A few critics suppose that the Bible advocates cannibalism in verses like these:

Jer. 19:9 I will make them eat the flesh of their sons and daughters, and they will eat one another's flesh during the stress of the siege imposed on them by the enemies who seek their lives.

Ezek. 5:10 Therefore in your midst fathers will eat their children, and children will eat their fathers. I will inflict punishment on you and will scatter all your survivors to the winds.

Lev. 26:29 You will eat the flesh of your sons and the flesh of your daughters.

By definition, an "advocation" constitutes an approval or an endorsement. These verses do not say, "You will eat your sons and daughters. Here are some recipes." They do predict what will happen as a judgment upon sin after an extended period of warnings and lesser judgments (so there can be no excuse of blaming God for these actions).

Critics might want to put themselves in the place of the average ANE inhabitant who had to scrape to survive -- or if they have trouble with that, in the place of those who survived a plane crash in the Andes many years ago, only because they ate human remains. Where in the hierarchy of morals does this grand taboo belong -- above or below the preservation of life? (For more on this issue, see this item off-site.)

That does leave one other passage often cited:

John 6:53-4 Jesus said to them, "I tell you the truth, unless you eat the flesh of the Son of Man and drink his blood, you have no life in you. Whoever eats my flesh and drinks my blood has eternal life, and I will raise him up at the last day.

This one is fairly clearly allegorical, as was shown at the Last Supper, when wine and bread were figuratively assigned these roles. Only by taking this passage from the larger context of Christ's ministry could this be stretched to endorse cannibalism, and then that only of one particular body.

On the matter of the Bible relating instances of actual cannibalism, and the mere telling of it being a moral issue, see here.

There's also a side issue with John that we can mention here; some argue that the metaphor of cannibalism would have been too shocking to Jesus' Jewish contemporaries to have been used by the historical Jesus. However, Eddy and Boyd in The Jesus Legend [219] raise the following in reply:

1. Jesus frequently used shocking metaphors designed to surprise his listeners.
2. By the criterion of dissimilarity (Jesus says something unlike what his contemporaries say and believe), statements like these are more likely to be genuine.
3. Conceptually, Jesus' language about his own blood (particularly at the Last Supper) finds its home within concepts of Jewish covenant theology.

A reader also noted:

In his "For Everyone" commentary series (which I have wisely been acquiring as soon as the parts have been published), our esteemed friend bishop Tom points out that Jesus' listeners might have picked up a connection with David's three daredevils.

As we remember, in the OT days these brave lads went and scooped Dave a hatful of water from the a well that was heavily under enemy forces. Dave didn't drink it, however, because these fellows had shown too great devotion towards him and risked their lives and limbs in the effort. It would have been like drinking their blood.

According to NT (Wright, that is), Jesus might be calling up this image. "I'm gonna do something for you that is going to cost me personally very much, but you need to receive it for yourselves. Unless you act unlike David and actually 'drink my blood' in this sense, what I do for you won't benefit you none."

-JPH