Vitamin B₁₂ in meat and dairy products

Abstract

Vitamin B₁₂ is synthesized exclusively by microorganisms; therefore, humans must absorb it from food. Excellent sources of B₁₂ are foods of ruminant origin, so dairy and meat products play an important role in efforts to meet the official daily B₁₂ intake recommendation of 3.0 μg. Concentrations of the vitamin vary within foods of ruminant origin, with the highest concentrations found in offal such as liver and kidney. In comparison, dairy products have much lower quantities of the vitamin. In bovine milk, the B₁₂ concentration is stable with regard to breed, feed, season, and stage of lactation, but in ruminant meat, the amount of B₁₂ can vary based on the feeding and husbandry of the animal as well as the cut of meat chosen and its preparation. Processing of ruminant food, including thermal treatment, usually diminishes the vitamin B₁₂ concentration. This review summarizes the vitamin B₁₂ content of foods and discusses the impact of food processing on vitamin content. The contribution of ruminant food sources to B₁₂ intake is specifically evaluated, with its bioavailability taken into account.

cobalamin, dairy products, meat, milk, vitamin B₁₂

INTRODUCTION

Vitamin B₁₂ (also known as cobalamin) is a chemical compound with vitamin properties. It consists of cobalt as the central atom and a corrin ring that encloses the metal atom. In comparison with other B vitamins, B₁₂ is not synthesized by animals, fungi, or plants. Exclusively, microorganisms (mainly anaerobes) or archaebacteria in the presence of cobalt are able to produce vitamin B₁₂. The most frequently occurring natural and active forms of B₁₂ are adenosylcobalamin (also known as coenzyme B₁₂) and methylcobalamin.¹ Hydroxocobalamin and industrially produced cyanocobalamin are inactive forms of vitamin B₁₂ that need to be metabolized in order to be used by humans.²

Although the intestinal flora of humans is able to synthesize vitamin B₁₂, humans are not able to absorb it since the location of synthesis (the colon) is too distal from the location of absorption (the small intestine).³ Therefore, vitamin B₁₂ has to be consumed through food. Foods of ruminant origin are an important source of vitamin B₁₂ due to the production of cobalamin by bacteria inhabiting the gastrointestinal tract of ruminants. After biosynthesis, the vitamin is stored in the muscles and the liver of the animal or secreted via the animal’s milk.⁴ Humans can benefit from these stores by consuming ruminant meat and milk. Daily intake of these foods helps humans meet the recommended daily amount of vitamin B₁₂ (3.0 µg/day).⁵ However, vegetarians and vegans who restrict or eliminate consumption of animal-source foods are at higher risk for developing a B₁₂ deficiency.⁶

In light of recommendations to limit red meat intake and the rising popularity of vegetarian and vegan lifestyles, it is worthwhile to examine both the concentrations and bioavailability of vitamin B₁₂ in animal-based meat and milk as well as to quantify the contributions these products make to daily intake of B₁₂. The present article provides an overview of the vitamin B₁₂ supply from meat and dairy products, with a particular focus on its concentration in and bioavailability from ruminant foods as well as the contributions these sources make to daily intake. The impact of food processing is additionally documented.

VITAMIN B₁₂ METABOLISM AND BIOLOGICAL FUNCTIONS

Cobalamins are considered to be the most complex cofactors in nature.¹ In food, their most important representative, vitamin B_12, is bound to proteins known as R-proteins or R-binders. Different steps and conditions are necessary for the effective uptake of cobalamin, including adequate synthesis of hydrochloric acid, proteases, and intrinsic factor (IF).² First, vitamin B₁₂ is released from food proteins by means of the proteolytic enzyme pepsin. This enzyme is secreted as pepsinogen in the stomach and activated by hydrochloric acid. Subsequently, the vitamin binds to the glycoprotein haptocorrin, which is secreted with saliva. In the small intestine, pancreatic proteases break down the B₁₂–haptocorrin bonds, thus enabling formation of the B₁₂–IF complex (B₁₂–IF).⁷ In a subsequent step, B₁₂–IF enters the apical surface of the cells of the distal ileum by binding to a specific receptor, cubilin, and it passes through the cells by being incorporated into cellular lysosomes.⁸ After entering the cell, B₁₂ is released from the IF. For further transport, cobalamin binds to another protein, either to haptocorrin, which is responsible for the transport of B₁₂ to the liver, or to transcobalamin, which builds a complex with vitamin B₁₂, holotranscobalamin (the active form of B₁₂). Holotranscobalamin is the only active B₁₂ fraction that can be incorporated into tissue cells.⁹

