Thursday 24 March 2016

Vitamin B12 - cobalamin

Basic Description

Vitamin B12, as the name implies, is part of the B complex of vitamins. Like the other B vitamins, it is involved in energy metabolism and other related biological processes.
However, that is where the similarity ends. The list of things that are unique about this vitamin is long, and includes the following facts:
  • Most B vitamins do not store well, but several years' worth of vitamin B12 can be stored in your body
  • Most B vitamins can be found in a wide variety of plant and animal foods, but since no plant or animal can make vitamin B12 (only microorganisms like fungi and bacteria can do that), it is typically only animal foods that contain B12 since plants cannot make or store this vitamin. However, mushrooms (since they are themselves fungi) often contain B12, as do fermented plant foods like tempeh or miso since they have been produced with the help of microorganisms. Most B vitamins are relatively small and have a fairly simple chemical structure, while vitamin B12 is larger and more complex.
  • Most B vitamins are more easily absorbed than vitamin B12,which has more complicated requirements for absorption.
  • In terms of physical amount, vitamin B12 has the lowest daily requirement of all the B vitamins, and it is needed in about 1/1000th the amount of some other B vitamins.
  • Vitamin B12 is the only vitamin that contains a metal element (cobalt). In fact, the cobalt contained in B12 is the reason that this vitamin goes by the chemical name cobalamin.
As the list above implies, optimal intake of vitamin B12 can sometimes be a challenge in human nutrition. Even though U.S. adults ages 20 and older average well above the Dietary Reference Intake (DRI) for B12, there are still subgroups within the U.S. that are more commonly at risk of B12 deficiency. For example, adults 51 and older can be at greater risk of B12 deficiency, presumably in relationship to decreased dietary intake and/or compromised digestive function.
The style of diet that you choose can have an major impact on your B12 nourishment. If you regularly consume land animal foods and fish in your meal plan, B12 intake is not very likely to be a problem. If you regularly consume fish but avoid land animal foods, B12 is still relatively unlikely to be a problem. With no fish or land animal foods in your routine diet, however, you are left with some fairly specific food sources of B12, namely, fermented foods such as tempeh and fungi (including mushrooms). We'll give you some practical steps for obtaining B12 nourishment in the Food Sources section.
We list eight excellent sources of vitamin B12 on World's Healthiest Foods. We also have three very good and four good sources of the vitamin. Although the number of good sources is smaller than for many foods, this should be plenty to ensure a strong supply of this critical nutrient.

Role in Health Support

Cardiovascular Support

Vitamin B12 plays several important roles in keeping our cardiovascular system on track. The first of these roles involves production of red blood cells. Red blood cells are critical for transporting oxygen throughout our bloodstream, and the oxygen-carrying pigment in the center of our red blood cells is called hemoglobin. A key building block for hemoglobin is a compound called succinylCoA, and without enough vitamin B12, we simply cannot make enough of this building block. (Methylmalonyl CoA mutase is the enzyme that allows this process to take place, and it only functions with the help of B12 in the form of adenosylcobalamin.)
The fact that B12 plays such a key role in red blood cell production means that deficiency of this vitamin can actually cause a form of anemia called B12 deficiency anemia. However, this form of anemia is relatively rare. Often, when it appears to occur, it is actually a by-product of pernicious anemia in which immune system antibodies interfere with the production or function of intrinsic factor (IF). IF is a glycoprotein produced by specialized stomach cells called parietal cells and it is required for proper metabolism of vitamin B12.
A second important role for B12 in cardiovascular support involves prevention of excessive homocysteine build-up. A long list of cardiovascular diseases have been associated with excessive accumulation of homocysteine in the bloodstream, including coronary heart disease, peripheral vascular disease, and stroke. Vitamin B12 helps normalize levels of homocysteine in the blood by allowing conversion of homocysteine to methionine. (This conversion process takes place through activity of the enzyme methionine synthase.)

