In the first HYBW we learned that:
1. The way the human body obtains energy is 'more complex than a very complex thing'.
2. There are millions of little men who organise the distribution of energy inside your body (estimates of the exact 1. quantity, 2. size and 3. origin of the little men vary, but most Very Clever People agree that there are 1. between 17 and a gazillion, 2. 'extremely small', 3. there are some questions that only Very Stupid People bother to ask).
3. The primary task of the little men is to make sure your body doesn't run out of energy, and to that end they will stockpile any extra energy (as fat), and will slow your body's system down if they notice that energy demands are exceeding supplies.
Because the little men are extremely lazy, when your body needs energy they will take the easiest route they can to get it. Once they have used up the muscles' (limited) glycogen reserves, they start to nick (although they prefer to call it 'borrowing') stuff from the little men who are repairing and maintaining your body. When all that stuff is used up they reluctantly reach for the (more difficult to open) containers marked 'muscle' and 'fat'.
Yes, you did read that right, MUSCLE and fat. When we consume less calories than our body expends, we don't just lose fat, we also lose muscle. And as muscle is one of the main 'energy burners in our systems, when we lose muscle, we also need less energy to support our body (this is one of the reasons the little men get rid of muscle when the 'starvation bells' start ringing).
And here we hit the vicious cycle that many serial crash dieters encounter. A person gets an invite to a function that's happening next month, and realises they no longer fit the clothes they want to wear. So they embark on one of the latest 'soup and celery' diets, and - after four weeks of utter misery - reach their target. During and after the function they resume their previous eating habits, and are dismayed to find they quickly put all the weight (and often more) back on again.
Well, if you've been paying attention, you're there with the answer already. 1. The little men have sensed a 'starvation' situation, so they've slowed down the body's metabolism. 2. When the calorie deficit has become an ongoing problem, they've obtained energy from muscle as well as fat. 3. When the 'starvation bells' stop ringing, the little men start stockpiling fat again, in a body whose metabolism is running slower and has less muscle than it did at the start of the diet.
This is a problem exacerbated by the fact that most people use weighing scales as the sole means of measuring their progress. But we'll talk more about that in HYBW #3.
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