Another mechanism of cobalamin absorption occurs in the presence of large amounts of vitamin B₁₂, e.g., after ingestion of vitamin B₁₂ supplements. Approximately ≤1% of free cobalamin can be absorbed by diffusion into the cells of the epithelial ileum.² Most unused vitamin B₁₂ is stored in the liver and muscles and has a half-life of 1–4 years.¹⁰

Vitamin B₁₂ combines major biological functions, i.e., methylcobalamin acts as an important carrier of methyl groups and as such it is, as co-factor of the enzyme methionine synthase, involved in the transformation of homocysteine to the metabolite methionine in the cytosol.¹¹ These processes and subsequent metabolic reactions are involved in the synthesis of neurotransmitters, phospholipids, DNA, and RNA.¹⁰ Adenosylcobalamin acts as a cofactor of methylmalonyl-coenzyme A (CoA) mutase, which converts methylmalonyl-CoA to succinyl-CoA in the mitochondria. This reaction is involved in the catabolism of cholesterol, fatty acids, and several amino acids.¹²

VITAMIN B₁₂ REQUIREMENTS AND B₁₂ DEFICIENCY

Vitamin B₁₂ status in humans depends on intake.^13–15 It is assumed that adults have a total cobalamin content of 2–3 mg.¹⁶ A pooled analysis showed that 1.4–5.1 µg of vitamin B₁₂ are excreted every day.¹⁷ In developed countries, B₁₂ intake is normally adequate. Nevertheless, B₁₂ deficiency is the most common vitamin deficiency requiring clinical therapy.³ There are a variety of causes of deficiency, including congenital disturbances of cobalamin metabolism and impairment of absorption, which can be caused by defects of the gastric mucosa, chronic atrophic gastritis, gastrectomy, malabsorption in the ileum, intestinal stasis, drugs, and intestinal parasites. Other causes include old age and pernicious anemia, which is characterized by autoimmune destruction of gastric parietal cells resulting in a lack of IF.^3,18 In developing countries, B₁₂ deficiency is common because of typically low consumption of animal foods, resulting in inadequate intakes.¹⁹ Worldwide, vegans and vegetarians may suffer from cobalamin deficiency due to their avoidance of B₁₂-containing foods.⁶ Newborns may also suffer from B₁₂ deficiency when their mothers suffer from the same deficiency.³

The main syndrome of vitamin B₁₂ deficiency is pernicious anemia, which is characterized by two primary symptoms: megaloblastic anemia and/or neuropathy. Megaloblastic anemia can be accompanied by anemia, enlarged red blood cells, hypersegmented neutrophil leucocytes, low white blood cell count, low platelet count, a sore tongue, and infertility. Neuropathy symptoms are characterized by degeneration of the spinal cord, loss of proprioceptive sensation, spastic weakness in the lower limbs, and, in some instances, depression or memory loss. However, the onset of B₁₂ deficiency is gradual due to large stores in the body. In the first instance, a subclinical deficiency is defined by a low level of serum B₁₂ and/or a raised methylmalonate concentration, followed by the less common clinical deficiency (megaloblastic anemia/neuropathy).¹⁸

In the United States, the recommended daily allowance of vitamin B₁₂ for females and males between the ages of 13 and >70 years is 2.4 µg/day.¹⁶ The nutrition societies of Germany, Switzerland, and Austria recommend a slightly higher dose of 3.0 µg/day⁵ (Table 1). However, some research findings suggest that even higher daily intake levels, in the range of 3.8–20.7 µg, are necessary to cover daily B₁₂ losses.¹⁷

Table 1

Recommended dietary intakes for vitamin B₁₂

Population	Recommended vitamin B12 intake (µg/day)
	Germany, Switzerland, and Austriaa	United Statesb
Infants
0–<12 month	0.4–0.8	0.4–0.5
Children
1–<13 years	1.0–2.0	0.9–1.8
Adolescents and adults
13 years and older	3.0	2.4
Pregnant women	3.5	2.6
Breastfeeding women	4.0	2.8

^aData from the German Society for Nutrition (2013).⁵

^bData from the Institute of Medicine (1998).¹⁶

VITAMIN B₁₂ CONTENT OF MEAT AND DAIRY PRODUCTS

Humans depend on the dietary intake of vitamin B₁₂. Although some plant foods such as certain edible algae and fermented soybeans (tempe) contain some B₁₂,²⁰ only animal-derived foods offer naturally sufficient amounts of cobalamin to meet the body’s requirements. The best sources are ruminant meat and milk due to the natural bacterial populations that synthesize vitamin B₁₂ in the rumen of these animals.^3,20 After its bacterial biosynthesis in the animal, vitamin B₁₂ is absorbed through the gastrointestinal tract and transported via the blood to body tissues and liquids including the liver, muscles, and milk.²¹

