Riboflavin (B2) deficiency is commonly associated with cracked lips, B12 deficiency is not so much. However sores at the edges of the lips may be a sign of borderline B12 deficiency, together with other symptoms too numerous to list here - mainly neurological, but including fatigue and migraine. A smooth tongue (sometimes with plaques called oral lichen planus) and ridged nails can also be signs.
vls, I think I can tell you why she thinks that B12 can be the culprit even though the blood work came back okay. That is because (unusually for a doctor) she obviously knows of research which was done by the Mayo Clinic several years ago, to test the accuracy of the blood test used to check for B12 deficiency.
They looked for clinical symptoms of B12 deficiency independently of the blood test in a sample of patients; treated them over a period of time while taking repeated blood tests, and looked to see how the clinical symptoms of B12 deficiency compared to the blood test. What they found was the blood test said that there was no deficiency even when there were numerous symptoms of deficiency. Not all the symptoms, necessarily, but certainly sub-clinical deficiency at least. So when the blood test said "within normal range" or "low end of normal", a patient was likely to have symptoms of deficiency. The reason apparently is that the blood test was a rough and ready way of obtaining results with a low level of reliability back when tests typically were of low sensitivity; as the ability to test with greater accuracy improved, the diagnostic scale was assumed to have also increased in accuracy, but did not.
Sadly, doctors who know of this research are as rare as hens teeth - obviously the research paper was not often cited, and died a death - and so the blood test is usually treated as accurate to this day.
Us Crohnies know better... it is a perennial problem for patients on these pages to have the symptoms, but the blood work says they are okay.
Zinc deficiency, like anaemia, is also something that comes up as a common cause of cracked edges of the mouth, and again is associated with chronic malabsorption same as B1, B2, B12 et al. Generally, if you are lacking in one, you will be lacking with some if not all of them, some if not all of the time. There is a complex interaction between all B complex vitamins, whereby under some circumstances some will improve the efficiency of other B vitamins, yet under different conditions (eg with carbohydrates) will decrease the effect. (The technical term for this is "antagonists" and "protagonists".) This is why taking a B Complex supplement may do no good whatsoever for ameliorating a B vitamin deficiency; the situation is so complex it is likely to be pot luck.
And just to confuse matters further, all are used by yeast, and are likely to be associated with yeast infection ! (And a diet high in refined carbohydrates/sugary foods. To digest these burns up a lot of these nutrients, and is a large part of the common Western dietary nutritional dysfunction profile.)
I do believe that is pretty much all that I know about
this subject !
