Have you ever heard someone tell you that fats burn in a carbohydrate flame? It actually sounds pretty cool, huh? But what does it mean? Essentially, what this statement tells us is that we NEED carbohydrates in our bodies to actually burn fat. This is seen at the cellular level and can be easily explained by looking at a few metabolic pathways more closely: Glycolysis, The Krebs cycle, and Beta Oxidation to be specific.
It’s important to know that fats burn in a carbohydrate flame primarily for the sake of endurance runners and other aerobic athletes who perform long bouts of exercise (e.g. marathons).
See, if you don’t “carb load” properly to fill your muscle’s glycogen stores and/or if your blood glucose levels are low, then you won’t be able to efficiently use fat as energy. This can be a huge problem for endurance athletes. Besides this, being unable to burn fats due to low or no glucose in your body may put your body at risk for ketoacidosis.
Okay, first thing’s first. How exactly do fats burn in a carbohydrate flame? To understand this we’ll need to look at the glycolytic pathway known as glycolysis. This is where glucose molecules are broken down and used as energy by yielding ATP, among other things (e.g. NADH).
There are many different substrates that a single glucose molecule goes through. Nevertheless, the last and final substrate in slow glycolysis is pyruvate (aka pyruvic acid).
Pyruvate then goes into what’s called the intermediate step where it’s converted into Acetyl-CoA. Acetyl-CoA then enters what’s called the Krebs cycle. It is here where the substrate oxaloacetate combines with Acetyl-CoA to create Citric Acid. From here it goes on to create another substrate and then another until the cycle finishes where it started. Thus, why it’s called the Krebs cycle.
The Krebs cycle is vital for creating ATP as it gives off NADH and FADH to the metabolic pathway called the Electron Transport Chain (ETC). It is here (in the ETC) where even more ATP is made. Thus, the Krebs cycle is extremely important for overall ATP production. Now that we have all of that out of the way, let’s get into why fats burn in a carbohydrate flame.
So, why do fats burn in a carbohydrate flame?
When the body is running low on carbs or if you’re near total carb depletion, you aren’t able to make much pyruvate. This is a problem as you need pyruvate to enter into the intermediate step to create Acetyl-CoA so it can enter the Krebs cycle.
When pryuvate levels are low, oxaloacetate in the Krebs cycle will be converted back into pyruvate via the enzyme pyruvate carboxylase. This allows more pyruvate to be made, which is an essential substrate for the production of ATP.
When this occurs, the Krebs cycle is no longer able to process Acetyl-CoA substrates as quickly as it could before. This is so because Acetyl-CoA MUST combine with oxaloacetate in the Krebs cycle in order for the cycle to actually work and produce NADH, FADH, and ATP.
So, what we have now is a slower Krebs cycle. Why is this a problem and what does this have to do with the fact that fats burn in a carbohydrate flame?
Well, fats are metabolized in the metabolic pathway called Beta Oxidation. As an activated fatty acid gets mobilized in Beta Oxidation via the rate-limiting enzyme hormone sensitive lipase (HSL), it goes through its own cycle, creating several other substrates.
The end result in Beta Oxidation is Acetyl-CoA. This substrate is then sent to the Krebs cycle so it can combine with oxaloacetate to create citric acid, among several other substrates.
This becomes a problem…
Oxaloacetate is being converted back into pyruvate in an attempt to create more intermediate steps. So, there is no place for all of the Acetyl-CoA’s from Beta Oxidation to go to as they need to combine with oxaloacetate to form citric acid.
The Krebs cycle is going much slower when carbohydrate levels are low. So, what happens to all of those Acetyl-CoA’s derived from Beta Oxidation that can’t enter into the Krebs cycle?
Well, they eventually accumulate in the blood and form what are called ketone bodies. This can be very dangerous as it can cause you to get ketoacidosis. This is a serious medical condition where the body produces excess blood acids.
This condition can come about if you’re on a dangerously low carb diet or if you’re not eating any carbs at all. This condition is somewhat common among individuals with diabetes.
Final thoughts on why fats burn in a carbohydrate flame…
Fats burn in a carbohydrate flame because you NEED carbs to actually use and burn fat. Without carbs, oxaloacetate converts back to pyruvate. This slows down the Krebs cycle and makes it to where it can’t accept all of Beta Oxidation’s Acetyl-CoA’s. This causes an accumulation of Acetyl-CoA’s in the blood which can cause you to develop ketoacidosis.
If you have plenty muscle glycogen or free flowing blood glucose, then glycolysis will be working quickly, producing many pyruvate substrates. This will cause the Krebs cycle to work fast as well. Oxaloacetate won’t have to convert back to pyruvate because it’s already making it due to all of the carbohydrates being processed in the pathway.
This, in turn allows the Acetyl-CoA’s from Beta Oxidation (fat metabolism) to be processed in the Krebs cycle by combining with oxaloacetate to form citrate, and so on and so forth throughout the cycle yielding NADH, FADH, and ATP. Thus, there will be minimal if any accumulation of ketone bodies in the blood.
The moral of the story here is to make sure that you eat your carbohydrates! Carb loading or even just making sure that you eat a healthy diet with a sufficient amount of good carbs in it will help you to ensure that you’ll be able to burn fat while you exercise, as well as improve your cholesterol and heart health in general, assuming that you stick with healthy dietary habits. So, get out there and go eat some carbs!
Kraemer, Robert. Physiology of Exercise. Lecture. 15 February 2017.