Meat

Livers from ruminant animals contain the largest amounts of vitamin B₁₂, and red meat is another excellent source. The national food composition databases of Denmark, Switzerland, Canada, and the United States present the following vitamin B₁₂ concentrations for raw meat: 0.7–5.2 µg/100 g for beef, 1.2–5.0 µg/100 g for lamb/mutton, 1.0–2.9 µg/100 g for veal, 0.4–2.0 µg/100 g for pork, and 0.2–0.6 µg/100 g for chicken. Regarding the vitamin B₁₂ content in the livers of different animals, concentrations vary from 59.0 to 110.0 µg/100 g for beef and 60.0 µg/100 g for veal. Values found in kidneys are as follows: 23.0–28.0 µg/100 g for veal and 27.0–31.0 µg/100 g for beef.^22–25 Most food composition databases do not disclose how the values were obtained; therefore, the observable differences within any particular type of meat cannot be explained. However, as discussed below, differences may be due to several factors including the methods of analysis, cooking methods, animal feeding and husbandry practices, and the cut of meat (i.e., type of muscle). Table 2 presents the concentrations of vitamin B₁₂ in raw and thermally processed meat, meat products, and organ meats, as documented in the literature.^21,26–40

Table 2

Vitamin B₁₂ concentrations in raw and thermally processed meat

Type of meat	Vitamin B12(µg/100 g)	Reference
Lamb
Lean meat, raw	1.12	Heyssel et al. (1966)26
Lean meat, raw	3.00–4.02	van Heerden et al. (2007)27
Muscle longissimus dorsi, raw	0.55–0.99	Ortigues-Marty et al. (2005)21
Muscle semi tendinosus, raw	0.67–1.04	Ortigues-Marty et al. (2005)21
Mince, raw	1.7	Williams et al. (2007)28
Mince, cooked	2.1	Williams et al. (2007)28
Lean meat, cooked	0.2–1.1	van Heerden et al. (2007)27
Chop, braised	0.45–2.31	Adams et al. (1973)29
Leg, roasted	1.26–2.22	Adams et al. (1973)29
Mutton
Leg roast and casserole, raw	2.8	Williams et al. (2007)28
Leg roast and casserole, cooked	1.9	Williams et al. (2007)28
Leg, roasted	0.83–2.29	Adams et al. (1973)29
Sheep, kidney, braised	52.6–110.4	Adams et al. (1973)29
Shoulder, boiled	0.42–1.67	Adams et al. (1973)29
Beef
Primal cuts of separable lean meat, raw	3.50–4.43	Bennink et al. (1982)30
Separable lean meat, raw	2.83	Bennink et al. (1982)30
Separable lean meat, thermally processed	2.71–2.77	Bennink et al. (1982)30
Muscle, raw	1.53	Watanabe et al. (1998)31
Liver, raw	62.51	Watanabe et al. (1998)31
Strip steak, raw	1.3	Leheska et al. (2008)32
Ground beef, raw	2.0	Leheska et al. (2008)32
Muscle rectus abdominis, raw	0.91–1.18	Ortigues-Marty et al. (2005)21
Muscle semi tendinosus, raw	0.36–0.58	Ortigues-Marty et al. (2005)21
Liver, raw	54.1–78.2	Ortigues-Marty et al. (2005)21
Muscle triceps brachii, raw	2.09	Ortigues-Marty et al. (2006)33
Muscle longissimus lumborum, raw	1.15	Ortigues-Marty et al. (2006)33
Muscle longissimus thoracis, raw	0.92	Ortigues-Marty et al. (2006)33
Mince-low fat, raw	1.9	Williams et al. (2007)28
Mince-low fat, cooked	2.4	Williams et al. (2007)28
Various pieces, raw	2.84–3.95	Szterk et al. (2012)34
Various pieces, fried	1.48–2.28	Szterk et al. (2012)34
Various pieces, grilled	1.03–1.19	Szterk et al. (2012)34
Tenderloin, roasted	1.15	Szterk et al. (2012)34
Brisket, boiled	1.06–1.36	Adams et al. (1973)29
Rump, braised	0.37–1.31	Adams et al. (1973)29
Sirloin, roasted	0.67–1.29	Adams et al. (1973)29
Shoulder, heated in various ways	0.50–2.22	Adams et al. (1973)29
Ox, liver, braised	53.5–111.5	Adams et al. (1973)29
Ox, kidney, braised	4.8–7.1	Adams et al. (1973)29
Veal
Lean meat, raw	1.6	Williams et al. (2007)28
Lean meat, cooked	3.0	Williams et al. (2007)28
Fricassee	0.52–1.58	Adams et al. (1973)29
Jellied	0.95–2.17	Adams et al. (1973)29
Goat
Ground meat, grilled	1.09–1.17	Johnson et al. (1995)35
Pork
Chop, braised	0.32–0.47	Adams et al. (1973)29
Leg, roasted	0.13–0.82	Adams et al. (1973)29
Muscle, raw	3.70	Watanabe et al. (1998)31
Lean meat, raw	0.45–0.61	Esteve et al. (2002)36
Poultry
Turkey, breast, roasted	0.41–3.29	Adams et al. (1973)29
Turkey, breast, raw	0.47	Molonon et al. (1980)37
Turkey, breast, cooked	0.44–0.70	Molonon et al. (1980)37
Chicken, red and white meat, cooked	0.19–1.30	Adams et al. (1973)29
Chicken, white meat, cooked	0.33–0.43	Doscherholmen et al. (1978)38
Chicken, red meat, cooked	0.71–0.83	Doscherholmen et al. (1978)38
Chicken, muscle, raw	1.73	Watanabe et al. (1998)31
Meat products
Sausages, cooked	0.3–1.54	Adams et al. (1973)29
Ham, baked	0.34–0.69	Adams et al. (1973)29
Ham, dry-cured	0.33–0.67	Lucarini et al. (2013)39
Ham, cooked	0.09–0.13	Lucarini et al. (2013)39
Ham, smoked	0.33	Lucarini et al. (2013)39
Cured meat products	0.41–2.20	Esteve et al. (2002)36
Frankfurt sausage	0.60–0.61	Esteve et al. (2002)36
Vienna sausage	0.33–0.85	Esteve et al. (2002)36
Turkey breast	0.48–3.41	Esteve et al. (2002)36
Mortadella	0.56–1.04	Esteve et al. (2002)36
Ham, cooked	0.23–0.45	Esteve et al. (2002)36
Salami	0.39–0.95	Guggisberg et al. (2012)40
Landjäger	0.59–0.81	Guggisberg et al. (2012)40
Salsiz	0.96–1.49	Guggisberg et al. (2012)40
Saucisson Vaudois	0.44–0.56	Guggisberg et al. (2012)40
Coppa	0.23–0.57	Guggisberg et al. (2012)40
Bacon, raw	0.36–0.62	Guggisberg et al. (2012)40
Ham, raw	0.26–0.68	Guggisberg et al. (2012)40
Mostbröckli	1.07–1.63	Guggisberg et al. (2012)40
Dried beef	1.12–2.61	Guggisberg et al. (2012)40

