Renegade Yogi

The Cori cycle, the neglected metabolic pathway

There is a well-documented phenomenon in your body that helps you burn fat while you are exerting yourself. But it is systematically avoided by most people when they are working out, because of science done years ago that has since been corrected. People avoid generating lactic acid and stay within their “aerobic threshold”.

But lactic acid is the key to a very inefficient (hence energy burning) and very fast metabolic pathway. If you’re not generating lactic acid, you are not burning as many calories as you could and you’re missing out on a lot of metabolic adaptation that could be making you stronger.

Necessary physiology (but not dull!)

Your body’s main source of energy is glucose. It stores this in the muscle in the form of glycogen, which is just a bunch of glucose stuck together.

Glycolysis converts glucose into pyruvate. This generates 2 ATP (the unit of energy in the cell). If the mitochondria aren’t too busy, the pyruvate gets used in the Krebs cycle, which generates about 30 ATP. That’s lots of energy, but the Krebs cycle is really slow. It cannot be done as quickly as glycolysis. And it’s also limited by how much oxygen can get to the cell (that’s why they call it “aerobic”). That’s why you start huffing when you’re working out (also to get rid of carbon dioxide).

Ok, so we’ve got this fast reaction, glycolysis, and this slow reaction, the Krebs cycle. When you need more energy than the Krebs cycle can provide, your cells are going to do a lot of glycolysis. It generates energy, but it also generates more pyruvate than you can use in the Krebs cycle. What happens to all the pyruvate generated through glycolysis? It builds up and gets converted to lactic acid (lactate).

Like I said before, lactate causes a burning sensation in the muscles. I like that feeling, but then again, I know it’s a good thing! But back to the story.

That lactic acid is released into the bloodstream and is taken in by the liver. The liver knows just what to do with it. It reverses the process that happened in the muscle: lactate -> pyruvate -> glucose. This costs a whopping 6 ATP. And that’s good, because it means you’ve just burned energy in your liver “without lifting a muscle”. The glucose then goes into the blood and is taken in by your cell. And the cycle starts over.

The cycle from glucose -> pyruvate -> lactic acid -> liver -> pyruvate -> glucose -> cell is called the Cori Cycle. Just to be clear, the Krebs cycle is still working all this time, but its energy output is negligible compared to the output of glycolysis. And since renewing the glucose with the Cori Cycle costs 6 ATP while glycolysis gives only 2 ATP, you’re burning a lot of energy. To use 2 ATP in your muscle, you need 6 more from your liver. Talk about burning calories!

But what happens when you take it easy and keep the intensity low? You don’t generate more pyruvate than you need, because you’re not using much glycolysis. Just enough to keep the slow and efficient Krebs cycle fed. You don’t generate lactic acid, you don’t send it to your liver, and you don’t burn 6 ATP. In other words, taking it easy is so efficient that you’re not burning as much energy as you can.

But there’s more!

Well after you’re done with your intense workout, your liver is still processing all of that lactic acid. So you’re still burning energy. And if you’ve managed to make a significant dent in the energy stored in your muscles (glycogen), that’s going to have to be replenished and then some. Your body will adapt by storing more glycogen in your muscles. This makes them bigger and oooh sexier. But if you take it easy, your muscles don’t use as much of their store, so they won’t feel the need to store more.

Now that you’re resting, you don’t need so much energy quickly, so the mitochondria are the main source of energy. And you need lots of energy to adapt those muscles. They’re going to be working overtime with all of that pyruvate you’ve built up that hasn’t been turned into lactic acid. They’ve got a lot of work to do, and many hands make light work. Your cells will respond by beefing up the number of mitochondria so that they can do this faster next time. That means faster recovery time. But if you take it easy, you don’t have a bunch of pyruvate built up. So the mitochondria don’t adapt.

And, of course, your liver is getting better at the Cori Cycle. Not more efficient (it still takes 6 ATP), but able to do it faster to keep up with demand. This means you can burn more energy faster. But the liver won’t adapt if you take it easy.

That’s why a 30 minute run, which once seemed difficult and made your muscles burn, doesn’t seem like that much anymore. Your body has adapted to it, gotten really efficient at it, and now you’re not burning so much energy. Your body has basically reached a comfortable steady state where your pathways are not taxed and so don’t need to adapt. Jogging, at this point, is about as efficient as walking.

The takeaway

If your goal is to burn fat, you need to up the intensity level to the point where your muscles burn. That burning sensation is lactate buildup. It’s hard at first, but you need to learn to work through the burn. It’s good for you.

Do this visualization to help internalize the goodness that is the burn!

1. Conjure up memories where you felt lactic acid burn.
2. Quickly replace those memories with the slim/sleek/ultrafit you. Let yourself feel some excitement and pride.
3. Repeat 5 or six times.
4. Take a break.
5. Check if it worked by conjuring up the lactic acid burn memories again. If you’ve successfully associated the new thought, you’re done! Otherwise, do it more.

You will need to constantly up the intensity as you adapt. But that’s ok, because it doesn’t mean that your workout will be longer. It means that you need to increase the number of reps in a given time or increase the resistance (or both). Your goal should be to burn as much energy as possible within a given workout. There is no reason to extend the time of your workout. You can guarantee that you have maxed out all of your energy systems (along the Cori cycle) and given your body lots of work to do after you’re done.

Jogging won’t cut it. You need to sprint as fast as you can. Until you burn. Then run some more.

Pushups and other bodyweight exercises are good, as long as you do enough and you do them quickly. And don’t stop when it starts to burn. Only short breaks between sets. I suggest you use a pyramid.

Weights and machines will increase the resistance. Make sure you feel the burn and keep going!

As a final word, I want to give you this tip: reps are only meaningful (in a calorie burning sense) when you feel a burn. Get to those reps as soon as you can.

Special Thanks

Thanks to Body by Science for clarifying how all of these systems tie together. It’s a great book and has much more physiology than I’ve mentioned here. It dispels common exercise fictions and shows how his exercise philosophy was systematically engineered from metabolic principles.