fats... require more calories to metabolize than what they provide
That's not Atkins' reasoning at all. Protein does indeed take some energy to process, though IIRC less than half of what it gives. But fat, like carbohydrate, takes very little; that's not the reason why his diet works.
AIUI, it's connected with body chemistry, and in particular, insulin. Insulin is a hormone that regulates blood glucose (sugar) level. After being eaten, carbohydrate is broken down and absorbed into the bloodstream as glucose, but the body can't use it without insulin. So as the blood glucose level rises, the pancreas produces insulin. This has several effects:
The upshot of the last two is that while you're using carbohydrate for energy, you're storing away fat instead of burning it. The only way you can start to burn fat is to run out of carbohydrate to burn.
- It enables the rest of the body to burn the glucose for energy. This helps to lower the blood glucose level again, keeping it stable.
- It enables excess fat and carbohydrate to be stored in the fat cells.
- It prevents fat being burned for energy.
This is why Atkins is different from most other diets: you can eat lost of fat and protein without storing it as fat, because without carbohydrate you're not producing much insulin, and without insulin you can't store fat.
As you say, in some ways Atkins is like returning to a pre-agriculture diet. Back then carbohydrate was an occasional treat, burned immediately, and people quickly switched back to burning fat again - these days, with our carbohydrate-rich Western diet, our bodies are in `sugar mode' most or all of the time. We get sudden spikes of blood glucose, and our pancreas has to work overtime to dump enough insulin in our systems to use it. That may well be why we're seeing a rise in cases of diabetes (the inability to regulate blood glucose properly). No carbohydrate means no spikes in blood glucose; fewer food cravings, less of the bloated, lethargic feeling after meals, steadier energy levels throughout the day, &c. So Atkins is healthier in other ways too.
There are a lot of exaggerated claims made for many diets, Atkins included; but in this case there does seem to be some sound biological reasoning for it. (I lost a stone and a half on an Atkins-like diet myself; it might not be suitable for everyone, but I can vouch for its effectiveness.)
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