Have you ever read or heard this phrase?
“Fat burns in the flame of carbohydrate”
It originates from a biochemist named Rosenfeld back in 1895. It was based on observations that cells could break down fatty acids into ketone bodies, but without sufficient glucose, the cells could not break them down fully into carbon dioxide and hydrogen.
We now know that fat actually burns in the flame of oxaloacetate.
Oxaloacetate can be derived from either glucose or amino acids. When we break down fats or carbohydrates for energy, we turn them into acetic acid, or acetate, which is a two-carbon unit. A shuttle called coenzyme A, which is made of pantothenic acid, carries the acetate around and together we call the complex acetyl CoA. Pantothenate is also called vitamin B5 and is found abundantly in many foods, particularly animal products.
In order to fully harvest energy from acetate, we need to send it through the citric acid cycle, also called the Krebs cycle or the tricarboxylic acid (TCA cycle). This cycle will break the acetate down into carbon dioxide and hydrogen. In doing so, it will also release high-energy electrons whose energy can then be harvested to synthesize ATP, a major usable energy currency of the cell. Entry into this cycle is dependent on a compound called oxaloacetate.
In the presence of glucose, we convert glucose to oxaloacetate. As oxaloacetate leaves the Krebs cycle, we can just use glucose to replenish it. In the absence of glucose, we do the opposite: we turn oxaloacetate into glucose. Thus, oxaloacetate gets depleted in the absence of glucose, unless we have some other source of it. We can make oxaloacetate from a variety of amino acids, but not from fats. Therefore, in the absence of dietary protein or carbohydrate, the only place to get oxaloacetate is to break down the lean proteins found in our muscles and internal organs.
As long as you’ve consumed sufficient protein, your body won’t break down your lean mass in order to make oxaloacetate.
Therefore, carbs are not necessary for fat metabolism.