One of the most popular ideas in weight loss right now is "trust your body's wisdom." The body knows what it wants. The body never lies. If you listen to signals like hunger and satiety, your body will never steer you wrong.
This is a lovely sentiment, and it's true that the body's is a great source of wisdom. Until it's not.
A new study presented at the annual meeting of the Endocrine Society shows how the brain can be tricked by the body into overeating and choosing fattening foods over healthier choices. 
The study set-up was simple enough: bring in hungry participants and ask them to choose between high-calorie, high-fat foods (e.g. pizza, cake, chocolate) and less fattening foods (e.g. salad, vegetables, and lean protein). Not surprisingly, hungry participants preferred the less healthy choices. Their bodies craved energy. The researchers then had participants makes similar choices, but 90 minutes after eating a meal. They weren't as hungry, and they made healthier choices. They "listened" to their bodies and choose a more appropriate snack, given their fullness.
But the researchers didn't stop there. They were interested in whether they could mimick the effects of fasting by manipulating participants' level of a hormone called "ghrelin." Grehlin stimulates appetite and plays a big role in the body's signals of hunger and cravings. It is typically regulated by things like how recently you ate your last meal and blood sugar level, making it a good signal of the need to eat. But it can also be influenced by many other things, including stress and sleep. This means that as much as you'd like to trust the body, the signal of hunger can be a lie.
Back to the lab: On one visit, participants who had recently eaten a full meal were injected with ghrelin. And this time, the participants behaved as if they were starving. They found the higher-fat, higher-calorie foods more appealing and were more likely to choose them--even though the body was actually quite satiated.
This injection was just a quick-and-dirty stand in for all the things that can push ghrelin levels up in the real world. If you're sleep deprived, your body is pumping out more ghrelin to get you to eat.  It's a poor substitute for sleep, but high-fat, high-sugar foods are a source of the energy you desperately need. The same is true for stress. And research shows that high-sugar foods-especially drinks, including soda-artificially boosts ghrelin levels.  This is one likely reason that soda and fruit juic consumption are both associated with obesity. The drinks themselves may not be a diet-breaker, but if you sip them all day, your body starts to lie to you. When it doesn, you'll be hungrier and more attracted to less healthy foods.
The bottom line: listening to your body is a fine ideal, but it needs to be balanced with mindful self-control. Know that not every craving is a message of wisdom from your stomach. Sometimes it's just a trick.
1. The Endocrine Society (2010, June 20). Stomach hormone ghrelin increases desire for high-calorie foods. Presented by T. Goldstone.
2. Spiegel K, Tasali E, Penev P, Van Cauter E. Brief communication: Sleep curtailment in healthy young men is associated with decreased leptin levels, elevated ghrelin levels, and increased hunger and appetite. Ann Intern Med. 2004 Dec 7;141(11):846-50.
3. Lindqvist A, Baelemans A, Erlanson-Albertsson C. Effects of sucrose, glucose and fructose on peripheral and central appetite signals. Regul Pept. 2008 Oct 9;150(1-3):26-32.