Taking a collagen supplement does not mean that collagen goes directly to your bones, joints, tendons, or skin. Before your body can use it, it must go through a much more complex process: digestion, absorption and distribution of its components.
In recent years, collagen has become one of the standout supplements. It is marketed as a help to care for joints, maintain the skin, strengthen bones, or support tendon and ligament health. But there is a fairly widespread idea that needs clarification: when we take collagen, our body does not receive “collagen ready to use” that automatically heads to a joint or a tendon. What we actually introduce are collagen proteins or collagen peptides that our digestive system processes.
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What happens when you take collagen?
The collagen found in supplements typically comes from animal tissues and is usually presented as hydrolyzed collagen or collagen peptides. When ingested, the digestive process begins. In the stomach and, subsequently, in the small intestine, the proteins are broken down into smaller fragments. Some end up as free amino acids, while others may cross the intestine in the form of small peptides.
These components enter the bloodstream and become available for the body to use. And here is the key: the body does not carry a label that says “these amino acids are for the knee” or “these are for the skin.” Once absorbed, the amino acids become part of the pool of nutrients available to the body. The body will use them where needed and according to its metabolic priorities.
So, does the collagen I take end up becoming collagen?
It can contribute to it, but not automatically. The body continually manufactures its own proteins. To produce collagen it needs amino acids, energy, vitamin C and other micronutrients and cofactors that participate in the process. Therefore, the amino acids from a collagen supplement can be used as raw material to manufacture new proteins, including collagen. But those amino acids do not have a predetermined destination.
Imagine we take 10 grams of collagen peptides. After digestion and absorption, their components join the body’s amino acid pool. Some of them may be used to synthesize collagen; others may be used to manufacture other proteins; and, depending on energy and metabolic needs, some amino acids may also be used as an energy source.
We cannot claim that a specific amount of the supplement ends up directly in bones, tendons, or joints. That’s why the idea that “I take collagen and that collagen goes to my knee” is an oversimplification.
And where does the 10% idea come from, then?
The claim that only around 10% of ingested collagen ends up allocated to bones, joints or tendons has gained traction. However, it is prudent to approach this figure with caution. There is no universal rule stating that exactly 10% of each collagen dose will improve those tissues. The fate of amino acids depends on multiple factors, such as nutritional status, the body’s needs, protein synthesis and breakdown, and the overall amount of proteins and nutrients available.
Therefore, it is more accurate to say that only a fraction of the components we ingest will end up participating in collagen synthesis in certain tissues, and that we cannot attribute a fixed percentage to bones, tendons or joints. Moreover, that an amino acid is used to manufacture collagen does not automatically mean that this process will yield a clinically noticeable improvement.
Collagen is not a “filler” for joints
This distinction is especially important when we talk about supplements aimed at athletes. A tendon, a ligament, cartilage or bone are living tissues that are continually subjected to processes of formation, repair and degradation. The body adjusts its metabolism in response to stimuli such as exercise, mechanical load, rest and nutrient availability.
That is why, if our aim is to care for our joints or tendons, it doesn’t make much sense to think solely about “supplying collagen.” The stimulus the tissue receives also matters.
Strength training, for example, provides a fundamental mechanical signal to maintain and adapt different structures of the locomotor system. Adequate energy and protein intake, along with proper micronutrients, also forms part of the context. The supplement, in any case, is just another piece.
That collagen doesn’t reach the joint directly doesn’t mean it’s useless.
Does this mean that taking collagen is useless?
No. Here is where it makes sense to avoid the opposite extreme. The fact that collagen doesn’t reach a joint directly doesn’t mean it’s useless. Some studies have observed that certain collagen peptides, consumed in specific amounts and over prolonged periods, may produce modest improvements in some markers or symptoms related to joint health. Its potential usefulness in other tissues and situations has also been explored. But these results should not be interpreted as: “I take collagen and my body automatically makes more collagen.” The reality is far more interesting.
Collagen peptides provide amino acids —with a high presence of glycine, proline and hydroxyproline— and certain peptides can also act as metabolic signals. The body may respond to these nutrients and signals by modifying certain processes related to the extracellular matrix and connective tissue metabolism.
But the effect depends on the dose, the type of product, the tissue, the person’s state, and the overall context of diet and training.
The protein doesn’t know the name of your injury
A good way to understand this is to think of amino acids as building blocks. When you eat collagen, you aren’t delivering to the body a finished piece of a joint. You are providing raw material and certain peptides that feed into a much broader system. The body decides what to do with them.
If it needs to manufacture collagen to renew tissues, it can use those amino acids. If it needs to synthesize other proteins, it will use them for that. And if there is an excess of certain nutrients relative to current needs, its metabolism will follow other pathways.
That is why, taking more collagen does not necessarily mean producing more collagen. The body does not function like a factory where simply increasing raw material automatically guarantees higher final production.
So what should we expect from a supplement?
A reasonable expectation would be to view collagen as a potential complementary tool, not as a miracle cure or a substitute for a proper diet. If a person follows a protein-sufficient diet, eats fruits and vegetables, maintains adequate vitamin C and other micronutrient intake, trains progressively and rests properly, the supplement can make sense in certain circumstances.
But no supplement alone can compensate for a poor diet, a prolonged energy deficit, lack of training or insufficient recovery.
And we mustn’t forget something fundamental: the body already produces collagen continually. We don’t need to ingest collagen for that process to exist. What supplementation does is provide an additional source of certain amino acids and peptides that the body can use.
The conclusion: less magic and more physiology
Next time you see a collagen supplement promising to “reach your joints,” it’s worth remembering what actually happens.
You take the supplement. You digest it. Its proteins are broken down. The amino acids and small peptides are absorbed and enter the bloodstream. From there, the body incorporates them into its metabolism and decides how to use them.
Some portion may contribute to the synthesis of new proteins, including collagen in different tissues. Another portion will have other metabolic destinations.
That’s why it isn’t correct to think that the ingested collagen is directly transformed into collagen in our tendons, bones, or joints, nor that there is a fixed percentage — like the famous 10% — that inevitably ends up there.
Collagen can be useful, but not because the supplement travels directly to where we need it. It is the body, with its complex mechanisms of digestion, absorption, distribution, and protein synthesis, that determines what happens to what we have ingested.
And perhaps that is the best way to understand supplements: they are not bricks we place directly in a tissue; they are nutrients the body incorporates into a much more sophisticated biological machinery.