The Kreb Cycle: How it effects our training: How Carbs work for us as Athletes, By: Jacquelyn Thompson
The Kreb Cycle
Written by: Jacquelyn Thompson | M.S. in Clinical Nutrition
The Kreb Cycle,
tricarboxylic Acid (TCA) cycle, or citric acid cycle (all three terms are used to describe this process) is one of the many pathways used by our body to produce energy. This pathway is oxidative or aerobic meaning oxygen is used in the process, allowing our body to produce up to 38 molecules of ATP at this time.
It is through this pathway that fat (as well as carbohydrates, and proteins or amino acids) can be utilized for energy, either in the form of the foods we eat or what our body already has in storage form. This is through the process of beta oxidation which provides the enzymes needed for the Kreb Cycle itself to take place. It is estimated that up to 90% of the food energy we eat is used in this manner in conjunction with oxidative phosphorylation.
This pathway is used more often while at rest, while performing long steady state activity, or during training or exercises considered to be LISS or low intensity steady state, such as walking on an incline, hiking, or cycling, utilizing fatty acids for lighter activity needs, and transitioning to glucose or glycogen as energy demands and intensity increase.
This is often why... you may see bodybuilders or highly conditioned athletes walking for long periods of time instead of killing themselves running for hours and hours to achieve the level of leanness they may be somehow able to maintain! (And before you think how unfair, keep reading). This is because they are (hoping to) able to utilize fatty acids (either from their diet or physical fat stores), and in the process reserving glucose and muscle glycogen for more intense training needs. This means they can burn fat when need be, and then reserve the use of their carbs (dietary) or glycogen (stored already in the muscles from earlier) as a fuel for other intense activities such as weight training, power lifting, or sprinting short distances, which requires ATP in the muscles for shorter, explosive bursts.
This can be done by calling on the glycolytic pathway with lactate being a limiting factor, meaning this usually lasts no longer than 1-2 minutes until lactic acid buildup causes a pain response before we are able to cause damage to ourselves.
This is partially why they glycolytic pathway is used in short bursts. As we are working harder two things happen:
- Oxidation is the bodies main means of burning fat. But it is a long process. When we are working hard physically, we are not able to take in oxygen quick enough for oxidation to occur and are then only using stored glycogen or glucose from our diet, in a less efficient way, to fuel these short bursts. (That's right, still not burning fat yet.)
- We are also working harder turning over ATP rapidly, at a higher rate than it can be replenished. We are then in a state of oxygen debt, until we (stop and gasp for several seconds to a minute) and are no longer in debt of this vital nutrient.
- This system is still used though for many aspects of exercise and performance (as it doesn’t take long to do 8-15 reps of a motion).
But our body can use another means of fueling our performance as well, one that does as burn fat as well as utilizes carbohydrates and proteins at the same time.
The use of the oxidative cycle via the Kreb Cycle allows for the body to burn at a steady yet consistent rate so long as there is oxygen present, utilizing a large majority of the food energy consumed to do so. This is partially why it is the primary system used while at lower levels of energy output, as once we are working harder two things happen:
Oxidation is the bodies main means of burning fat. But it is a long process. When we are working hard physically, we are not able to take in oxygen quick enough for oxidation to occur.
We are also working harder and turning over more ATP at a higher rate than it can be replenished.
This is done by converting fats, proteins, and glucose or glycogen, and shuttling them into the mitochondria, along with the aid of several nutrients like biotin, h2o, Acetyl-CoA, and a few others, to convert NAD+ to NADH and shuttle pyruvate into the cell. All of this combined creates a cycle of ATP creation resulting in up to 38 molecules of ATP potentially being synthesized by these enzymatic processes.
The actual act of converting pyruvate into useable energy in the form of ATP is a highly involved process. Not everything can simply pass through the cell membrane, so they must be converted into a manner that is allowed to enter-basically being taken apart or restructured in manners that are passable, then reconstructed once within the mitochondria into useable building blocks, which are then reassembled through several more steps with the aid of coenzymes, until ATP is formed, water and carbon dioxide are off-gassed, and ATP is kicked back out of the mitochondria and out of the cell to be used as explosive little energy bursts when muscle fibers contract.
Or for a longer explanation:
During citrate synthase, Pyruvate is taken, and can either be used as an immediate energy source, putting off lactic acid, or it can be acetyl-CoA adds two carbons to oxaloacetate to create citrate.
This is then followed by aconitase which via isomerization removes H2o then adds it back once through the cell membrane.
During isocitrate dehydrogenase CO2 is lost and NADH is produced.
Succinate dehydrogenase ensures FAD+ is reduced to form FADH2 and then after H2O is added back in across the double bond, after which more NADH can be produced.
This in turn produces pyruvate which is converted back to acetyl-CoA again in the process.
As all of this occurs via coenzymes (made of amino acids) and the process of electrons being added in via the transport chain, fatty acids are broken down and utilized for fuel.
Glucose is used to contribute carbon and hydrogens as well as provide fuel sources as well, all which helps in the process of creating ATP for the use of energy.
How do we utilize this info though in terms of maintaining a healthy weight, supporting energy needs, and health and fitness endeavors?
While the means of activating or utilizing the oxidative pathway may take longer in terms of the activities that engage this pathway, it allows for fat utilization, less stress on the system, less impact to be placed on joints, ligaments, and tendons, and for those who may be in a deficit, allows for nutrients to be timed and utilized in theory, in as optimal a manner as possible. While this takes longer and calls for more time management, many prefer the longevity and repetition this offers in pursuit of their physique or weight loss goals, as it is easier to repetitively do and less taxing on the system, less likely to result in an injury, and allow for more consistency which in terms of weight loss is highly important!
Others may enjoy or be able to plan better for a shorter 10-15 minute HITT session, but this depends on if this is something they can maintain doing without stressing their system (as cortisol is a catabolic hormone, this could result in further breaking down muscle tissue, a counter-intuitive result), or being too exhausted to perform in other areas of their training. In general, an individualized approach that is both maintainable, sustainable, and able to be consistently executed with as minimal added stress to the system will show long term benefits and should be determined based on individual needs. This being said, the info above should help those who may be unsure about what route will serve them and their goals the best way they see fit.
By understanding how what we do effects the body as a system entirely, from the foods we eat, to the way they fuel our function and needs, we are then able to decide as we see fit the route that works best for us to move more efficiently towards our goals, and ideally shift into creating a lifestyle that is not only maintainable, but enjoyable as well.
References
Frederick C. Hatfield, P. (2019). ISSA Fitness: The Complete Guide, Ninth Edition. Carpinteria, CA: International Sports Sciences Association.
Sareen S Gropper, J. L. (2018). Advanced Nutritoin and Human Metabolism. Boston, MA: Cengage Learning.
Comments
Post a Comment