Vitamin B12 and Plant-Predominant Diets (2024)

Abstract

Vitamin B12 deficiencies are common in individuals consuming plant-predominant diets,including those who consume diary and/or eggs. Deficiencies can lead to megaloblasticanemia and peripheral neuropathy, among other multi-system manifestations. The prevalence,assessment and prevention of vitamin B12 deficiency in patients followingplant-predominant diets will be discussed.

Keywords: vitamin B12, plant-based diet, vegan, vegetarian

“Because of the increased risk of deficiency, most patients consuming plant-based dietsshould be encouraged to use a vitamin B12 supplement.”

While plant-based diets are rich in nutrients such as fiber, vitamin C, folate, andmagnesium, nutrient deficiencies including that of vitamin D, iron, omega-3 polyunsaturatedfatty acids, and vitamin B12 can occur.¹ Vitamin B12 is unique in that it is only synthesized by microorganisms and thusessentially devoid in foods of plant origin, except through contamination with soil, exposureto foods containing vitamin B12, or if the plant-based food is fortified.² Vitamin B12 is essential for neurologic function, red blood cell production, and DNA synthesis.³ Deficiencies can lead to megaloblastic anemia and peripheral neuropathy, among other,multi-system manifestations.⁴ Usually, hematologic manifestations and anemia precede neurologic signs; however,because a plant-predominant diet is rich in folate, early hematological symptoms may bemasked, making monitoring and prevention of deficiency of upmost importance.⁵

Although vitamin B12 deficiency was once thought to be extremely rare except in very strictvegan/vegetarian diets, it is now known that deficiency is more common, even in thosevegetarians consuming eggs and dairy.² Another common misconception is the idea that vitamin B12 deficiency takes many yearsto develop, whereas we now know that it can occur within the first 2years of initiating aplant-predominant diet.⁶ The prevalence, assessment, and prevention strategies of vitamin B12 deficiency inpatients following plant-predominant diets will be discussed.

Assessment

The recommended laboratory evaluation for patients with suspected vitamin B12 deficiencystarts with a serum vitamin B12 level.^4,7,8 Serum vitamin B12 thresholds diagnostic fordeficiency vary widely, and it has been found that deficiency conditions may be present atconcentrations above commonly used cutoffs (156pg/mL).^9-11 Extremely low levels (< 70pg/mL)usually reflect deficiency accompanied by clinical manifestations, but there is a largerange of uncertainty (100–300pg/mL) in which further testing with serum or urinemethylmalonic acid or holotranscobalamin II may be useful.⁸ Due to the lack of reference method or gold standard for assessing vitamin B12deficiency, it may be prudent to treat all patients with mild deficiency while evaluatingfor and treating other diagnoses.⁸ Likewise, because of the prevalence of vitamin B12 deficiency in patients following aplant-predominant diet, a supplement should be recommended for most patients.

Prevalence

One of the first studies to link vitamin B12 deficiency to plant-based dietary patterns waspublished in 1955 by Wokes et al.¹² The study systematically compared a group of British, Dutch, and US vegans withomnivores from the same countries. The study found that many of the vegans had significantlylower vitamin B12 concentrations than their meat-eating counterparts. A second study,published a year later by Dhopeshwarkar et al.,¹³ observed a similar trend in lactovegetarians vs nonvegetarians in India.

While the prevalence of vitamin B12 deficiency in the United States/United Kingdom generalpopulation is approximately 6% in patients less than 60years of age and 20% in patientsolder than 60, the prevalence in patients consuming plant-predominant diets ishigher.^2,3,14 In a systematic review of literaturebased on serum concentrations of vitamin B12 among vegetarians, a deficiency was presentranging from 0 to 86.5% in adults and elderly, up to 45% in infants, from 0 to 33.3% inchildren and adolescents, and from 17 to 39% among pregnant women.¹⁴ As discussed previously, this research was based on the use of serum vitamin B12levels which likely underestimates the true rate of deficiency, due to the inaccuracy ofthis assessment method.¹⁵ Likewise, this study was conducted in vegetarians and included patients who consumedairy, eggs, or both.

