Ammino acids are proteins not fats. Glucose and fats are consumed all the time by the body unless it's straved for oxygen. Then it burns glucose only.
If the blood sugar level falls, the consumption of fats also decreases. E
Eremicus, I bolded "fatty acids" not amino acids. And, if blood sugar falls, the pancreas will release glucagon which will stimulate the liver to release an enzyme lipase to break triglycerides into glycerol and free fatty acids for use in metabolism (lipolysis).
Normally fatty acids are needed for metabolism of steroid dependent cells, such as those cells that produce sex hormones. Other hormones are fat based too. But, I'm not going to list them all.
But, to maintain homeostasis, the body will generate the process off gluconeogenesis when blood sugar falls and glycogen stores are insufficient. The body must do this to protect the central nervous system which is completely glucose dependent. And does not require insulin. Blood sugar must stay at least 70 mg/dl for proper nervous cell function. If the blood sugar drops too low, the central nervous system will collapse.
So, in reality, if blood sugar falls, fat metabolism increases. This is how we lose weight. Our bodies break down fat for fuel so our central nervous system doesn't die. I'm not saying it doesn't break down protein. It does, but with exercise, the muscle cells are stimulated and more fat will break down instead of protein. But, we have to be careful not to work the muscles too hard, or they'll break down from trauma and cause a person to go into rhabdomyolosis and acute kidney failure.
It's all about cellular metabolism, cellular respiration and the generation of adenosine triphosphate. It's how the krebs cycle works.
I don't advocate eliminating all carbs. They're brain food! (And I'd worry about ketoacidosis.) But, by reducing carb intake, the body will break down fat to feed it.
