Both Sanguinarians and self-defined "psi-vamps" use the term "feed" to refer to their consumption of blood or "energy." Nearly all human Vampire-Identified People agree that "the need to feed on something" is their sole significant defining characteristic. There is far less agreement on exactly what benefits and effects result from "feeding."
The term itself derives from the Modern Vampire Myth, in which supernatural (fictional) vampires usually can't consume ordinary food and drink, but must have a regular intake of blood for basic nourishment. Although the supernatural vampire is said to "require blood to exist," rarely if ever are such vampires in danger of starving to death without blood. If deprived of blood, they may risk losing control and reverting to a bestial or depraved mental state in which they endanger themselves or do things they regret later on. But they don't fear dying, or even experiencing permanent harm, from blood starvation. This makes the blood-need of Modern Myth (fictional) vampires far more similar to an addiction than a nutritional requirement.
The word "feeding" refers to a living organism. We don't "feed" cars with gas or "feed" electrical appliances with electricity. "Feeding" also implies providing an organism with that substance which it requires to stay alive. Babies require light, physical touch and a hygienic environment in order to thrive, but we only talk about "feeding" a baby when we give it food, without which it will certainly die. But all Vampire-Identified People must eat ordinary food and drink water, no matter how much blood or "energy" they have available to them.
In addition to this, most Sanguinarians consume human blood, but fresh human blood tends to pass through the digestive tract largely unprocessed, meaning that any nutritional content it may have (such as proteins or iron) is mostly lost. Further, few Sanguinarians consume a large amount of blood at one time, or "feed" with any frequency approaching even once per day. Obviously, that level of blood consumption can't nutritionally sustain an adult human being.
Yet, the connection of blood with "feeding" is so strong that I have never, to date, heard of a Sanguinarian who introduced blood into his or her system by any other means than direct ingestion (drinking)--who used, for example, injection, as was depicted in the 1995 film, The Addiction. If blood-need is physiological, wouldn't injection be a more efficient way of treating it? If blood-need is nutritional, why do such small quantities satisfy the craving? The issue becomes even more complicated when some Sanguinarians assert that only the communion of taking blood from a consenting human donor, sometimes during sex, is truly satisfying to them. If blood is a purely medical or physical need, why does the context make such a difference?
Nevertheless, all Sanguinarians and self-defined "psi-vamps" consider what they do to be "feeding," as though it had a nutritional effect. They resist comparing themselves as addicts. In some ways, they may be analogous to insulin-dependent diabetics, who have to externally supply a vital substance that their system lacks. However, diabetics and others dependent on such daily medical regimens will die if they are deprived of their prescribed substance. If even the most severe blood-needing Sanguinarians risked death for lack of blood intake, we would expect some to be hospitalized due to blood starvation. (I have heard rumors of this, but no verified cases.) However, although many Sanguinarians report "withdrawal" symptoms so severe as to make them wish they could die, they apparently are not in danger of dying solely for lack of blood. The "need" that Vampire-Identified People experience is for a fix, not a meal.
The major concerns for almost all Sanguinarians and self-defined "psi-vamps" are how to "feed" more efficiently, how to find "donors," or sources, and how to guarantee a regular supply of what they need. These are very similar to the priorities of drug addicts. Some Sanguinarians are rather hostile to suggestions that anything else but fresh, living blood from a live donor could satisfy their needs. The possibility that some in-depth research might produce a blood-substitute for Sanguinarians, like methadone, that would render "blood feeding" unnecessary may not be that attractive a prospect.
I firmly believe that the term "feeding" is misleading and self-serving and should be abandoned by Vampire-Identified People. It's the wrong paradigm to describe what Vampire-Identified People "need" and prevents an honest exploration of the mechanisms behind it.