Evidence-Based Nutrient Recommendations

Explanation of Vitamin B12 Recommendations


by Jack Norris, RD


Please see our most current vitamin B12 recommendations in Daily Needs.

The U.S. Recommended Dietary Allowance (RDA) for vitamin B12 for adults is 2.4 µg (6), set in 1998. This RDA was based on how much B12 is needed to prevent macrocytic anemia. In the interim, research on vitamin B12 needs has suggested that macrocytic anemia is not the first indicator of B12 deficiency and has focused instead on elevated methylmalonic acid (MMA) and homocysteine as more sensitive markers.

Fortified Foods

A 2010 paper by Bor et al. (13), showed that in healthy people aged 18-50, a vitamin B12 intake of 4 to 7 µg/day was associated with the lowest methylmalonic acid and homocysteine levels (6). They reviewed other research from the past 10 years, primarily on older populations, that indicates the ideal B12 intake is 6 to 10 µg/day. Their review covered the following papers:

  • Vogiatzoglou A, Smith AD, Nurk E, et al. Dietary sources of vitamin B-12 and their association with plasma vitamin B-12 concentrations in the general population: the Hordaland Homocysteine Study. Am J Clin Nutr 2009;89:1078-87.
  • Bor MV, Lydeking-Olsen E, Moller J, Nexo E. A daily intake of approximately 6 micrograms vitamin B-12 appears to saturate all the vitamin B-12-related variables in Danish postmenopausal women. Am J Clin Nutr 2006;83:52-8.
  • Kwan LL, Bermudez OI, Tucker KL. Low vitamin B-12 intake and status are more prevalent in Hispanic older adults of Caribbean origin than in neighborhood-matched non-Hispanic whites. J Nutr 2002;132: 2059-64.
  • Tucker KL, Rich S, Rosenberg I, et al. Plasma vitamin B-12 concentrations relate to intake source in the Framingham Offspring study. Am J Clin Nutr 2000;71:514-22.

Vitamin B12 is more easily absorbed from fortified foods (because it is not bound to protein), so vegans will not need quite as high an intake as meat-eaters. Because of this, I will not distinguish between the ages for vegans, who are getting all their B12 from fortified foods or supplements.

Of the papers above, Bor et al. (2006) and Vogiatzoglou et al. (2009) correlated B12 intake to methylmalonic acid (MMA) and homocysteine (HCY). They found that 6 and 6-10 µg/day, respectively, were associated with the lowest levels of MMA and HCY. Because these were older populations whose protein-bound B12 ability was probably decreased, and because, according to the Institute of Medicine, absorption of non-protein-bound B12 does not appear to decrease with age (6), I will stick with Bor et al.’s 2010 finding which were on people ages 18 to 50.

Fortified foods represented 29% of intake in the Bor et al. (2010) study.

According to the Institute of Medicine, the second of two doses of B12 eaten 4-6 hours apart is absorbed as well as the first (6).

Therefore, a straight calculation that vegans need at least 2 doses of 2 µg per serving per day is easy to make. If you cannot find fortified foods with 2 µg per serving, you should eat more servings or take supplements.


Unlike the studies above that measured B12 intakes from food and compared it to MMA and HCY levels, there is little research looking at what level of supplements people need. Instead, we will have to primarily extrapolate from known absorption rates to get the ideal levels of small-dose supplements that will be linked to the lowest MMA and HCY levels.

In a 2018 study from Italy, vegetarians and vegans were divided into two groups and supplemented with cyanocobalamin for 12 weeks (Del Bo’, 2019). One group of 9 vegetarians and 9 vegans took a 50 µg pill per day. A second group of 9 vegetarians and 9 vegans took one dose of 2,000 µg per week. All supplements were taken sublingually. Vitamin B12 status in both groups normalized across all markers including serum vitamin B12 levels, homocysteine, and methylmalonic acid. Based on this study, 50 µg per day should be enough to improve vitamin B12 status. It provides evidence that our recommendations of 1,000 µg twice per week is enough.

In a 2010 study from India, vegetarian and somewhat B12-deficient subjects with high homocysteine levels were given doses of either 2 µg or 10 µg of B12 each day for a year (12). While both doses lowered homocysteine, neither 2 µg nor 10 µg was enough to lower it to an ideal level of less than 8 µmol/l. See Table 2.

