[nookie]

"I have noticed that on days when I eat very little, even though I was not hungry that day, I get a pretty nasty headache the next day"

Well, the most common reason is lack of water. Practically every food contains a lot of water and burning fat is a more inefficient process. So if you don't consume enough calories then you must drink more water/salt than usual.

[jerrya]

Thanks, that's very likely and I have been much more conscious of drinking water in the past few days, and my doc called this morning to tell me I was dehydrated on the blood test from last week (although I put that down to a 10 hour fast before the test.)