in the methionine cycle, and result in raised homocysteine levels. there is much evidence linking elevated homocysteine concentrations with an increased risk of cardiovascular disease, and homocysteine lowering treatments have led to improvements in cardiovascular reactivity and coagulation factors. in adults with metabolic syndrome, individuals with low levels of vitamin b12 had higher levels of homocysteine compared to healthy subjects. it is thus possible that vitamin b12 deficiency enhances the risk of developing cardiovascular disease in individuals who are obese. alternatively, low levels of vitamin b12 may increase the levels of proinflammatory proteins which may induce ischaemic stroke. it is important to screen vitamin b12 deficiency in obese individuals, due to its importance in energy metabolism, and relationship with homocysteine and its potential to modulate weight gain. more studies are needed to test for the causality of vitamin b12 and obesity using genetic markers. a few studies have also reported no deficiency of vitamin b12 in obese individuals. finally, a recent literature review conducted over 19 studies, found no evidence of an inverse association between bmi and circulating vitamin b12. previous clinical and population-based studies have indicated that vitamin b12 deficiency is prevalent amongst adults with type 2 diabetes. kaya et al., conducted a study in women with polycystic ovary syndrome, and found that obese women with insulin resistance had lower vitamin b12 concentrations compared to those without insulin resistance. similarly, in a study conducted in european adolescents, there was an association between high adiposity and higher insulin sensitivity with vitamin b12 concentrations. individuals with a higher fat mass index and higher insulin sensitivity (high homeostatic model assessment index) had lower plasma vitamin b12 concentrations. furthermore, a recent study conducted in india reported that mean levels of vitamin b12 decreased with increasing levels of glucose tolerance e.g. individuals with type 2 diabetes had the lowest values of vitamin b12, followed by individuals with pre-diabetes and normal glucose tolerance, respectively. however, b12 levels of middle aged-women with and without metabolic syndrome showed no difference in vitamin b12 levels between those with insulin resistance (ir) and those without. it is believed that malabsorption of vitamin b12 in diabetic patients, is due to individuals taking metformin therapy (an insulin sensitizer used for treating type 2 diabetes). furthermore, obese individuals with type 2 diabetes are likely to develop gastroesophageal reflux disease, and take proton pump inhibitors, which further increased the risk of vitamin b12 deficiency. a recent literature review conducted over seven studies, found that there was limited evidence to show that low vitamin b12 status increased the risk of cardiovascular disease and diabetes. however, the review did not identify any associations between vitamin b12 and cardiovascular disease in the remaining four studies. currently, no data supports vitamin b12 supplementation on reducing the risk of cardiovascular disease. in a dose-response meta-analysis of five prospective cohort studies, it was reported that the risk of coronary heart disease (chd) did not change substantially with increasing dietary vitamin b12 intake. of these five studies, three of the studies stated a non-significant positive association and two of the studies demonstrated an inverse association between vitamin b12 supplementation and coronary heart disease (only one of the studies was significant). anemia vitamin b12 deficiency is one of the main causes of anemia. in countries where b12 deficiency is common, it is generally assumed that there is a greater risk of developing anemia. however, the overall contribution of vitamin b12 deficiency to the global incidence of anemia may not be significant, except in elderly individuals, vegetarians, cases of malabsorption and some genetic disorders. anemia is defined as a condition in which there are not enough red blood cells, as the tissues and organs of the body do not get enough oxygen. megaloblastic anemia caused by vitamin b12 deficiency is characterized by red blood cells that are larger than normal and are unable to deliver oxygen to the body's organs. the clinical case indicates an altered synthesis of dna, in which vitamin b12 is essential for the production and maturation of red blood cells in the bone marrow. adult patients often report to medical attention symptoms related to anemia, such as feeling tired and weak, breathlessness, exercise intolerance, feeling faint, headaches, paleness, dry lips and a disturbance of taste. pernicious anemia is the most common cause of vitamin b12 deficiency anemia in adults, which results from malabsorption of vitamin b12 due to a lack or loss of intrinsic factor. there are relatively few studies which have assessed the impact of haematological measures in response to b12 supplementation. one study in 184 premature infants, reported that individuals given monthly vitamin b12 injections (100 µg) or taking supplements of vitamin b12 and folic acid (100 µg/day), had higher haemoglobin concentrations after 10–12 weeks, compared to those only taking folic acid or those taking no vitamin b12 injections. in deficient mexican adult women and pre-schoolers, it was found that vitamin b12 supplementation did not affect any haematologic parameters. aging in the elderly, vitamin b12 deficiency has been associated with the development of macular degeneration, and the risk of frailty. macular degeneration is the leading cause of severe, irreversible vision loss in older adults. several risk factors have been linked to macular degeneration, including family history, genetics, hypercholesterolemia, hypertension, sunlight exposure and lifestyle (smoking and diet). it has been shown that daily supplementation of vitamin b12, b6 and folate over a period of seven years can reduce the risk of age-related macular degeneration by 34% in women with increased risk of vascular disease (n=5,204). however, another study failed to find an association between age-related macular degeneration and vitamin b12 status in a sample of 3,828 individuals representative of the non-institutionalized us population. frailty is a geriatric condition which is characterized by diminished endurance, strength, and reduced physiological function that increases an individual's risk of mortality and impairs an individual from fulfilling an independent lifestyle. frailty is associated with an increased vulnerability to fractures, falls from heights, reduced cognitive function and more frequent hospitalisation. the worldwide prevalence of frailty within the geriatric population is 13.9%, therefore there is an urgent need to eliminate any risk factors associated with frailty. poor