Table 2. B12 Levels in India (2010) Study
2 µg B12 / day 10 µg B12 / day
Plasma Hcy (µmol/l)
Baseline 19.7 18.5
4 months 14.2 12.9
12 months 12.9 11.6

Absorption rates of different doses of cyanocobalamin have been measured and appear in Table 3.

Table 3. B12 Absorption Rates
Food / Oral Dose % absorbed µg absorbed (avg)c
fortified milk5 65%a
fortified bread5 55%
mutton5 65%
chicken5 60%
trout5 39%
eggs5 24-36%
liver6 11%
.1 µg 7 77% 0.08
.25 µg7 75% 0.19
.5 µg7,8 71-90% .35-.45 (.4)
.6 µg7 63% 0.38
1 µg69 50-80% .5-.8 (.6)
2 µg79 40-50% .8-1 (.9)
5 µg610 20-55% 1-2.8 (1.7)
107,9 15-16% 1.5-1.6
25 µg6 5% 1.25
50 µg8,9 3% 1.5 (1.5)
1,000 µg6,8 1-1.5%b 10-15 (12.5)
aNumber represents only the fortified portion of B12 in the milk • bEven without intrinsic factor • cAverage of all measurements (i.e., not the mid-range)

The data in the table is somewhat mixed. A 5 µg supplemental dose of cyanocobalamin appears to saturate the intrinsic factor absorption mechanism. Thus, B12 absorption drops to 1-1.5% for any additional B12 ingested above 5 µg. But in the results in the table, the total amount absorbed from 5 to 50 µg is all about the same.

Considering all the evidence, we should assume that absorbing 2 to 3 µg per day is ideal. Depending on how absorption rates are calculated, to receive the same absorption rates from supplements in one dose, someone would require 25 µg to 240 µg per day (11).

Because these extrapolations are imprecise and because it seems much too high that someone would require 240 µg per day, based on anecdotal evidence and the fact that vegans getting an average of 5.6 µg of B12 per day had healthy homocysteine levels on average (14), we’ve adjusted the upper limit down to 100 µg.

Some people like to take large dose supplements, so let’s do the math on those. The people in Bor et al. (2010) were absorbing 14 to 24.5 µg of B12 per week. If you assume a 1.25% absorption rate from high dose supplements, you would need 1,120 to 1,960 µg per week. To be safe, we recommend 1,000 µg twice per week.


A 2003 study compared 500 µg per day via the sublingual and oral routes (15). The results were that sublingual was no better than oral B12 at raising vitamin B12 levels or improving B12 activity (as measured by homocysteine and methylmalonic acid levels). The report did not specifically state whether the tablets were chewed or not (it’s probably safe to assume they were swallowed whole). If someone has any reason to believe their stomach acid is not strong enough to dissolve a B12 table or capsule, or for any other reasons believes it is not dissolving, then chewing a tablet is advised.

People Over Age 65

Summary: Based on the studies below, it appears that 500 – 1,000 µg per day of cyanocobalamin is the ideal amount for people over 65 years of age with low levels of B12 to take in a daily dose. For older adults who have been supplementing and have recent labs confirming their markers of B12 status (MMA and homocysteine) are normal, supplementation of 50 to 100 µg per day or 1,000 µg at least twice per week should be sufficient. Talk to your health care provider for personalized supplementation recommendations.

There have been at least four relevant studies for how much vitamin B12 people over 65 years need, based on a once daily supplement. To my knowledge all of these supplements were cyanocobalamin.

A 2005 clinical trial from the Netherlands found that among people aged 70-94, who had vitamin B12 deficiency but were otherwise healthy, 16 weeks of 500 µg/day of cyanocobalamin was required to get MMA levels in the healthy range. Other doses tested were 2.5, 100, 250, and 1,000 µg (16).

A 2002 observational study from Canada of 242 people aged 70-94 without vitamin B12 deficiency found that those taking a daily supplement had significantly lower MMA levels (173 vs. 188 µmol/l; p = .042). However, there were many even in the daily supplement group who had elevated MMA levels. The range of supplements was from 2.6-37.5 µg/day with intakes being spread out about evenly over the range (17).

In a 2013 clinical trial from the UK in 100 people aged 65-86 with poor B12 status, 500 µg/day of cyanocobalamin was more successful than 10 µg or 100 µg at lowering urinary MMA levels in 75-85% of the participants over 8 weeks (18).