Differences according to method of analysis

Although the various methods of determining the vitamin B₁₂ content in foods correlate well, the results may differ because less selective methods may also detect biologically inactive cobalamins. Typically, the B₁₂ concentration is determined by bioassays utilizing microorganisms that require vitamin B₁₂. This is the predominant method in the literature.^{26,28,29,32,35,36} Vitamin B₁₂ can also be assessed with a radioisotope dilution assay,^21,30,33,38 which yields results comparable to those for the microbiological assay but may, in some cases, give a slightly higher estimate of the B₁₂ content.^30,41,42 In two of the investigations reviewed, a chemiluminescence analyzer was used to measure B₁₂ concentrations.^31,39 This seems to be both more selective than and well-correlated with the microbiological method used to test most of the foods.³¹ Nevertheless, the B₁₂ concentration detected in raw pork and chicken muscle (not in beef muscle) was significantly higher with chemiluminescence than with microbiological analysis (pork 3.70 versus 2.42 µg/100 g; chicken 1.73 versus 1.54 µg/100 g).³¹ Another method for B₁₂ determination in foods that seems to be more sensitive than the microbiological assay is high-performance liquid chromatography.^27,34,40

Animal differences

As mentioned previously, the vitamin B₁₂ concentration in ruminant meat such as beef and lamb is usually higher than in the meat of monogastric animals such as pigs and poultry due to the larger bacterial populations that synthesize vitamin B₁₂ in the rumen of these animals.^3,20 A comparison of muscle samples from Limousin steers, Charolais heifers, and Charolais and Montbeliard cull cows showed no significant influence of animal breed on the B₁₂ concentration of raw meat.³³ However, the older the animal, the higher the vitamin B₁₂ concentration tended to be. The levels in beef are, therefore, generally higher than in veal, and mutton usually contains more vitamin B₁₂ than lamb.⁴³

Differences according to meat cuts

The type of muscle has a substantial impact on the vitamin B₁₂ concentration, with the orientation of the muscle’s energy metabolism being the most probable predictor of cobalamin concentration. Oxidative muscles contain more vitamin B₁₂ than glycolytic muscles, supposedly due to the larger number of mitochondria in which adenosylcobalamine is preferentially located.^21,33 In an investigation by Ortigues-Marty et al.,²¹ the oxidative-type rectus abdominis muscle (flank) contained 1.08 µg vitamin B₁₂ per 100 g compared with only 0.5 µg vitamin B₁₂ per 100 g in the glycolytic semi-tendinosus muscle (eye of round). Additionally, vitamin B₁₂ is water soluble and associated with proteins. Therefore, cuts of meat with low total lipid content have higher concentrations of B₁₂ than cuts with high total lipid content.³³ Different cuts of meat can thus possess substantially different concentrations of cobalamin.