vitamin b status has been shown to be associated with an increased risk of frailty. two cross sectional studies have reported that deficiencies of vitamin b12 were associated with the length of hospital stay, as observed by serum vitamin b12 concentrations and methylmalonic acid (mma) concentrations . furthermore, another study looking at elderly women (n=326), found that certain genetic variants associated with vitamin b12 status (transcobalamin 2) may contribute to reduced energy metabolism, consequently contributing to frailty. in contrast, a recent study by dokuzlar et al., found that there was no association between vitamin b12 levels and frailty in the geriatric population (n=335). given that there are limited studies, which have assessed the relationship between vitamin b12 and frailty status, more longitudinal studies are needed to clarify the relationship. neurological decline severe vitamin b12 deficiency is associated with subacute combined degeneration of the spinal cord, which involves demyelination of the posterior and lateral columns of the spinal cord. symptoms include memory and cognitive impairment, sensory loss, motor disturbances, loss of posterior column functions and disturbances in proprioception. in advanced stages of vitamin b12 deficiency, cases of psychosis, paranoia and severe depression have been observed, which may lead to permanent disability if left untreated. studies have shown the rapid reversal of the neurological symptoms of vitamin b12 deficiency, after treatment with high-dose of vitamin b12 supplementation; suggesting the importance of prompt treatment in reversing neurological manifestations. cognitive decline elderly individuals are currently assessed on vitamin b12 status during the screening process for dementia. studies investigating the association between vitamin b12 concentrations and cognitive status have produced inconclusive results. it has been shown that elevated mma concentrations are associated with decreased cognitive decline and alzheimer's disease. in addition, low vitamin b12 and folate intakes have shown associations with hyperhomocysteinemia, which is associated with cerebrovascular disease, cognitive decline and an increased risk of dementia in prospective studies. there are limited intervention studies which have investigated the effect of supplementation of vitamin b12 and cognitive function. a cochrane review, analysing two studies, found no effect of vitamin b12 supplementation on the cognitive scores of older adults. a recent longitudinal study in elderly individuals, found that individuals had a higher risk of brain volume loss over a 5-year period, if they had lower vitamin b12 and holotc levels and higher plasma thcy and mma levels. more intervention studies are needed to determine the modifiable effects of vitamin b12 supplementation on cognition. osteoporosis there has been growing interest on the effect of low serum vitamin b12 concentrations on bone health. studies have found a connection between elevated plasma thcy and an increased risk of bone fractures, but is unknown whether this is related to the increased levels of thcy or to vitamin b12 levels (which are involved in homocysteine metabolism). results from the third nhanes conducted in the united states, found that individuals had significantly lower bone mass density (bmd) and higher osteoporosis rates with each higher quartile of serum mma (n= 737 men and 813 women). given that poor bone mineralization has been found in individuals with pernicious anemia, and that the content of vitamin b12 within bone cells in culture has shown to affect the functioning of bone forming cells (osteoblasts); it is possible that vitamin b12 deficiency is causally related to poor bone health. randomized intervention trials investigating the association of vitamin b12 supplementation and bone health have yielded mixed results. two studies conducted in osteoporotic risk patients with hyperhomocysteinemia and individuals who had undergone a stroke, found positive effects between supplementation of b vitamins on bmd. however, no improvement in bmd was observed in a group of healthy older people. further, controlled trials are needed to confirm the impact and mechanisms vitamin b12 deficiency has on bone mineralization. causes vitamin b12 deficiency can be caused by impaired absorption, inadequate dietary intake, or increased requirements. impaired absorption explains most cases of vitamin b12 deficiency, but it can also result from other factors. impaired absorption impaired absorption of vitamin b12 leads to pernicious anemia a type of megaloblastic anemia. pernicious anemia comes about from the lack of gastric intrinsic factor produced by parietal cells in the stomach, and needed in the ileum for the absorption of vitamin b12. pernicious anemia is the most common cause of vitamin b12 deficiency. any disruption that leads to the loss of parietal cells can lead to malabsorption, and includes atrophic gastritis, a condition that often affects the elderly, and gastric surgeries that involve the removal of all or part of the stomach, such as roux-en-y gastric bypass surgery. surgical removal of the small bowel such as in crohn's disease results in short bowel syndrome and the inability to absorb vitamin b12. impaired absorption can also result from blind loop syndrome where an overpopulation of bacteria in the small intestine absorb the vitamin. some infections such as giardiasis, and diphyllobothriasis caused by parasites can also cause malabsorption. forms of achlorhydria (including that artificially induced by drugs such as proton pump inhibitors and histamine 2 receptor antagonists) can cause b12 malabsorption from foods, since acid is needed to split b12 from food proteins and salivary binding proteins. this process is thought to be the most common cause of low b12 in the elderly, who often have some degree of achlorhydria without being formally low in intrinsic factor. this process does not affect absorption of small amounts of b12 in supplements such as multivitamins, since it is not bound to proteins, as is the b12 in foods. long-term use of ranitidine hydrochloride may contribute to deficiency of vitamin b12. untreated celiac disease may also cause impaired absorption of this vitamin, probably due to damage to the small bowel mucosa. in some people, vitamin b12 deficiency may persist despite treatment with a gluten-free diet and require supplementation. the diabetes medication metformin may interfere with b12 dietary absorption. a genetic disorder, transcobalamin ii deficiency can be a cause. nitrous oxide exposure, and recreational use. chronic exposure to toxigenic molds and mycotoxins found in water damaged buildings. b12 deficiency caused by helicobacter pylori was positively correlated with caga positivity and gastric inflammatory activity, rather than gastric atrophy. inadequate intake