[cocoadog]

I haven't seen a specific study but I have read assertions along the same lines: either low-fat or low-carb can work.

I believe the insulin-centric explanation (I'm a low carb eater and this is the perspective I tend to see) would be something like:

- Low carb and high fat: The fat goes to your fat cells (that is, whatever fat you don't burn for fuel immediately after eating), but since your insulin is low the fat is readily burned off relatively soon.

- Low fat and high carb: The carbs cause an insulin spike, and insulin inhibits fat burning, BUT since you haven't stuffed much fat into your fat cells, there isn't much to burn off when your insulin eventually comes back down. You need to be relatively insulin sensitive, so that your insulin level does come back down. Even if you're insulin resistant, this diet might still work if you eat only a tiny amount of fat.

- High fat and high carb: Fat goes to your fat cells and insulin levels are high, which inhibits fat burning, so you gain fat faster than you burn it.

(Edits: I thought Markdown lists would work but they didn't.)

[maloney]

"Low carb and high fat: The fat goes to your fat cells (that is, whatever fat you don't burn for fuel immediately after eating), but since your insulin is low the fat is readily burned off relatively soon."

- The fat you eat does not have to be stored as fat, the raised insulin levels are necessary for the body to store energy as fat.

"Low fat and high carb: The carbs cause an insulin spike, and insulin inhibits fat burning, BUT since you haven't stuffed much fat into your fat cells, there isn't much to burn off when your insulin eventually comes back down. You need to be relatively insulin sensitive, so that your insulin level does come back down. Even if you're insulin resistant, this diet might still work if you eat only a tiny amount of fat."

- Carbs can be converted to fat via lipogenesis.

[spion]

And in fact they have to be converted to fat. There are only two other options:

stored in muscles as glycogen - however, the muscle stores can only store up to 1600 calories so they may be full. Also, once sugar gets into a muscle it can't get out - it can only be used by that particular muscle.

used by the brain - however the brain doesn't need more than 500 kcal per day. (and can live with as little as 120 kcal / 30g of glucose provided that the rest of the energy is supplied by ketones)

So... exercise doesn't help lose weight, but it sure helps prevent weight gain when eating carbs. With the amount of carbs we eat nowadays and the low amounts of physical activity, its no wonder at all we store a lot of fat.