Goddesses, Demons, and Earth Spirits do not live off of regular food and water as do humans. Not much is said about demons so this may not appy to them. Goddesses in heaven do not need any food to live. They are sustained by a power source in heaven itself. Once on Earth it's a different story, since the regular source of power is no longer present. The goddesses must draw off of the same power in the mortal realm which sustains the Earth spirits. Goddesses can't directly process the energy themselves though, so the energy is routed through Yggdrasil which converts it into a form which can sustain them. This isn't the only method that they can gain energy, they can also use their respective alternate source of energy.
Alternate energy sources can be used in place of the regular energy sources but are limited to how much power a goddess really gains from them. In other words it keeps them alive, but isn't anywhere near as much as they usually draw. In Belldandy's case she doesn't even consume anything, she simply sleeps. The goddesses also seem to enjoy partaking in their alternate energy sources on a regular basis, wether they require it or not.
- Skuld : eating ice cream
- Belldandy : sleeping
- Urd : drinking sake
- Peorth : unknown
Like travel mediums, alternate energy sources reflect the current era in which a goddess exists. Ice cream for instance hasn't been around for all that long (no more than 100 years or so, and sake for no more than 10,000). Past goddesses would probably have alternate energy source like eating gruel. Yuck!