DNA Production

Vitamin B12 is a necessary co-factor for the production of DNA, the genetic material that acts as the backbone of all life. This process requires folate and vitamin B6 as well, and disruptions of any of these nutrients can lead to problems.
The diagnosis of vitamin B12 deficiency is often dependent on problems with DNA production. When vitamin B12 is low, normally rapidly dividing blood cells are not able to effectively reproduce their DNA, leading to abnormally big cells. This phenomenon, called macrocytosis, is often the first way doctors suspect problems with the vitamin.

Brain and Nervous System Health

Along with the heart, liver, muscles, and kidneys, the brain is an organ that utilizes a large amount of energy in a form called aerobic energy. Aerobic energy means oxygen-requiring energy production in specialized cell parts called mitochondria. As described earlier in the Cardiovascular Support section, one role that B12 plays is maintenance of hemoglobin in red blood cells to allow successful transport of oxygen. This process is especially important in brain health.
Another role of B12 described in the Cardiovascular Support section was prevention of excessive homocysteine build-up in the blood through conversion of homocysteine to methione. However, one aspect of this process not described earlier is the simultaneous recycling of a molecule called SAMe (S-adenosylmethionine) that takes place along with homocysteine conversion. SAMe has sometimes been referred to as the "universal methyl donor" because of its unique ability to provide special chemical groups—called methyl groups—in many different places where they are needed. One such place is the brain and nervous system, where movement of methyl groups is a key process. Some of the nervous system messengers (neurotransmitters) cannot be produced without the help of enzymes called methyltransferases, and these enzymes in turn cannot be produced without the availability of methyl groups. This area of methyl metabolism is another key way in which vitamin B12 plays a major role in the health of our brain and nervous system.
These nervous system connections to B12 help explain some of the clinical symptoms associated with B12 deficiency. When levels of vitamin B12 get very low, nerve damage can ensue. The insulation sheath around nerve fibers begins to break down, making it harder for signals to get to more distant areas of the body (called peripheral areas). As you might guess, symptoms first become apparent in the hands and feet. While the exact mechanisms are not fully understood, researchers know that severe B12 deficiency can cause these "peripheral neuropathies" and that restoring optimal supplies of B12 can keep these problems from becoming more severe.

Support of Energy Metabolism

While mentioned earlier, it's important to underscore the role of B12 in support of oxygen-based energy production (called aerobic energy). At the heart of this process is a metabolic cycle called the citric acid cycle and included within this cycle is a molecule called succinyl-coA. Since vitamin B12 is important for maintaining proper supplies of succinyl-coA in the citric acid cycle, it is important for supporting all aerobic energy metabolism.

Other Potential Health Benefits

Still under debate by researchers is the exact role of B12 in support of bone health. On the one hand, B12 deficiency appears to be associated with increased risk of osteoporosis. This connection involves the positive role of B12 (in several of its cobalamin forms) in supporting the activity of the osteoblast (bone-forming) cells. At the same time, B12 also appears to help regulate activity of tumor necrosis factor (TNF). TNF overactivity can result in too much bone breakdown and remodeling by a second type of bone cells called osteoclasts. Too much osteoclast activity—regardless of the reason for its occurrence—is also associated with increased risk of osteoporosis. Despite these logical connections between B12 deficiency and osteoporosis risk, however, actual research findings are inconsistent in making the B12 connection to bone status.

