, as it is in the obese, all it does is make them want to eat more.

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And to make it even worse, eating more doesn't necessarily satisfy the craving created by smelling those delicious smoked ribs, because ...

Why do some people feel the need to gobble up everything in their line of sight, while others can eat a third of that and yet stay satisfied? Are the moderate eaters just pretending to be happy with their stupidly tiny portions, secretly bursting into tears every time they see a Doritos commercial?

Well, there might be an actual, physical reason, and that reason is all about the mouth: Some of us have crappier taste buds than others. Namely, obese people, and specifically obese children.

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It appears that fat kids tend to have worse taste buds than their leaner counterparts, and this could be a significant factor as to why they became fat in the first place. The theory is that because of their crappy tasting capacities, it takes more food to experience the same "taste hit" as a person with a normal tongue.

In an effort to verify this, researchers gave both obese and non-obese children different taste strips to lick and identify. These strips were infused with base flavors (sweet, sour, salty, bitter, and savory), and they all had different intensities of taste. Their scores were ranked on a scale of 0 to 20, and the fat kids performed significantly worse than non-obese ones; on average, they only scored 12.6, while others scored a solid average of 14. While the obese children had no problem identifying what a given taste was, they did have trouble identifying its strength level. In other words, they tasted the same things, only not as intensely.

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Now, this is the sort of thing that might not have mattered back at a stage of evolution when it was really hard to get fat -- there was a time when how much we ate depended on how much we were able to kill/gather/grow. But now that virtually everyone reading this has ready food waiting just a refrigerator away, little physical flaws like this suddenly make it way harder for some of us to know when to stop cramming Hot Pockets into our cheeks.

Humans have immune cells that carry the awesome name of invariant natural killer T-cells. Their job is to help prevent metabolic trouble, and they fight obesity as part of that. But their numbers don't scale up with your fatness -- the opposite happens. The more fat you gain, the fewer fat-fightin' natural killer T-cells remain. And once they've lost the battle, there is no backup.

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In practice, this means that maintaining your weight is easy as pie when you're near your ideal weight, because at that point you have an army on your side. Once you hit a certain weight, you're left fighting alone, like a city whose police force just quits when crime gets too out of hand. Yes, your body's fat regulation system works just like the plot of RoboCop.

It's not all bad, though: Research indicates that not only can the body's T-cells be reactivated with either weight loss or various therapies, but they can actually be used as a super weapon against certain metabolic ailments, such as diabetes.

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Scientists connected participants of all shapes and sizes to MRI scanners and showed them pictures of food when their blood glucose was down. When subjects were shown pictures of high-calorie junk food, their brains gave them the equivalent of "drop everything else and go get that cheeseburger right now," just as they should. The reward centers lit up, the impulse control office went quiet -- all part of a system designed to keep you from starving yourself.

But here's where the deck gets stacked against the fat among us. When glucose levels were returned to normal, the thin subjects' brains stopped screaming for sugar. The reward centers went silent, the control center lit up. With the obese? Their brains stayed in hunger mode:

Via Yale.edu

The blue is the reward centers, the orange is the decision-making. So to recap, when blood sugar is low, everybody wants junk food. That's expected. But when the blood sugar of the obese was brought up to normal, their brains were making the same demands as if it were low.

Research doesn't specifically state whether this means that people's brains become different once they get fat, or that people get fat because they have different brains -- an important distinction, in case you were hoping things would get easier once you lost the weight. Unfortunately, if you're fat, then it's the same either way: Your hunger alarm simply never turns off.

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Check out XJ's $0.99 science fiction novella here. He also has a blog, or you can follow him on Twitter.

For more reasons why everything is working against you, check out Fat Is Officially Incurable (According to Science) and 6 Of Your Favorite Things That Are Secretly Making You Fat.

If you're pressed for time and just looking for a quick fix, then check out The 6 Most Impressively Nerdy Marriage Proposals.

And stop by LinkSTORM to learn how you can kick fat right in the genitals.

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Speaking of the diabolical nature of your body, here are 5 Physical Traits That Determine if a Character Is Evil.