Vitamin B

The term "vitamin B6" describes three related compounds: pyridoxine, pyridoxal, and pyridoxamine. These vitamins act as coenzymes involved in the synthesis and metabolism of proteins and amino acids. Vitamin B6 is also important for a variety of other physiologic functions such as carbohydrate metabolism and steroid hormone regulation. Important food sources of this vitamin include meat, poultry, fish, milk, nuts, whole and fortified grain products, potatoes, and bananas [34].

Vitamin B6 has long been thought to decrease nausea and vomiting during pregnancy, and it is still widely used for this common condition [35, 36]. Vitamin B6 was also a component in the once widely used medication doxylamine-pyridoxine (marketed as Debendox® and Bendectin®). This drug was removed from the market in 1983, when its use was suspected to cause limb defects [37]. It is noteworthy, however, that this putative detrimental effect of doxylamine-pyridoxine on birth defects was not confirmed in subsequent research, and the combination of doxylamine and pyridoxine is currently available for treating nausea and vomiting in pregnancy under the trade name Diclectin® in Canada [38, 39].

Indeed, some studies provide evidence that vitamin B6 supplementation during pregnancy can decrease the severity of nausea and vomiting without dangerous side-effects. For example, Vutyavanich and colleagues conducted a randomized, double-blind, placebo-controlled trial in which pregnant women received either oral pyridoxine hydrochloride (30 mg/day) or a placebo [40]. Women receiving the pyridoxine reported less severe nausea (P = 0.0008), and had a trend toward having fewer vomiting episodes (P = 0.06). Other investigators have found similar results, especially in women experiencing severe nausea [41]. However, there are very few reports of well-controlled trials in this area, and conclusions drawn by systematic reviews of the literature are mixed. For example, whereas Jewell and Young reported in 2003 that evidence supports an effect of vitamin B6 on decreasing severity of nausea, Thaver and colleagues reported in 2006 that there is not enough evidence to conclude that vitamin B6 supplementation in pregnancy has clinical benefits [42, 43]. Without a doubt, the conduct of additional clinical trials is warranted on this topic, as vomiting and nausea represent serious complications for many pregnant women.

14.4.3.1 Recommendations

The American College of Gynecology in 2004 in issuing its most recent guidance on treatment of morning sickness during pregnancy stated that, "taking vitamin B6 ... is safe and effective and should be considered a first-line treatment." The IOM has established the UL for this vitamin to be 100 mg/day during pregnancy. It should be noted that, although no detrimental effects have been associated with high intakes of vitamin B6 from foods, very large oral doses (2,000 mg/day or more) of supplemental pyridoxine are associated with the development of sensory neuropathies and dermatological lesions. Thus, this level of supplementation should always be avoided.

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