DISCLAIMER – THIS ONE IS MORE SCIENCY-Y.
Our body is impressively adaptable and can use a broad array of metabolic pathways to produce energy. If we think of our body as a car, our cells as the engine and food as our fuel, then we’re a bit like a hybrid. We can use multiple different kinds of food to make ATP (the energy currency derived from metabolism). The metabolism of different macronutrients is complex, therefore, some compounds lead to faster energy production.
That cellular engine we were talking about earlier, more specifically is inside cells and called the mitochondria – the energy powerhouse of the cells. This is where that all important energy currency, ATP, is generated. For us to be able to produce ATP we need to use theKrebs cycle which breaks down Acetyl-CoA. To get this Acetyl-CoA, we need to break down either pyruvate or fatty acids that are obtained from breaking down carbs, fats or proteins.
Generating pyruvate or fatty acids occurs inside cells, but outside of mitochondria. To be turned into ATP they must be inside. That means they have to get across the mitochondrial membranes. For long chain fatty acids this requires a unique protein called carnitine.
Mitochondrial fatty acid entry
Long-chain Fatty acids cannot enter inner mitochondrial membranes directly. First, they need to be broken down into something called Acyl-CoA. Acetyl CoA requires a protein to help it across the mitochondrial membrane, so a carnitine carrier system exists for transport. The binding of Acyl-CoA and carnitine forms acylcarnitine, which activates the carnitine and initiates the transport.
Carnitine activation requires low insulin (which will become important later). The acylcarnitine movement into mitochondria is made possible by a protein (Carnitine palmitoyl transferase I (CPT I)) that exists on the mitochondrial outer membrane. Another protein (Carnitine palmitoyl transferase II) that is in the inner mitochondrial membrane, then pulls apart the Acyl-CoA from Carnitine, freeing the fatty acid within the cell where it can be broken down (beta-oxidised) to provide energy. The same proteins that separated carnitine from acyl-CoA and put acylcarnitine into the mitochondria put the carnitine back into the cytosol outside of the mitochondria so it can be used again.
The takeaway– Long chain fats have to go through a fair bit of processing before we get energy from them.