Curcuminoids
Polyphenols
The yellow of turmeric, and just about the only thing that colors it.
Very poorly absorbed on its own. With black pepper and some fat, absorption is several times higher.
Effects and benefits
The pigment of turmeric has a famous flaw: on its own, it barely reaches the blood. Pepper changes everything, multiplying that passage by twenty in one trial. As extracts, meta-analyses show a lower marker of inflammation.
Curcumin is poorly absorbed, quickly transformed by the liver and eliminated: after a high dose, very little is found in the blood. The piperine in pepper slows this elimination; in one trial, 20 mg of piperine multiplied the curcumin measured in the blood by twenty (Shoba et al., 1998).
Meta-analyses of trials found a drop in CRP, a marker of inflammation, with curcumin extracts (Sahebkar, 2014). Others looked at knee osteoarthritis pain, with results close to those of common anti-inflammatories in small trials (Daily et al., 2016); it is not a reason to stop a treatment.
Caution concerns supplements, not the spice: ANSES has recorded liver injuries linked to turmeric supplements, especially those formulated to reach the blood better, and advises against them for people with liver disease or taking certain medicines (ANSES, 2022).
Sources: Shoba et al., Planta Med, 1998 · Sahebkar, Phytother Res, 2014 · Daily et al., J Med Food, 2016 · ANSES, 2022