Water-soluble vitamin deficiency and toxicity: B9, B12 and vitamin C: Pathology review

35 year old Emily comes to the clinic because she’s been experiencing tingling and numbness in both hands and feet. Emily also mentions that she’s been following a vegan diet for the past 12 years, but she’s never taken any vitamin supplements. You immediately decide to run a blood test, which reveals that Emily has elevated levels of both homocysteine and methylmalonic acid. In addition, a peripheral blood smear shows megaloblasts and hypersegmented neutrophils.

Next comes 38 year old Joseph, who’s complaining about the fact that his gums are swollen and bleed easily. On further questioning, Joseph tells you that he’s been homeless for several years. Upon physical examination, you notice that he is underweight, and has multiple bruises, especially on his legs. You decide to take a look at his scalp, and find tiny red spots associated with small twisted hairs that look like corkscrews.

Based on the initial presentation, both Emily and Joseph seem to have some form of water- soluble vitamin deficiency or toxicity. Water- soluble vitamins include the B-complex vitamins and vitamin C. And just like all vitamins, they need to be derived from food, and inadequate dietary consumption can result in deficiency. So, in a test question, look for individuals who come from lower income countries, are at an advanced age, engage in chronic alcohol intake, or have an eating disorder like anorexia nervosa.

Okay, now, another high yield fact is that water-soluble vitamins get easily excreted in the urine. On the other hand, fat-soluble vitamins get stored in fat cells. And that’s why their toxicity, also known as hypervitaminosis, is much less common than that of fat-soluble vitamins, which instead get stored in fat cells. Keep in mind that hypervitaminosis can indeed occur when there’s excess intake of vitamin supplements, highly fortified foods, or medications containing a vitamin derivative.

Okay, now in this video, we’re gonna be focusing on the water-soluble vitamins B9, B12, and C! Let’s start with vitamin B9, also known as folate, which is mainly found in leafy greens, and nowadays many countries fortify foods like grains and cereals with folate. Now, the folate present in these foods is generally in the polyglutamate form, which means that it’s linked or conjugated to a chain of the amino acid glutamate. However, polyglutamate folate is too big to be absorbed by the gut. So, an enzyme in the jejunal mucosa, called intestinal conjugase, cuts down or deconjugates the polyglutamate residues into the smaller monoglutamate form.

Monoglutamate folate is then absorbed by the enterocytes of the jejunum, and an enzyme called dihydrofolate reductase converts it into tetrahydrofolic acid, or THF. Finally, THF is released by the enterocytes via a transporter protein into the portal circulation, where THF can travel to the liver. Now, some THF continues into the systemic circulation to ultimately reach the rest of the body tissues, while some gets stored in the liver. Keep in mind that, if we don’t get enough folate, these stores will get used up and depleted in about 4 months, leading to folate deficiency.

Now, what you need to remember is that folate deficiency can be caused by inadequate dietary intake, which typically occurs in people that don’t consume enough leafy greens. Another important cause is increased body demand, which mainly occurs during pregnancy, as folate is necessary for fetal development. As a consequence, folate deficiency can lead to neural tube defects, such as spina bifida, which is a congenital defect of the spine, as well as anencephaly, which is when the baby is born missing part of the skull and brain.

To prevent this, pregnant individuals should take supplements of folic acid, which is a synthetic form of folate, especially during the first trimester. Finally, folate deficiency can arise when there’s impaired intestinal absorption, which may be caused by medications like phenytoin, which inhibits the intestinal conjugase; trimethoprim and methotrexate, which inhibit dihydrofolate reductase; and finally sulfasalazine, as well as chronic alcohol intake, which can decrease the expression of the transporter protein that carries tetrahydrofolic acid from the enterocytes to the portal circulation.

Okay, let’s move on to vitamin B12, aka cobalamin, which is found in animal and dairy products such as eggs, meat, or milk. Normally, these food products are broken down in the stomach by pepsin. Bear in mind though that pepsin’s inactive precursor pepsinogen can only be activated in very acidic environments, and that’s one of the reasons why the stomach secretes gastric acid.

Okay, so once active, pepsin helps digest animal and dairy food products, releasing vitamin B12. Then, the stomach parietal cells produce a protein called intrinsic factor, which binds to the free vitamin B12 in the stomach. This complex then travels within the intestinal lumen without being absorbed. When the complex reaches the terminal ileum, the enterocytes recognize the intrinsic factor and absorb the whole complex.

Finally, once inside the enterocytes, the complex dissociates, and vitamin B12 is released into the portal circulation. Some of it then enters the systemic circulation to ultimately reach the rest of the body tissues, while some gets stored in the liver. But unlike folate, the liver can store large quantities of B12, so it could take 3 to even 10 years to deplete these stores and develop B12 deficiency.

Now, an important cause of B12 deficiency is inadequate dietary intake, which is often seen in long time vegans who don’t take vitamin supplements. But remember that the most common cause of vitamin B12 deficiency is impaired intestinal absorption, and you need to know the different ways this could happen. One cause is atrophic gastritis, in which there’s chronic stomach inflammation that destroys the gastric mucosa over time. As a result, there’s reduced secretion of gastric acid, which impairs the activation of pepsinogen into pepsin, and so there’s very little release of vitamin B12 from digested food.

In addition, with atrophic gastritis there’s decreased production of intrinsic factor, so the little B12 available can’t be absorbed. Another potential scenario is an individual who underwent a gastric bypass, since food passes through the stomach too quickly, so there’s not enough release of B12, and again there’s decreased production of intrinsic factor. Another high yield cause of decreased B12 absorption is pernicious anemia, in which the body produces antibodies against intrinsic factor or parietal cells, destroying them.

Also, any conditions affecting the terminal ileum, like Crohn disease or ileal resection, can prevent B12 absorption. Finally, parasitic infections like Diphyllobothrium latum or fish tapeworm, as well as bacterial overgrowth in the small intestine can interfere with B12 absorption, as these organisms would take up most of the available B12 for themselves.

All right, now examiners love to compare folate and B12 deficiency, so be sure to keep an eye out for certain clues! Okay first, folate is used to synthesize purines and pyrimidines, which are the building blocks for DNA replication and cell division. On the other hand, vitamin B12 is an important cofactor for DNA synthesis. So as a result, both folate and B12 deficiency affect the rapidly dividing cells in the bone marrow, which can lead to anemia or low hemoglobin and red blood cells, leukopenia or low white blood cells, and thrombocytopenia or low platelets. And when all three blood cell lines are affected, it’s called pancytopenia.

In response, the bone marrow tries to compensate by releasing large, immature red blood cells called megaloblasts, which have a mean corpuscular volume or MCV larger than 100 fL, and the final result is macrocytic, megaloblastic anemia. The bone marrow also starts releasing large, immature, and hypersegmented neutrophils, which means they have six or more lobes in their nucleus, when they normally have just three or four.

Now, through a variety of mechanisms, vitamin B12 also plays a role in the synthesis of myelin, which wraps and protects our nerves. So a very important difference with folate is that B12 deficiency can lead to demyelination, resulting in neurological dysfunction. This may lead to subacute combined degeneration, which refers to progressive demyelination of the spinal cord, and presents with decreased vibratory and position sense, numbness, weakness, and may lead to spastic paresis, with stiff muscles and jerky movements.

In addition, B12 deficiency may cause peripheral neuropathy, or nerve damage, which can manifest as burning, tingling, prickling, and pain in the hands and feet. And over time, people with prolonged B12 deficiency may develop irreversible nerve damage. On the other hand, folate deficiency does not cause neurological dysfunction, and that’s something you absolutely have to remember!