The myth of switching to fat metabolism

There are numerous instances I’ve watched sports activities on TV and commentators present their insights on the regulation of gasoline use. “And then you definitely swap to fats metabolism” is likely one of the generally heard catch phrases. Final weekend this occurred once more once I was watching biking. However it’s a delusion that we’ve got a swap that enables us to pick out completely different fuels. How does it work? Please go this one on to TV commentators.

Once we train the muscle contracts and we want vitality for this. The vitality is offered within the type of adenosine triphosphate or ATP. The quantities of ATP saved in our physique are very small and due to this fact it’s essential to have programs in place that may regenerate ATP in a short time.

The 4 essential “vitality programs” are

These programs range of their velocity with which they’ll generate ATP. In addition they differ of their capability and a few processes require oxygen.

Throughout larger depth train the demand for ATP may be very excessive and though ATP ranges by no means drop very a lot there generally is a change within the ratios of ATP and the breakdown merchandise ADP and AMP (the place ATP is damaged down and we lose one or two phosphate molecules). These are indicators to hurry up vitality manufacturing. It’s, for instance, a set off for glycolysis to hurry up. This implies increasingly fast carbohydrate use.

The top product of glycolysis is pyruvate and that is the substrate (gasoline) for cardio metabolism. Nevertheless, if an excessive amount of pyruvate is offered and can’t be eliminated quick sufficient by cardio metabolism this may finally decelerate glycolysis. Subsequently, some pyruvate is changed into lactic acid, in order that pyruvate concentrations keep low and glycolysis can proceed. The manufacturing of lactic acid is thus serving to us, not harming us! (regardless that the manufacturing of lactic acid will finally end in an acidification of the muscle which can affect muscle perform and ache sensations we expertise).

Cardio metabolism is absolutely fuelled by acetyl-CoA, a metabolite that may be derived from carbohydrate or from fats. Within the mitochondria, the vitality factories of the cell, pyruvate is changed into acetyl-CoA and this may then be used within the so known as Tricarboxylic Acid Cycle or TCA cycle (Krebs cycle).

Acetyl-CoA also can come from the breakdown of fatty acids (fats) by a course of known as fats oxidation. As soon as carbohydrate and fats are damaged all the way down to acetyl-CoA the processes don’t distinguish anymore between acetyl-CoA from carbohydrate and acetyl-CoA from fats.

So, if there was a swap this swap must exist earlier than carbohydrate or fats is changed into acetyl-CoA and this merely doesn’t exist. These pathways are at all times energetic and acetyl-CoA is at all times produced from each sources.

Nevertheless, typically pyruvate is produced at excessive charges as a result of the depth is larger and extra pyruvate will probably be transformed into acetyl-CoA. That is largely pushed by activation of glycolysis. In different situations glycolysis proceeds slowly as a result of there may be much less demand for ATP and there could also be a big provide of fatty acids. In such instances extra acetyl-CoA will come from fatty acids.

To date, we talked about carbohydrate and fats, however there are completely different sources of carbohydrate and completely different sources of fats. Some carbohydrate is saved within the liver, some might come from our vitamin and a few is saved within the muscle. The liver and dietary glucose will probably be transported by way of the blood to the muscle. We are able to distinguish these sources in research and we are able to see that with growing intensities the contribution of each plasma glucose and muscle glycogen enhance, however by far the most important enhance and contribution is from the available muscle glycogen.

Fats comes from completely different sources too. Some fats is saved within the muscle. That is known as intramuscular fats or intramuscular triglycerides (IMTG). Regardless that endurance athletes could also be lean they really have barely bigger IMTG shops of their muscle than much less skilled people (a subject for a future weblog). Fats also can come from adipose tissue and be transported to the muscle as fatty acids within the blood. These fatty acids are then taken up within the muscle and might endure beta oxidation within the cell. Fats from meals might be a much less essential vitality supply as a result of it reaches the circulation very slowly and when it reaches it, it’s within the type of giant fats particles that have to be damaged all the way down to fatty acids first earlier than they can be utilized. 

Fats oxidation from blood fatty acids and IMTG enhance from low to average intensities and reduce when the train depth will increase from average to excessive. Peak fats oxidation can typically be noticed round 65% VO2max, though that is extremely particular person as we mentioned in earlier blogs about FatMax (Finding your fat burning zone; What is FatMax?).

With this in thoughts, one can’t ‘swap’ from carbohydrate to fats burning. Each processes proceed, however the magnitude of contribution adjustments. Listed below are two summaries to switch “And then you definitely swap to fats metabolism” that you could now use when watching endurance occasions:

• The contribution of fats and carbohydrate to offer vitality adjustments through the race, relying on the depth.

• Because the depth decreases, there could also be a transient enhance in fats metabolism to offer vitality.

1. There are primarily 4 vitality programs

2. They’re all used on the similar time the entire time

3. The principle driving drive for substrate use is the depth of train and the demand for ATP

4. Muscle glycogen can ship vitality by way of glycolysis and aerobically by way of oxidation

5. Carbohydrate use (each from plasma glucose and muscle glycogen) enhance step by step with growing depth

6. Muscle glycogen is by far crucial substrate at larger intensities

7. Fats oxidation will increase from low to average intensities and reduces from average to excessive intensities

8. Fats is derived from plasma (largely from adipose tissues) in addition to from shops contained in the muscle (IMTG)

9. There’s NO SWITCH to go from carbohydrate to fats metabolism or the opposite means round.