How Do We Acquire and Use Energy From Food? Part 2

Welcome back to our discussion of energy systems! I appreciate you coming back and your desire to learn! That’s the whole goal of this website: to learn ya’ somethin’! Upon reading a comment from the first post, a reader enlightened me on my neglect to go into detail about what ATP is or what is stands for. So, before I dive into the final energy system, aerobic glycolysis, I’m going to briefly talk about what ATP is! Let’s begin!

ATP Revisted

ATP stands for Adenosine TriPhosphate. This is the molecule our body synthesizes from all these different energy systems in order to make us move in all the ways that we do. Chemically, it is composed of a DNA molecule known as Adenine (in this case, adenosine), ribose, and phosphate groups.

Adenine is one of the four components that create DNA (Only four things known as nucleotides make up your entire DNA sequence! That’s amazing!). Adenine then binds (connects) to ribose, a sugar molecule. Finally, this sugar is bound to a chain of 3 molecules known as phosphate groups.

What makes ATP the OG energy molecule is those phosphate groups. These are known as “High-energy bonds” that, when broken off the ATP molecule, release A TON of energy that our muscles, cells, etc. use to do all the activities that we do.

When a phosphate group is removed from ATP, it becomes ADP (Adenosine DiPhosphate) and AMP (Adenosine MonoPhosphate) when two groups are removed. Here’s a nice visual from Khan Academy¹ to summarize what I mean by molecules, phosphate groups, etc.

Untitled design (8)

Phew. Okay. That covers ATP. Now! Onto the star of the metabolic show, aerobic glycolysis!

Aerobic Glycolysis

Why do I refer to this energy system as the star of the show? This is the system that not only provides the most energy, but it is also in use the most amount of time because typically, we aren’t jumping, sprinting, etc. We only do that for a relatively short period of time (even though it may feel like it never ends).

When we’re just walking, sitting, working, doing normal people stuff, we’re using this energy system. ADDITIONALLY, this is the primary energy system in use when we’re doing light to moderate-intensity exercise for a long period of time.

What’s the reason behind this? Well, for everyday stuff, we’re not in dire need of energy at that very moment like we may be if we’re sprinting away from a bear or angry girlfriend (which are equally dangerous).

Our bodies are built for survival. If it doesn’t need energy ASAP, it’s going to break it down slower but provide more of that energy on a per-cycle basis. What I mean by this is for each “cycle” completed of aerobic glycolysis, we get more energy molecules, meaning more energy for us! Woohoo!

After the lactic acid cycle is depleted/unable to work further, this system kicks in for the remainder of the exercise. Interestingly, long-distance runners can actually notice when their metabolism “switches” to aerobic glycolysis. It’s characterized by fatigue, tiredness, and a feeling of “hitting the wall”. They feel this way because energy isn’t being produced as quickly as we need it.

Also a fun fact, this system is aerobic which means it requires oxygen to start working. Ever notice that you start breathing more the longer you exercise?? You’re taking in that oxygen for a reason. Your body knows when it needs oxygen, and so your brain will tell you to breathe more to take in more oxygen! BOOM!

Why does it take so long to acquire this energy? Aerobic glycolysis relies on fat consumed in the diet or from body fat stores once dietary fat is consumed in order to synthesize glucose and/or ATP. I say ‘and/or’ because when we use fat as energy, it actually breaks into its two components (glycerol backbone and three fatty acid chains, refer to this article on fat for a refresher on the structure of fats).

Glycerol produces a small amount of glucose while the fatty acid chains cannot be converted into glucose; so they have their own metabolic pathway to produce ATP. Creating glucose from sources other than carbohydrate (protein, glycerol, lactate) is known as gluconeogenesis². We actually saw this during the lactic acid cycle! Lactate becomes glucose during the cycle!

Back to the question, fat, as an energy source takes a while because of those damn fatty acid chains. These chains are composed of a lot of carbon atoms that go through a lot (a lot!) of steps to become usable energy. This metabolic pathway is known as Beta-oxidation or fatty acid oxidation.

Why Does This Matter?

Well I’m not going to teach you something if it’s not important! Also, this information will be on the test next Thursday, so make sure you study it.

It’s important because if you do long-duration exercise, you will be using this energy system for most of the time. Additionally, this is the system in use most of the time throughout daily life!

Yeah. So what?

So what? SO WHAT?! This is a sign for you to see that dietary fat is not bad for you. It’s an energy source that is very important for prolonged energy production! Also, if you know that you’re going somewhere without food for a few hours, having fat in a meal prior will help you stay energized. ‘Energized’ does not equal ‘full’ though, keep that in mind. Combat stomach emptiness with fiber and protein!

But, if you need energy for a long time because you won’t get to eat, having some fat from nuts, peanut butter, oils, avocados, seeds, etc. will keep you moving forward! THAT’S why this is important, dammit.

Here is a helpful graph from Precision Nutrition³ to summarize what these last two posts were about. I encourage you to read that linked article too. It’s super informative!

As you can see, ATP stores in the muscle are used up almost instantly, followed by the ATP-PC system (Creatine Phosphate) in purple, then the lactic acid system in green after about 2 minutes. Finally, aerobic glycolysis kicks in for the remainder of the activity at the expense of exercise or activity performance aka “Hitting the wall”.

Image result for energy system use over time

Takeaways

  • ATP is the primary energy molecule made of adenosine, a sugar molecule, and phosphate groups
  • Aerobic glycolysis kicks in after the lactic acid system and continues pumping out energy for the duration of exercise or the activity being performed.
  • Dietary fat and body fat are the primary fuel sources for aerobic glycolysis (Does not mean you can sit on your ass and claim you’re burning body fat. It doesn’t work like that.
  • When used for energy, fat is broken into two components that enter two different metabolic pathways (gluconeogensis for glycerol and beta-oxidation for fatty acids)
  • If you can understand what system is used, you can better prepare meals for exercise or if you’re going to be out for the day!

Do you like posts like this where I explain nutrition science topics?? I love talking about this stuff because I feel that science needs to be communicated to the public more often and in a better way. That’s one purpose of this blog if you couldn’t tell by now! Let me know what you think in the comments! All feedback welcome! Thanks for reading!

References

¹Basic concepts in bioenergetics: phosphoryl group transfers and ATP hydrolysis

²Glucose Can Be Synthesized from Noncarbohydrate Precursors

³All About High Intensity Interval Training (HIIT)

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