Turmeric, an important component in curry, is commonly used as a spice and food additive to give foods their characteristic yellow color. Turmeric consists of the curcuminoids curcumin (Cur), demethoxycurcumin (DMCur), and bisdemethoxycurcumin (BDMCur). DMCur and BDMCur are curcuminoids remained largely unexplored. This paper will focus on this yellow polyphenol, Cur, extracted from the rhizome of turmeric (Curcuma longa) [1], as well as its nanoencapsulated forms. As a bioactive compound, Cur is known for its potential health benefits such as anti-tumor [2], anti-inflammatory [3], anti-oxidant [4,5], antimicrobial [6], anti-hypertensive and anti-hyperlipidemic properties [7]. Cur can modulate many signaling pathways such as inhibiting NF-kB phosphorylation, thereby regulating inflammatory related signaling pathways [8]. As a result, Cur is widely used in functional foods, pharmaceuticals, cosmetics and health care products.

However, Cur has a low water solubility, stability and poor bioavailability [9]. It is difficult for free Cur to reach high enough concentrations after ingestion in the body. People need to take large amount of free Cur orally to achieve an effective concentration. Besides, due to the rapid metabolism of Cur in the intestine and liver, there is a low Cur level in serum, thus again lowering the bioaccessibility of Cur. Moreover, the efflux effect of P-gp and multi-drug resistance-associated protein (MRP2) in intestinal epithelial cells leads to the further pumping out of the absorbed Cur limiting the absorption of Cur. The emerging carriers/nanocarriers can be effective in improving the stability, solubility, and bioaccessibility of Cur, resulting in an effective anti-inflammatory, antioxidant, and antibacterial functions. In addition, 20–100 nm protein nanocarriers can penetrate deeply in mucus and promote prolonged retention of the encapsulated Cur in the intestine, leading to a high endocytosis by intestinal epithelial cells and significantly higher bioavailability of Cur.

In recent years, several reviews on Cur delivery systems are published. For instance, Saman et al. [10] focused on the mucoadhesive delivery system of Cur and analyzed the interactions between Cur and gastrointestinal components, epithelial cell enhancers and efflux inhibitors. Rita et al. and Hanie et al. summarized the effectiveness of Cur delivery systems in clinical trials [8,11], explaining their therapeutic effects on various diseases. Michele et al. and Suchiwa et al. analyzed the factors affecting the bioavailability of Cur and systematically reviewed the positive effects of delivery systems made of different materials on improving the oral bioavailability of Cur [12,13]. However, the responsive Cur delivery systems and the interaction between the Cur-loaded nanocarriers and gut microbiota have not been investigated. For this reason, this review mainly discusses Cur delivery systems responsive to different environmental stimuli, e.g. pH-responsive, enzyme-responsive, targeted-to-specific cells or tissues, etc., and the metabolism of Cur nanocarriers and the relationship between Cur nanocarriers and gut microbiota. In addition, the biocompatibility and restrictions of existing Cur delivery systems in food industry have also been concerned.