Influence of feeding and animal husbandry

Ruminants normally do not have a vitamin B₁₂ source in their feed but they do benefit from the bacteria in their rumen that utilize dietary cobalt to synthesize vitamin B₁₂.⁴⁴ Thus, vitamin B₁₂ synthesis in ruminants varies depending on the amount of cobalt in the feed. Since a sufficient amount of B₁₂ is essential for the animals’ health, recommendations exist to ensure an adequate cobalt content of the animal diet.^44,45 With the feed commonly provided, ruminants should be adequately supplied with cobalt and, therefore, with vitamin B₁₂.⁴⁶ The results of a study by Ortigues-Marty et al.²¹ indicate that diet-induced differences in the B₁₂ concentrations of ruminant meat are essentially related to the amount of available cobalt in the animal diet. However, unlike liver concentrations, B₁₂ concentrations in muscle were only minimally affected by the feed. The authors presumed that the liver has to be depleted to a certain degree before levels in muscle are reduced.²¹

Influence of thermal processing

Vitamin B₁₂ is water soluble and sensitive to light as well as to oxidizing and reducing agents such as ascorbic acid, sulfite, and iron salts. Consequently, the thermal processing of fresh meat, such as by cooking, roasting, and braising, influences the available cobalamin content.⁴² There are two conflicting effects of thermal processing on vitamin B₁₂ concentrations in meat. First, there is a concentrating effect, due to moisture and lipid decreases that occur during cooking. Second, since vitamin B₁₂ is water soluble, it can be lost with the water, especially when moist-heat methods are used. In addition, vitamin B₁₂ is sensitive to light and oxidation; thus, losses may occur when cooking time is augmented and as a result of certain preparation conditions.^33,37 Several investigations compared the concentrations of vitamin B₁₂ in raw meat and in meat that was cooked using dry-heat as well as moist-heat methods.^27,30,33,37 Study results varied, but they predominantly showed an increase in B₁₂ content on a fresh-weight basis. Using dry-heat cooking methods, an increase of 15–37% of the initial value was described.³³ However, on a dry matter basis, some significant decreases in vitamin B₁₂ concentrations were observed, which could be attributable to the long duration and/or high temperature of cooking. Cobalamin losses of 10–40% caused by cooking are documented in the literature.^30,33 Differences occur based on the cooking method used. However, cooking at a lower temperature results in higher B₁₂ concentrations than cooking at higher temperatures.³⁷ In conclusion, cooked meat seems to offer similar or higher B₁₂ concentrations per 100 g edible portion than raw meat due to the moisture and lipid losses that occur during cooking, even though an important part of the vitamin B₁₂ content might be lost with the water and destroyed by the cooking method used. This becomes obvious when values are compared on a dry-weight basis.

Influence of meat maturation

Between the time of slaughter and consumption, meat is stored; during this period, the meat matures. This period of maturation is widely used to enhance the meat’s quality, particularly its tenderness but also its flavor. One study examined the effect of the duration of maturation on vitamin B₁₂ content, and no effect was demonstrated.³³

Milk and dairy products

The national food composition databases of Denmark, Switzerland, and Canada present the following vitamin B₁₂ concentrations: bovine milk, 0.08–0.49 µg/100 g; goat milk, 0.07–0.10 µg/100 g; cheese, 0.34–3.34 µg/100 g; yogurt, 0.12–0.60 µg/100 g; and cream, 0.17–0.50 µg/100 g.^22–24 As mentioned above, concentrations can vary a great deal depending on the method of analysis. Efforts to optimize the analytical methods for B₁₂ determination are ongoing. Table 3 summarizes the vitamin B₁₂ concentrations in a variety of dairy products.^47–53

