What is the first-pass effect?
The first-pass effect (first-pass metabolism) describes the breakdown of a compound in the gut wall and liver before it reaches the general circulation. What passes from the gut into the blood runs first through the portal vein to the liver; some fat-soluble compounds take the lymphatic route instead and bypass it. Part of it is converted and excreted there, so less reaches the blood than was taken.
How does the first-pass effect work?
After swallowing, a compound is absorbed in the small intestine. But blood from the gut does not flow straight into the body; it first travels through the portal vein to the liver. The liver is the main detoxifying organ. With its enzymes it chemically modifies foreign substances, usually making them more water-soluble so they can be excreted via the kidneys or bile. Similar enzymes are already active in the gut wall.
Whatever does not survive this "first pass" unchanged never reaches the blood. The share that does determines bioavailability. Some drugs are therefore designed as injections, suppositories or under-the-tongue tablets, because that bypasses the liver at first.
The first-pass effect with curcumin
Curcumin is a classic example. The liver recognises it as a foreign compound and very quickly couples it to glucuronic acid or sulphur groups. This process is called glucuronidation. The result is highly water-soluble breakdown products that are excreted quickly. Accordingly the half-life of free curcumin is short in animal and cell experiments (in rat studies in the range of minutes; there is no reliable single value for humans).
There is a second bottleneck even before the first-pass effect: curcumin is fat-soluble and dissolves poorly in the gut. Both hurdles together explain why, with plain turmeric powder, often less than 1 % of the compound reaches the blood. Fat with a meal improves uptake but does not change the breakdown in the liver.
How can the first-pass effect of curcumin be influenced?
Summary of the approaches described in our articles.
| Approach | Effect on first pass | Limitation |
|---|---|---|
| Approach Fat with the meal | Effect on first pass mainly solves the solubility problem | Limitation liver breakdown remains |
| Approach Piperine (black pepper) | Effect on first pass inhibits breakdown enzymes in liver and gut | Limitation can also inhibit the breakdown of medication |
| Approach Soluble formulations (micelles, cyclodextrin complex) | Effect on first pass more compound enters the body in dissolved form | Limitation uptake rises, breakdown still takes place |
Substances that inhibit liver enzymes can also change the first-pass effect of medicines and raise their blood levels. This applies, for example, to piperine. If you take medication, discuss supplements with your doctor first. This is not medical advice.
Frequently asked questions about the first-pass effect
Does the first-pass effect affect all drugs? All substances absorbed through the gut. How strongly depends on the compound: some are broken down almost completely, others hardly.
Is the first-pass effect bad? No, it is a normal protective mechanism of the liver. It becomes a problem only when a compound is broken down too fast to arrive.
Does curcumin with pepper help? Piperine inhibits breakdown enzymes and can raise curcumin uptake. It can also change the breakdown of medication, which is why some products deliberately do without piperine.
