I'm listening to Gary Taubes' "Why We Get Fat And What To Do About It" on audio book borrowed from the library. I love consuming a book this way, employing my commuting time to good use. The book discusses the cause and effect of getting fat. It's kinda hard to get your head around. He argues against "calories in, calories out" being the cause of getting fat. Rather, he states that we eat a lot because we are getting fat. Huh?
One example in the book is a study on rats. The ovaries were removed from female rats and they ate a lot and got fat. Some of these rats were calorie restricted. They too became fat by becoming sedentary. It would seem that the rats got fat because they either ate too much or moved too little. But Taubes says that this is simply stating the obvious and doesn't describe the underlying cause of the obesity. It's like asking why a room becomes crowded. A smart-ass would say because more people entered than left the room. Certainly, but why?
In the rat study, they also injected some of these rats with estrogen, a sex hormone that would have been produced by the ovaries. The rats with the supplemental estrogen did not eat more food and did not become fat. The explanation for these results is that the hormone estrogen has an inhibitive effect on fat absorption. Fat cells have an enzyme whose job is to absorb fat from blood into fat cells. Estrogen regulates or inhibits how effective these enzymes are, like a traffic cop. The role of this hormone also explains why it is common for menapausal women to gain weight when they produce less estrogen.
So, the rats didn't get fat simply because they ate too much or moved too little. They got fat because estrogen was removed from their bodies which allowed their fat cells to hoard fat from their bloodstream. This removal of fat from their blood caused a caloric deficit that would have otherwise been utilized by other cells in the body. These rats needed to offset this loss of energy by either eating more food or moving less. They ate more because they became fat.