Table 3

Vitamin B₁₂ concentrations in milk and dairy products

Dairy product	Vitamin B12 (µg/100 g)	Reference
Milk
Cow	0.2–0.7	Renner (1982)47
Sheep	0.30	Sieber (2012)48
Goat	0.07	Scott and Bishop (1986)49
Buffalo	0.3	Souci et al. (2008)50
Horse	0.3	Souci et al. (2008)50
Human	0.05	Souci et al. (2008)50
Skim	0.3	Souci et al. (2008)50
Ultra-high temperature	0.38	Souci et al. (2008)50
Buttermilk	0.095–0.23	Scott and Bishop (1986)49
Cream	0.3	Scott and Bishop (1988)51
Fermented milk
Yogurt	0.17–0.43	Scott and Bishop (1986)49
Cheese
Edam, gouda	1.4–1.9	Scott and Bishop (1988)51
Emmentaler	3.1	Sieber et al. (1988)52
Blue	1.0–1.2	Scott and Bishop (1988)51
Gruyère	2.0	Sieber et al. (1988)52
Parmesan	1.5–1.9	Scott and Bishop (1988)51
Cottage cheese	2.0	Souci et al. (2008)50
Curd	0.38	Sieber et al. (1999)53

Vitamin B₁₂ in milk is mostly bound to proteins.⁵⁴ Hydroxycobalamin, adenosylcobalamin, and methylcobalamin represent the major derivatives of B₁₂ in bovine milk and hard cheese.^55,56 In bovine milk, B₁₂ concentration is stable with regard to breed, feed, season, and stage of lactation (except for colostrum, which contains higher concentrations of B₁₂).³ However, milk processing results in severe losses of B₁₂. In light of this problem, various experiments have been performed by different working groups and are presented in the sections below.

Thermal processing and storage of milk

Arkbage et al.⁵⁷ conducted experiments that included the heating and boiling of milk. Temperatures of 76°C for 16 s and 96°C for 5 min did not cause losses of B₁₂. In other studies, however, high losses were observed, e.g., boiling milk for 2–5 min and 30 min resulted in vitamin B₁₂ reductions of 30% and 50%, respectively,^58,59 as were minor losses, e.g., <10% loss of vitamin B₁₂ after pasteurization and 0–20% loss after ultra-high temperature treatment.⁴⁷ Storage of ultra-high temperature-treated milk at 7°C for 18 weeks did not lead to losses of B₁₂, whereas storage at 23°C and 35°C for 18 weeks was accompanied by a significant reduction of up to 33% of vitamin B₁₂ concentration. Vitamin retention was strongly dependent on the oxygen concentration in the packaging.⁶⁰

Milk fermentation and storage of yogurt

The addition of starter cultures such as Lactobacillus bulgaricus and Streptococcus thermophilus for the fermentation of milk to yogurt decreased the original amount of vitamin B₁₂ by 25%.⁵⁷ Similar results were observed in other studies.^61–63 Storage of yogurt (unopened cup) at 4°C for 14 days caused further losses of 33%. In total, only 40% of the initial amount of B₁₂ in milk was present in the prepared and stored yogurt.⁵⁷

Milk fermentation and storage of cheese

During the cheese-making process, the whey fraction is removed, leading to a severe loss of vitamin B₁₂ due to its water solubility. In one study of six fermented dairy products, cottage cheese contained only 16% of the B₁₂ concentration that was initially present in the milk from which it derived, and packaging and storage for 10 days did not alter the vitamin concentration.⁵⁷

During the production of two hard cheeses, 44–52% of the B₁₂ originally present in the milk was removed via the whey fraction.⁵⁷ A cheese-ripening process of 32 weeks led to further B₁₂ losses of 10% for one hard cheese and no further losses for the other. During the production of blue cheese, 38% of the initial B₁₂ content was removed with the whey fraction. After 5–6 weeks of ripening and 8 weeks of storage, the vitamin B₁₂ content did not decrease further.⁵⁷

Similar results were observed when producing Camembert, blue cheese, Port-Salut, and Gruyère cheeses; in the whey fraction, only 43.0–60.5% of the initial vitamin B₁₂ concentrations in the milk were found.⁶⁴ Moreover, further losses occurred within the first few days after production and continued during the ripening process in mold cheeses (losses from 38% to 44%). In the cheese rind of Port-Salut, small quantities of B₁₂ were formed. Greater B₁₂ increases in cheese as well as on the surface were measured in Gruyère due to the application of propionic bacteria. Storage of the cheeses at temperatures between 2°C and −20°C did not change the vitamin B₁₂ concentrations.⁶⁴

B₁₂-consuming and B₁₂-producing bacterial strains

Lactic acid bacteria require vitamin B₁₂ in order to grow. As a result, they contribute strongly to the findings described in this article. In particular, L. bulgaricus and S. thermophilus, which are commonly used for milk fermentation, were found to be very efficient vitamin B₁₂ consumers.⁶³ In that experiment, the retention of vitamin B₁₂ in yogurt after storage was found to be low; this was most likely due to the storage temperature of 4°C, which is not low enough to inhibit the metabolic activity of lactic acid bacteria. Consequently, the bacteria continue to use B₁₂ for growth. Similar results and assumptions were published by Swedish researchers who observed severe losses of B₁₂ after milk fermentation, especially in yogurt and kefir.⁶¹