In a 2002 study from Seattle on 23 people 65 years and older with B12 deficiency but otherwise healthy, 1,000 µg of B12 was required to get the average MMA level into the normal range, as compared to 10 and 100 µg. This study was continuous in that first they put everyone on 10 µg for 6 weeks (moved average MMA from 581 to 400 nmol/l), then 100 µg for 6 weeks (moved average MMA from 400 to 380 nmol/l), and then 1,000 µg for 6 weeks (moved average MMA from 380 to 200 nmol/l). The final 6 weeks resulted in a big drop after the 2nd six weeks resulted in only a small drop (19).


Last updated August 2013

5. Russell RM, Baik H, Kehayias JJ. Older men and women efficiently absorb vitamin B-12 from milk and fortified bread. J Nutr. 2001 Feb;131(2):291-3.

6. Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline. Washington, DC: National Academy Press; 1998.

7. Mollin DL. Radioactive vitamin B12 in the study of blood diseases. Brit Med Bull. 1959;15(1):8-13.

8. Estren S, Brody EA, Wasserman LR. The metabolism of vitamin B12 in pernicious and other megaloblastic anemias. Adv Intern Med. 1959;9:11.

9. Chanarin I. The Megaloblastic Anemias 3rd Ed. Oxford, UK: Blackwell Scientific Publications; 1990.

10. Heyssel RM, Bozian RC, Darby WJ, Bell MC. Vitamin B12 turnover in man. The assimilation of vitamin B12 from natural foodstuff by man and estimates of minimal daily dietary requirements. Am J Clin Nutr. 1966 Mar;18(3):176-84.

11. Calculations: To absorb 2 µg in one dose: 1.7 from 5 µg plus 1.5% of 20 µg. Worst case scenario: To absorb 3.0 µg in one dose: 3.0 / 1.25% absorption rate = 240 µg.

12. Deshmukh US, Joglekar CV, Lubree HG, Ramdas LV, Bhat DS, Naik SS, Hardikar PS, Raut DA, Konde TB, . Wills AK, Jackson AA, Refsum H, Nanivadekar AS, Fall CH, Yajnik CS. Effect of physiological doses of oral vitamin B12 on plasma homocysteine: a randomized, placebo-controlled, double-blind trial in India. Eur J Clin Nutr. 2010 May;64(5):495-502.

13. DBor MV, von Castel-Roberts KM, Kauwell GP, Stabler SP, Allen RH, Maneval DR, Bailey LB, Nexo E. aily intake of 4 to 7 microg dietary vitamin B-12 is associated with steady concentrations of vitamin B-12-related biomarkers in a healthy young population. Am J Clin Nutr. 2010 Mar;91(3):571-7.

14. Haddad EH, Berk LS, Kettering JD, Hubbard RW, Peters WR. Dietary intake and biochemical, hematologic, and immune status of vegans compared with nonvegetarians. Am J Clin Nutr. 1999;70(suppl):586S-93S.

15. Sharabi A, Cohen E, Sulkes J, Garty M. Replacement therapy for vitamin B12 deficiency: comparison between the sublingual and oral route. Br J Clin Pharmacol. 2003 Dec;56(6):635-8.

16. Eussen SJ, de Groot LC, Clarke R, Schneede J, Ueland PM, Hoefnagels WH, van Staveren WA. Oral cyanocobalamin supplementation in older people with vitamin B12 deficiency: a dose-finding trial. Arch Intern Med. 2005 May 23;165(10):1167-72.

17. Garcia A, Paris-Pombo A, Evans L, Day A, Freedman M. Is low-dose oral cobalamin enough to normalize cobalamin function in older people? J Am Geriatr Soc. 2002 Aug;50(8):1401-4.

18. Hill MH, Flatley JE, Barker ME, Garner CM, Manning NJ, Olpin SE, Moat SJ, Russell J, Powers HJ. A vitamin B-12 supplement of 500 μg/d for eight weeks does not normalize urinary methylmalonic acid or other biomarkers of vitamin B-12 status in elderly people with moderately poor vitamin B-12 status. J Nutr. 2013 Feb;143(2):142-7.

19. Rajan S, Wallace JI, Brodkin KI, Beresford SA, Allen RH, Stabler SP. Response of elevated methylmalonic acid to three dose levels of oral cobalamin in older adults. J Am Geriatr Soc. 2002 Nov;50(11):1789-95.

20. Effect of two different sublingual dosages of vitamin B12 on cobalamin nutritional status in vegans and vegetarians with a marginal deficiency: A randomized controlled trial. Clin Nutr. 2019 Apr;38(2):575-583. 

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