Drain all the blood out, the body will die.

What we don't know:

Everything else.

Take saturated fats - once the great and absolutely certain diet demon, and still condemned as such by the AHA (which hasn't been right about anything yet). To quote the recent conclusions of an international expert panel on saturated fats, published in the American Journal of Clinical Nutrition:

"We do not yet have enough evidence to give dietary recommendations for individual SFAs, but the evidence is useful for advising food manufacturers, eg. stearic acid can be used as a replacement for trans fatty acids where appropriate, although the evidence is not sufficient to determine whether it is superior to other SFAs because multiple pathways may be involved and clear data on clinical endpoints are not available."

In other words, "We really don't know a damned thing".