Bacterial strains that produce vitamin B₁₂ have also been identified, including Propionibacterium strains, which are regarded as the most efficient vitamin B₁₂ producers.⁶⁵ In particular, the addition of Propionibacterium shermanii to the starter cultures of different milk beverages increased the concentration of vitamin B₁₂ in the products.⁶⁶ Poonam et al.⁶⁷ listed some dairy Propionibacterium strains and the concentrations of vitamin B₁₂ they produce. However, most Propionibacterium strains are not suitable for fermentation of milk; rather, they play an important role in the industrial production of synthetic B₁₂. This area of research has not yet been fully explored, but it is indisputable that these microorganisms have an enormous potential to naturally enrich fermented dairy foods with vitamins or other important micronutrients.

Bioavailability of vitamin B₁₂ from food of animal origin

In recent years, scientists have endeavored to evaluate the bioavailability of B₁₂ from different food sources. Their aim was, and still is, to optimize the recommended daily allowance of vitamin B₁₂, particularly for children and the elderly who need vitamin B₁₂ to develop, maintain, and improve their cognitive performance.⁶⁸ Vitamin B₁₂ research also focuses on vegans and vegetarians, since their abstinence from foods of animal origin is associated with different states of vitamin B₁₂ deficiency.⁶ With regard to the bioavailability of vitamin B₁₂ from foods, study results vary greatly. The administration of 0.25 µg radioactive-labeled B₁₂ via water and milk resulted in absorption rates of 55% (water) and 65% (milk) in elderly people.⁶⁹ Similar absorption rates were documented for B₁₂ in chicken meat and mutton (Table 4).^26,38,70,71 The B₁₂ status in vegetarians was positively correlated with dairy product intake (especially milk) but not with egg or seafood ingestion.⁷² Meat, fish, and poultry also showed positive impacts on B₁₂ levels in human plasma, albeit less intensively than dairy.¹³ A similar observation was reported from a Norwegian study that showed the concentration of B₁₂ in human plasma correlates with the ingestion of B₁₂ via dairy products and fish but not with intake via eggs or meat.¹⁴ The latter finding is surprising since the bioavailability of B₁₂ in meat and liver has been shown to be high.^26,38 The following factors are assumed to play a role in this finding: 1) the B₁₂ intake calculated does not conform with actual intake since the cooking of meat influences the available B₁₂ concentration^30,58; 2) the decreasing rates of absorption with increasing B₁₂ concentrations (see description below) favors the repeated consumption of low doses of B₁₂ (as in regular consumption of milk products) instead of one large dose (as in one large portion of meat)⁷³; and 3) people with reduced gastric secretion possibly have problems digesting collagen-rich foods such as meat, which leads to a reduced release of B₁₂.¹⁴ Contrary to this last observation, a study of Kenyan schoolchildren demonstrated that consumption of a meal including either meat or milk 5 days per week for 9 months significantly improved plasma B₁₂ concentrations.⁷⁴ After 2 years of intervention, the prevalence of children with low plasma vitamin B₁₂ levels (<148 pmol/L) was reduced in both groups, changing from 55.6% to 4.5% in the meat group and from 41.0% to 8.9% in the milk group.

Table 4

Bioavailability of different amounts of B₁₂ from food sources

Food source	Amount of B12 (µg)	Bioavailability (%)	Reference
Cooked mutton	0.9a	56–77	Heyssel et al. (1966)26
	3.0a	76–89
	5.1a	40–63
Chicken meat	0.4–0.6a	65	Doscherholmen et al. (1978)38
	0.8–1.3a	63
	1.3–1.9a	61
Liver	38.0	4.5/9	Heyssel et al. (1966)26
Milk	0.25a	65	Russell et al. (2001)69
Egg	1.1–1.4/100g		Squires and Naber (1992)70
Scrambled egg yolks		8.2
Scrambled whole eggs		3.7
Boiled eggs		8.9
Fried eggs		9.2
Fish	2.1a	42	Doscherholmen et al. (1981)71
	4.1a	38
	9.2a	42
	13.3a	30

^aTotal amount of vitamin B₁₂ for intake.

