It’s possible you aren’t getting enough salt, depending on what foods your 500 calories include. Most processed foods include salt but if you aren’t eating those you may not be getting enough and if you’re drinking a really lot of water or other fluids, you could deplete your salt supplies. Hyponatremia is a condition caused by insufficient sodium in your blood. It’s a more common condition than I realized until it affected me a couple years ago. I was taking a blood pressure medication that had a diuretic in it and drinking enough water (because I was thirsty) to deplete my sodium levels. After blood and urine tests to diagnose it, the doctor prescribed a different medication without the diuretic in it. (This was long before 5:2 and I wasn’t dieting.) He also told me to not drink any water unless I was really thirsty. I had to go the next morning for another blood test and my salt levels had increased some by then.

Headache was the first symptom, then nausea and lightheadedness. I felt pretty bad by the time I went to the doctor. He probably didn’t tell me to drink salt water because of the high blood pressure, but I imagine that would have relieved it more quickly.

Salt does indeed have an effect on blood pressure. During the time losing 17 pounds on the 5:2 plan, I’ve had to reduce my blood pressure meds a couple times because it was too low and I felt lightheaded. When I checked it, both numbers were very low. Dissolving half a teaspoon of salt in a cup of water and drinking it, brought it up to normal levels in less than an hour. There have been studies that dispute the salt/high blood pressure connection, but the vast majority of scientific research shows that there is a connection. That said, I think on a 500 calorie FD it’s possible to not get enough salt, especially because most of us drink more fluids to keep us from feeling hungery.

Most headaches when dieting aren’t caused by hyponatremia, but it’s something to be aware of because it can be very dangerous. Usually hyponatremia is caused by drinking excessive amounts of water in a short time, like many athletes do during marathons when they’re overheated or trying to stay ahead of salt depletion. Or from taking a diuretic like in my case.