Summary of Food Sources

Microorganisms—and especially bacteria and fungi—are the only organisms definitively known to produce vitamin B12. There has been longstanding debate over algal production of B12, which includes debate over the potential role of sea vegetables to provide B12 (as well as debate over dietary supplements like spirulina). However, we interpret the research in this area to show that sea vegetables cannot be counted on for B12 support, not because there is no possibility of B12 production in sea vegetables, but because the form of B12 in sea vegetables is not a usable vitamin form.
Even though land animals and fish cannot make vitamin B12 in their cells, they are often able to save up B12 produced by bacteria and concentrate it in their cells. For this reason, many land animal foods and many seafoods are nutrient-rich in B12. In fact, all but one of our WHFoods ranked sources of B12 come from animal foods or fish. Because plants do not concentrate or utilize vitamin B12 in the same way as animals, plant foods do not become nutrient-rich in B12 unless they have been fermented (like the fermentation of soybeans into tempeh) by B12-producing bacteria or fungi. Excluded from this statement are fungi (for example, mushrooms) since scientists classify them in their own separate category from plants. But if we adopt a less technical perspective and include mushrooms as plant foods, they would also have to be included as sources of B12. At WHFoods, crimini mushrooms are our only ranked non-animal derived food source for B12.
Our recommended daily intake level for B12 is 2.4 micrograms, and one serving of any of the following WHFoods will provide you with 100% or more of this amount: sardines, salmon, tuna, cod, lamb, or scallops. You'll get over 50% with a single serving of beef or shrimp, about one-third of the daily amount from one cup of yogurt, and between 10-25% from one serving of cheese, chicken, turkey, eggs, or cow's milk.
In contrast with these animal and fish foods, one cup of crimini mushrooms will only provide you with about 3% of the daily recommend amount. This relatively low contribution from mushrooms (a non-animal food) raises the important question of B12 nourishment for individuals who don't regularly consume animal foods or fish. In the broadest sense, individuals who focus primarily on plant foods in their meal plan are often referred to as "vegetarians." However, this term can have a variety of different meanings. "Pesca-vegetarians," for example, consume fish along with plant foods. "Lacto-vegetarians" consume dairy foods along with plants foods. "Lacto-ovo vegetarians" consume not only dairy foods but also eggs along with plant foods. If a person eats plant foods exclusively, the term usually used to describe his or her meal plan is "vegan." Most healthcare providers—including most nutritionists—currently recommend that persons who exclusively consume plant foods take steps to ensure their B12 nourishment by adding foods fortified with B12 or B12-containing supplements to their daily routine. As a general rule, we support this approach, although we realize that there can be exceptions.
Nutritional yeast grown on a molasses medium is an example of a food-based quasi-supplement that would provide a vegan source of vitamin B12. One widely available brand has more than twice the Dietary Reference Intake (DRI) for B12 in one and one-half tablespoons of yeast. Not all nutritional yeasts are rich in vitamin B12, however, and you'll need to check labels for details.
Before leaving the topic of B12 and food sources, we want to go one step further in explaining some ongoing speculation about the relationship between B12, bacteria, and human nutrition. As described earlier, bacteria and other microorganisms are the only life forms that can be described as definitively able to produce B12. Interestingly, however, research studies have shown that bacteria capable of producing B12 can live inside our human intestinal tract. (One example of a bacterium known to produce B12 and also able to colonize parts of our digestive tract is Propionibacterium shermanii.) Furthermore, it seems likely that B12-producing bacteria are able reside in the very last segment of our small intestine known as the terminal ileum.The terminal ileum is especially important for vitamin B12 nourishment since it is the primary site for B12 absorption. In this last segment of our small intestine, however, there aren't nearly as many bacteria as are present in our large intestine. (We're talking about a minimum of 10,000 times less, and probably more like one million times less.) So exactly how much B12 contribution could potentially be made by B12-producing bacteria in the terminal ileum is an open question. While we don't see any justification for relying on bacterial production of B12 in our intestines as a source of this vitamin, it is also impossible for us to totally rule out this possible pathway for B12 nourishment and hopefully we will get some further clarification here in future research.
Nutritional yeast grown on a molasses medium is an example of a food-based quasi-supplement approach that would provide a vegan source of vitamin B12. One widely available brand has more than twice the Recommended Dietary Allowance (RDA) for B12 in one and one-half tablespoons of yeast. Note that not all nutritional yeasts are rich in vitamin B12, and that you'll need to check labels for details.
The National Academy of Sciences currently recommends that people over the age of 50 receive much of their vitamin B12 from supplements or fortified foods. Currently, about 40% of the vitamin B12 that Americans eat comes from these non-food sources. In addition to the fortified yeast discussed above, soy products and breakfast cereals often contain this type of added vitamin B12.

Click Here For More Articles

No comments:

Post a Comment