Some people have a terrible habit to bite their nails. A grown man recently admitted at my Shabbos table that he has a bad habit to bite his nails and that he swallows his nails after biting them. Similarly, some people bite the skin on their lips and swallow that as well. This raises an interesting question regarding the kashrus of human nails and skin.
There is a machlokes Rambam and Rashba (Ran in Kesubos) whether the prohibition to eat human flesh - בשר מהלכי שתים, is a Torah violation (issur aseh), or a Rabbinic violation. The Rama (Y.D. 79:1) rules in accordance with the Rambam that human flesh is prohibited from the Torah. In the parenthesis citing the soures for the Rama it mentions that Tosafos and the Rosh seem to understand that it is not a Torah prohibition. The Gr"a also provides some background for the two opinions. This is all in regard to the flesh of a live human being. The Shach (3) points out that the flesh of a dead body is an issur hana'ah which is certainly d'oraysa and derived from egla arufa. The Aruch Hashulchan rules in favor of the Rambam because since human flesh is tamei on a Torah level, it must also be prohibited to eat since we usually don't find tu'mah without their being a prohibition.
It isn't clear whether nails and skin have the same status as flesh. The Darchei Teshuva cites a sefer who quotes telling that he was approached by a childless woman who wanted to swallow the circumcised skin (orla) of a baby as a segula to have a child. He ruled that this would be prohibited according to the Rambam from the Torah. He assumed that skin would be included in the status of flesh. However, the Pri Megadim (sifsei da'as 3) quotes from the Rashba in a teshuva that the skin of a person is permitted since even by kodshim where the meat is certainly forbidden, it is permitted to use the skin. Furthermore, it should be similar to hair on which there is no prohibition, but then writes that perhaps there would be a Rabbinic prohibition.
The gemara in Nidah 55a clearly states that hair and nails aren't included in flesh in the realm of tu'mah. Only flesh of a dead body is a source of tu'mah, not teeth, hair or nails because they are either not created with the person (teeth), or they regenerate. Based on this it would seem clear that in the realm of eating human flesh, hair, teeth and nails would also not be included. Regarding skin the gemara has 2 approaches. In the first approach the gemara assumes that in the world of tu'mah, skin is not like flesh because it regenerates, but in the second approach the gemara assumes it is like flesh since it doesn't regenerate and therefore has tu'mah like flesh. It would seem that the question of whether skin is regarded as flesh is a point of dispute between the two approaches of the gemara. The Rambam (Tu'mas Meis 3:11) rules in accordance with the second approach, that human skin unless it is thoroughly worked like a hide, would be a source of tu'mah on a Torah level, therefore it is legitimate to assume that when chazal say that for humans skin is like flesh - עורן כבשרן, it would mean in all areas of halacha. Based on this, human skin is forbidden to eat m'doraysa.
On the first version of the gemara that doesn't consider human skin tamei on a torah level, yet the Rabbis rendered it tamei to prevent mistreatment of the skin of one's parents; Tosafos asks, why did they need to make it tamei m'drabonon since anyway there is a Torah prohibition to benefit from a dead body? Tosafos offers 2 answers: 1. The Rabbonon realized that people take tu'mah more seriously than issur, so although there is a Torah prohibition, they felt that by rendering it tamei it would be more of a deterrent. 2. The prohibition to benefit from a dead body applies only to flesh, not to skin. Tosafos supports this notion from the fact that the source to prohibit benefit from a dead body is egla arufa which is learned from kodshim and by kodshim itself there is no prohibition on the skin. It seems from Tosafos that regardless of whether the tu'mah on human skin is d'oraysa or d'rabonon (2 approaches of gemara), all would agree that the issur to benefit from a dead body would not include the skin.
Since we pasken like the second version of the gemara, we are faced with somewhat of a discrepancy in categorizing skin. In the world of tu'mah it is like flesh which is tamei, but in the context of benefiting from a dead body it is not considered like flesh and there is no prohibition. It is unclear whether the prohibition to eat human flesh includes skin. If we model after tu'mah it should, but if we model after the prohibition to benefit from a dead body it should not. It seems more logical to learn issur from issur rather than issur from tu'mah which seems to be the assumption of the Rashba cited by pri megadim who doesn't consider human skin prohibited by the Torah.