Orthostatic intolerance (OI) is a very common problem for people with Chronic Fatigue Syndrome (CFS). OI symptoms include dizziness, faintness, brain fog, nausea and sweating.
Living with Chronic Fatigue Syndrome becomes that much more difficult when orthostatic intolerance is added to the multisystem dysfunction. But there are some things that may help reduce OI symptoms.
Drink lots of liquids, up to two liters of water a day. Though our society is busy trying to cut down salt, don't you do it. Using more salt causes your body to retain fluids in your blood vessels and this will keep blood pressure at a healthy level.
"Avoid large meals, especially at night. The digestive process requires added blood flow, drawing fluids away from other body functions (like thinking and moving large muscle groups). Instead, eat smaller meals throughout the day. Also avoid consuming alcohol because it causes dilation of the veins. (Most people with CFS and/or OI find they can’t tolerate alcohol anyway.)"
http://www.cfids.org/cfidslink/2009/070105.asp