The Krebs Cycle Explained: How Mitochondria Turn Food Into Cellular Energy
Key takeaways
- The Krebs cycle, citric acid cycle, and TCA (tricarboxylic acid) cycle are three names for the same pathway, which runs inside the mitochondria.
- Its main job is to harvest high-energy electrons from food-derived molecules and load them onto carriers—NADH and FADH2—for the electron transport chain.
- It sits in the middle of energy production: glycolysis and fatty-acid breakdown feed it; the electron transport chain follows it to make ATP.
- The cycle is also a biosynthetic hub and a source of signaling molecules—it does far more than make energy.
- You can’t “supplement” the Krebs cycle directly, but supporting mitochondrial health—through exercise and research-backed compounds like standardized Urolithin A—supports the environment it runs in.
Almost every discussion of cellular energy eventually lands on the same place: a loop of reactions discovered by Hans Krebs in 1937, running quietly inside the mitochondria of nearly every cell in your body. If mitochondria are the cell’s power plants, the Krebs cycle is the central turbine room. It rarely gets explained in plain language, so here is a clear, jargon-light tour of what it is, what it produces, and why it keeps coming up in conversations about aging and cellular energy.
What is the Krebs cycle?
First, the names. The Krebs cycle, the citric acid cycle, and the TCA (tricarboxylic acid) cycle all refer to the same thing. It is called the citric acid cycle because the first molecule formed in each turn is citrate (citric acid), and the TCA cycle because citrate is a tricarboxylic acid. Scientists use the terms interchangeably.
At its heart, the cycle is a loop of eight enzyme-driven reactions that takes in a two-carbon molecule called acetyl-CoA and systematically strips energy out of it. The “cycle” part matters: the molecule that starts each turn (oxaloacetate) is regenerated at the end, so the loop can run continuously as long as fuel keeps arriving (Akram, Cell Biochem Biophys, 2014).
Where it sits in the bigger picture
The Krebs cycle does not work alone. It is the middle act of a three-part energy story:
1. Breakdown and delivery. Carbohydrates are broken down through glycolysis into pyruvate; fats are broken into fatty acids. These are converted into acetyl-CoA, the universal fuel that enters the cycle. This is why both carbs and fat can ultimately power the same machinery.
2. The Krebs cycle. Acetyl-CoA enters, and over one turn the cycle releases carbon dioxide (which you breathe out) and—crucially—transfers high-energy electrons onto two carrier molecules: NADH and FADH2. A small amount of energy (as GTP/ATP) is also made directly.
3. The electron transport chain. NADH and FADH2 carry their electrons to the inner mitochondrial membrane, where the electron transport chain uses them to pump protons and ultimately produce the bulk of the cell’s ATP through oxidative phosphorylation. The Krebs cycle, in other words, is the step that loads the carriers the final stage depends on.
What one turn produces
It helps to see the inputs and outputs of a single turn laid out plainly.
| Per turn of the cycle | Amount | Where it goes |
|---|---|---|
| Acetyl-CoA (input) | 1 | Combines with oxaloacetate to form citrate |
| NADH produced | 3 | Delivers electrons to the electron transport chain |
| FADH2 produced | 1 | Delivers electrons to the electron transport chain |
| GTP/ATP produced | 1 | Direct, small energy yield |
| CO2 released | 2 | Exhaled through the lungs |
Because each molecule of glucose yields two acetyl-CoA, the cycle typically turns twice per glucose molecule. The electron carriers it generates are responsible for the large majority of ATP your cells ultimately produce.
More than an energy factory
Here is the part that surprises many people: the Krebs cycle is not only about making energy. Its intermediate molecules—citrate, succinate, fumarate, malate, and others—are drawn off to build fatty acids, amino acids, and other essential components, and several of them act as signaling molecules that influence gene expression and cellular behavior. Modern research describes the cycle as a central metabolic and signaling hub rather than a simple energy conveyor belt (Martínez-Reyes & Chandel, Nat Commun, 2020). This dual role is one reason mitochondrial health has such wide-reaching effects across the body.
The Krebs cycle and aging
The cycle itself does not “wear out,” but the mitochondria that house it can become less efficient over time. Research in cells and animals suggests that aspects of mitochondrial function and quality control tend to decline with age, which can affect how smoothly energy production runs (review of mitochondrial dynamics and aging — PRECLINICAL). It is worth stating plainly: this is an area where animal and cell evidence is strong and human evidence is still developing. The takeaway is not that the Krebs cycle fails, but that keeping mitochondria healthy helps preserve the environment it runs in.
Where does Urolithin A fit?
You cannot take a pill that “speeds up” the Krebs cycle, and no supplement should be described that way. What some compounds are studied for is supporting the broader health and quality of mitochondria—the compartment where the cycle operates.
Standardized Urolithin A is one example. It is a postbiotic derived from dietary ellagitannins (found in pomegranates, walnuts, and berries) and is studied mainly as a mitophagy activator—supporting the cell’s recycling of damaged mitochondria. In preclinical research, it induced mitophagy and improved markers of muscle function (Ryu et al., Nat Med, 2016 — PRECLINICAL). In humans, a first-in-human study found it safe and associated with a mitochondrial gene-expression signature in muscle (Andreux et al., Nat Metab, 2019 — HUMAN), and randomized trials reported modest improvements in some measures of muscle performance (Singh et al., Cell Rep Med, 2022 — HUMAN). None of these studies measured the Krebs cycle directly; the connection is that healthier mitochondria provide a better setting for energy metabolism overall. If you’d like a standardized daily option, SOMA Urolithin A gummies deliver a 1,000 mg serving.
How to support the engine room
The most powerful inputs are the ordinary ones. Regular exercise—both aerobic and resistance training—is the best-established way to stimulate mitochondrial renewal and support energy metabolism. A varied diet supplies the fuel and the vitamin-derived cofactors (such as B vitamins) that Krebs-cycle enzymes rely on. Sleep and stress management support the recovery processes that keep mitochondria working well. Supplements studied for mitochondrial support can complement these habits but never substitute for them.
Frequently asked questions
Are the Krebs cycle and citric acid cycle the same thing?
Yes. The Krebs cycle, the citric acid cycle, and the TCA cycle are three names for the same pathway. The differences are historical and chemical naming conventions, not different processes.
Where in the cell does the Krebs cycle happen?
Inside the mitochondria, specifically in the fluid-filled matrix. The electron transport chain that follows it is embedded in the nearby inner mitochondrial membrane.
Can a supplement boost my Krebs cycle?
No supplement “boosts” the cycle directly, and claims like that should be treated with skepticism. Some nutrients and compounds are studied for supporting overall mitochondrial health, which is the environment the cycle operates in.
Why do I breathe out carbon dioxide from this cycle?
As the cycle strips energy from acetyl-CoA, it releases carbon atoms as carbon dioxide. That CO2 travels in the blood to your lungs and is exhaled—a direct link between cellular metabolism and breathing.
Support your cellular health, every day
SOMA delivers a high-potency 1,000 mg dose of Urolithin A in one simple daily gummy — no guesswork, no converter-status lottery.
Shop SOMA Urolithin A Gummies →These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.