HYPOGLYCEMIC HEADACHE 

The majority of headaches can be ascribed to tension, sinusitis, migraine, muscular pain and a long list of other causes, but there is another possibility: that hypoglycemia [low-blood sugar] can cause headaches. 

For patients with undiagnosed recurrent headaches, check for hypoglycemia with a three-hour glucose tolerance test (GTT). If hypoglycemia is detected, then dietary intervention (high-protein, low-carbohydrate) should be tried to see whether the headaches resolve. 

Two caveats to this advice however: Most patients with hypoglycemia have other prominent symptoms (hunger, sweating, dizziness) and most references suggest a five-hour GTT as a more accurate tool - with the requirement that the headache develops during the hypoglycemic phase of the test.