The craze over skin-smoothing collagen has spread to "nabe" hotchpotch, with restaurants serving up the protein-rich fare — usually in the form of pig's knuckles — getting prominent play on TV and in magazines.

Besides eateries serving nabe containing collagen, the high-protein connective tissue found between animal bones, drugstores are also selling the gristle in supplement form.

Health-conscious consumers snap up such products even though nutritionists claim in books and blogs that ingesting collagen is not necessarily the most effective way to erase wrinkles. That is because consumers can be easily deceived and should instead be more skeptical, said Fusako Baba, an honorary professor specializing in consumer psychology at Asia University in Tokyo.