NOVA

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It's one of the largest organs in your body you can't live without. Yet many people desperately want to lose their fat. But fat's been given a bad rap. They think of fat as being an evil substance, but fat is life. Fat is critical to our health, it is releasing a whole host of hormones important for our bones, for our brains, for our reproductive organs. Fat is actually a very sophisticated endocrine organ. For millions of years people who were able to hold onto their fat had a selective advantage and now all of a sudden we've asked people to get rid of their fat. But our bodies never evolved to do that. -

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So what happens when we have too much fat? Diabetes, cholesterol issues, heart issues. It just sneaks up on you. It's like smoking cigarettes, it kills you slowly. You don't feel the pain. I was aware that I was gaining weight, but I didn't know what to do about it. I'd follow every single thing my mom would say, I'd exercise for literally an hour daily. -

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Do we control our fat or does it control us? Scientists are finding hormones and genes that act on the brain to influence our size and shape. And those biological processes are well beyond willpower. -

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The implications are huge. Obesity is the number one chronic disease that we're dealing with. -

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How can science weigh in our weight problem? The pieces are starting to fall together, signals coming from the body, the brain, and the genes. It's very complex, but that's exactly what you would expect for a system which is so critical to survival. -

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The truth about fat. (dramatic music) On NOVA.