There is almost a 0% chance that the GP, as (presumably) an educated and well-fed citizen of a first-world country, has a serious vitamin deficiency. That's no

There is almost a 0% chance that the GP, as (presumably) an educated and well-fed citizen of a first-world country, has a serious vitamin deficiency. That's not the failure case that he's avoiding. My guess would be that that "weird feeling," in most cases, is a sugar rush from eating something with a high glycemic index; do that a lot [1] and it puts pressure on your liver and kidneys and otherwise causes non-optimal results. Not as serious as an iron deficiency, certainly, but worth paying attention to. [1] Most people do. White bread has a high glycemic index.