A second review reported vitamin B12 deficiency rates from studies that identifieddeficiency using methylmalonic acid, holotranscobalamin II, or both.² In this study, a deficiency was documented in 11–90% of adults and elderly, 62% ofpregnant women, 25–86% of children, and 21–41% of adolescents. Of interest, the prevalenceof deficiency among children in the United States was estimated to be 55% in general, but asub-analysis revealed that prevalence was 67% in children who followed a vegetarian diettheir entire life vs 25% among other vegetarian children.¹⁶ The authors hypothesized this finding was likely due to low maternal B12 stores thatresulted in low stored B12 in offspring. Although a wide range of reported B12 deficiencywas found in adults and children, there was heterogenicity in the definition of deficiencyused in many of the studies. For example, the lowest rates of deficiency (11%) were reportedin a study that used criterion for deficiency that was 2 to 3 times higher than what wasused in the other included studies. The study also found that B12 deficiency was more likelyin vegans than vegetarians.

Prevention

While it may be possible for patients consuming plant-predominant diets to obtain adequateamounts of vitamin B12 through intake of eggs, dairy, and fortified products, as evidencedby the studies described above, most patients do not. The vitamin B12 content of eggs andmilk is quite low, and appreciable loss in content or bioavailability is demonstrated whenmilk is processed and eggs are cooked.¹ Nutritional yeast is a product commonly fortified with vitamin B12 and oftenrecommended for use in patients consuming plant-predominant diets to reduce the risk ofdeficiency. Approximately 2 tablespoons of nutritional yeast contain the amount equal to theUS RDA (2.4mcg/day) for adults.¹⁷ While the product can add B12 to the diet, it must be consumed consistently toprevent deficiency and supplements have been shown to be more effective in correcting anexisting vitamin B12 deficiency than use of nutritional yeast.¹⁸ Furthermore, nutritional yeast is not endorsed as an adequate practical source of B12by the American Dietetic Association.¹⁹

Cyanocobalamin is the most economical and commonly encountered vitamin B12 supplement inthe United States. Oral doses range from 100 to 5000mcg. The absorption of vitamin B12depends on the dose and frequency of administration.²⁰ In general, the higher the dose, the lower the absorption rate.²¹ Various oral and intramuscular formulations are available, and high-dose oralsupplementation has been found to be as effective as parenteral administration.²² There is no universal dosing recommendation for cyanocobalamin for health maintenancein patients consuming a predominantly plant-based diet. Recommendations for adults rangefrom 250mcg by mouth daily to 500–1000mcg by mouth several times per week.^19,23 Despite the common misconception that avitamin B12 deficiency takes decades to develop, it is known that vitamin B12 deficiency canoccur as early as 2years after initiating a plant-predominant diet.^18,24,25 Therefore, supplementation withcyanocobalamin should be initiated, ideally, with the onset of the plant-based diet.

Conclusions

Vitamin B12 deficiency in patients consuming plant-predominant diets is high, even amongthose vegetarians who consume dairy, eggs or both. The risk for deficiency is higher invegan patients and in those who have consumed plant-based diets from birth. Because of theincreased risk of deficiency, most patients consuming plant-based diets should be encouragedto use a vitamin B12 supplement. Oral cyanocobalamin at doses of at least 250mcg dailyshould be recommended. If laboratory measures are used to assess or monitor B12 status, itis important to note that serum vitamin B12 thresholds diagnostic for deficiency varywidely, and it may be prudent to supplement patients with mild deficiency (< 300pg/mL)or employ more specific tests (methylmalonic acid and holotranscobalamin II). Whilenutritional yeast can be helpful in obtaining vitamin B12 in the plant-predominant diet, asupplement is preferred for patients with existing deficiency. Lastly, because deficienciescan present within the first 2years of initiating a plant-based diet, patients should beencouraged to begin supplementation at the onset of the diet.

Footnotes

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