For synthetic vitamin B₁₂, the scientific literature presently reports a bioavailability of <4% in humans and animals, which is quite low.⁷⁵ The bioavailability of natural B₁₂ isomers from raw and processed cows’ milk (pasteurized or microfiltered) compared with synthetic cyanocobalamin and with a B₁₂-free control diet was investigated in a Canadian crossover study with pigs. It was found that dietary concentrations of cobalamin correlated inversely with bioavailability.⁷⁶ This effect was previously described for humans as well.⁷ The more B₁₂ that is consumed, the less will be absorbed. This phenomenon is due to saturation of the vitamin B₁₂–IF receptors in the small intestine, which is estimated to be reached at concentrations of 1.5–2.0 µg per single meal under physiological conditions.^73,77 It is suggested that 50% of 1 µg of ingested vitamin B₁₂ is absorbed, dropping to 10% when 10 µg is ingested and to only 5% when the intake is 20 µg. However, very high doses of cobalamin enable 1% absorption of B₁₂ by passive diffusion (e.g., 5 µg of a 500-µg supplement⁶⁵). Doets et al.¹⁷ recently reviewed daily vitamin B₁₂ losses and bioavailability using a factorial approach. Data from eight studies that included 83 participants showed an increase in vitamin B₁₂ absorption with increasing doses of vitamin B₁₂ intake (food of animal origin). A meta-analysis published by the same group found an 11% increase of plasma vitamin B₁₂ concentration in humans after intake of the vitamin was doubled (mainly through B₁₂-enriched foods). This effect was stronger in elderly participants (13%) than in adults (8%).⁷⁸ In general, the Institute of Medicine takes into account an average dietary B₁₂ absorption rate of 50% by healthy individuals with normal intestinal functions.¹⁶ Further clarification of these data is needed.

Contributions of meat and milk to total vitamin B₁₂ intake

One portion (120 g) of cooked meat may meet 5–132% of recommended daily B₁₂ intake (3.0 µg) when calculations are based on the values given in Table 2. Dietary guidelines recommend daily consumption of three servings of dairy products.^79,80 Based on Table 3, three glasses of milk (250 mL/glass) would supply 1.5–5.25 µg vitamin B₁₂. One portion of milk, one of cheese (hard variety, 60 g), and one of yogurt (150 g) would provide 1.6–4.3 µg/d vitamin B₁₂.

Table 5 summarizes the extent of the contributions made by meat and meat products as well as milk and dairy products to vitamin B₁₂ intake in different countries.^81–85 The contributions vary according to national consumption habits and fortification practices. For example, in the United States, beef and lamb meat as well as liver and burgers/patties are among the top 10 food sources of vitamin B₁₂. However, cereals are the single top contributor to vitamin B₁₂ intake because they are fortified with the vitamin.⁸⁴ In countries such as Switzerland, in which fortification of cereals is not as extensive as in the United States, the portion of vitamin B₁₂ obtained from meat and dairy products is naturally greater.⁸¹ Available data suggest that meat and dairy products are important contributors to vitamin B₁₂ intake in many Western countries. Thus, it is not surprising that a study of female university students found a decrease in the average B₁₂ intake that was below recommendations (from 3.9 to 1.2 µg) when more than one foodstuff from the meat and poultry group was eliminated.⁸⁶

Table 5

Contribution of meat and meat products and of milk and dairy products to total vitamin B₁₂ intake according to country

Country	Contribution of meat and meat products to total B12 intake (%)	Contribution of milk and dairy products to total B12 intake (%)	Reference
Switzerland	58	27	Schmid et al. (2012)81
France	42	9	Volatier and Dufour (2007)82
The Netherlands (young children)	NA	58	Vissers et al. (2011)83
The Netherlands (adults and elderly)	NA	44–46	Vissers et al. (2011)83
United States	20–40 (beef and lamb meat, liver, burgers)	NA	Sharma et al. (2013)84
United States (19–50 years)	25 (lean beef)	NA	Zanovec et al. (2010)85
United States (>50 years)	20 (lean beef)	NA	Zanovec et al. (2010)85

Abbreviations: NA, not available.

CONCLUSION

Through consumption of meat, milk, and dairy products, it is possible to meet a substantial portion of human requirements for vitamin B₁₂. In particular, meat from ruminants (containing between 0.36 and 4.43 µg of B₁₂) and liver are valuable sources of the vitamin. The vitamin B₁₂ concentrations in milk and dairy products are lower than in meat; nevertheless, they contribute substantially to meeting the recommended intake of cobalamin in many Western countries. The processing of both meat and milk, especially thermal treatment of these foods, leads to severe losses of vitamin B₁₂. However, compared with raw meat, cooked meat offers similar or even higher B₁₂ concentrations per 100 g edible portion due to moisture and lipid losses that occur during cooking. In terms of ruminant food processing, dairy fermentation plays a unique role since the fermenting bacteria are not only B₁₂ consumers but, in some cases, B₁₂ producers. This finding offers a broad field of new possibilities for naturally enriching ruminant foods with cobalamin, thereby ensuring an appropriate B₁₂ supply among the world’s population.

Acknowledgments

Funding. No external funding was received for this work.

Declaration of interest. The authors have no relevant interests to declare.