Updated on 3/21/2005
You don't say why you want to fast once a week. I have experimented with fasting one day a week as a useful physical and psychological discipline. Many people experience a clearer mental state and increased energy after a short-term fast. When I did my weekly one-day fasts, I consumed nothing but water or herb tea. The first thing I would ask is how much water you were drinking during your fast. You should be drinking at least eight 8- to 12-ounce glasses of water or unsweetened herbal tea. Drinking less than that might explain your headaches.
I hope you were not fasting in an effort to lose weight. Fasting isn't an effective diet tool - it alters your metabolism in a way that actually may make it harder for you to lose weight. Most people compensate for the deprivation of the fast by increasing their caloric consumption afterward.
If you're a coffee drinker or normally consume beverages that contain caffeine, your fasting headaches could be the result of caffeine withdrawal. If that's the case, I would suggest weaning yourself off caffeine before trying to fast again.
Not everyone does well with fasting - thin people and those who are particularly sensitive to cold have a more difficult time. If that's the case, you probably should reconsider fasting. I would also caution about being overly active during a fast. Not only should you rest your digestive system, but you should also take time to rest your entire body. Too much physical activity during this time may also be a cause of headache.
Andrew Weil, M.D.