if sugar gives you a headache, this may be related to a gastrointestinal issue (e.g. gluten sensitivity, dietary histamine sensitivity, small intestinal bacterial overgrowth possibly causing its own sensitivities...) The specifics of the cause in your case are between you and your doctor, but i'd like more people to know, especially since these sorts of problems disproportionately affect people with autism and ADHD, long a core Hacker News demographic, and most of us have trouble paying attention to everything our body is feeling in exquisite detail.

other signs of digestive problems in this area may include:

- a pattern of early-afternoon fatigue and brain fog (this is mediated by histamine, which in turn can be caused by a food allergy, a non-allergy food sensitivity, bacteria in the stomach that don't belong there, or the like)

- feeling generally flushed and hot after eating food (more histamine!)

- waking up in the middle of the night, ~4am, especially if you're feeling too hot (also a sign of excess histamine)

- often having an upset stomach, and lots of stomach acid

- increased sensitivity to heat or exercise (mast cell degranulation because your body is already on edge)

- generalized hard-to-place fatigue, brain fog, depression (the whole histamine-response pathway is shot through with various neurotransmitters)

- serious nasal congestion contributing to sleep apnea, and extreme fatigue

now, this is going to be different from person to person, but your doctor's not going to know anything's wrong if you don't tell them!

(and if something like this is the case, it's also the sort of condition that might sometimes has an instant miracle cure, because if you stop releasing the feel-bad neurotransmitters every time you eat, then you suddenly stop feeling terrible!! go figure.)

I had really high carb tolerance but doing keto for many years has plummeted it and I'm very sensitive now (can still train it). Another example doing keto for many years you also have lower alcohol tolerance.