Choline intake, plasma riboflavin, and the phosphatidylethanolamine N-methyltransferase G5465A genotype predict plasma homocysteine in folate-deplete Mexican-American men with the methylenetetrahydrofolate reductase 677TT genotype

Insulin resistance and glycine metabolism in humans

Plasma glycine level is low in patients with obesity or diabetes and the improvement of insulin resistance increases plasma glycine concentration. In prospective studies, hypoglycinemia at baseline predicts the risk of developing type 2 diabetes and higher serum glycine level is associated with decreased risk of incident type 2 diabetes. Consistently, plasma glycine concentration is lower in the lean offspring of parents with type 2 diabetes compared to healthy subjects. Among patients with type 2 diabetes, hypoglycinemia occurs before clinical manifestations of the disease, but the pathophysiological mechanisms underlying glycine deficit and its potential clinical repercussions are unclear. Glycine participates in several metabolic pathways, being required for relevant human physiological processes. Humans synthesize glycine from glyoxylate, glucose (via serine), betaine and likely from threonine and during the endogenous synthesis of L-carnitine. Glycine conjugates bile acids and other acyl moieties producing acyl-glycine derivatives. The glycine cleavage system catalyzes glycine degradation to carbon dioxide and ammonium while tetrahydrofolate is converted into 5,10-methylene-tetrahydrofolate. Glycine is utilized to synthesize serine, sarcosine, purines, creatine, heme group, glutathione, and collagen. Glycine is a major quantitative component of collagen. In addition, the role of glycine maintaining collagen structure is critical, as glycine residues are required to stabilize the triple helix of the collagen molecule. This quality of glycine likely contributes to explain the occurrence of medial arterial calcification and the elevated cardiovascular risk associated with diabetes and chronic kidney disease, as emerging evidence links normal collagen content with the initiation and progression of vascular calcification in humans.

Common Genetic Variants Alter Metabolism and Influence Dietary Choline Requirements

Nutrient needs, including those of the essential nutrient choline, are a population wide distribution. Adequate Intake (AI) recommendations for dietary choline (put forth by the National Academies of Medicine to aid individuals and groups in dietary assessment and planning) are grouped to account for the recognized unique needs associated with age, biological sex, and reproductive status (i.e., pregnancy or lactation). Established and emerging evidence supports the notion that common genetic variants are additional factors that substantially influence nutrient requirements. This review summarizes the genetic factors that influence choline requirements and metabolism in conditions of nutrient deprivation, as well as conditions of nutrient adequacy, across biological sexes and reproductive states. Overall, consistent and strong associative evidence demonstrates that common genetic variants in choline and folate pathway enzymes impact the metabolic handling of choline and the risk of nutrient inadequacy across varied dietary contexts. The studies characterized in this review also highlight the substantial promise of incorporating common genetic variants into choline intake recommendations to more precisely target the unique nutrient needs of these subgroups within the broader population. Additional studies are warranted to facilitate the translation of this evidence to nutrigenetics-based dietary approaches.

Potential Links between Impaired One-Carbon Metabolism Due to Polymorphisms, Inadequate B-Vitamin Status, and the Development of Alzheimer's Disease

Alzheimer's disease (AD) is the major cause of dementia and no preventive or effective treatment has been established to date. The etiology of AD is poorly understood, but genetic and environmental factors seem to play a role in its onset and progression. In particular, factors affecting the one-carbon metabolism (OCM) are thought to be important and elevated homocysteine (Hcy) levels, indicating impaired OCM, have been associated with AD. We aimed at evaluating the role of polymorphisms of key OCM enzymes in the etiology of AD, particularly when intakes of relevant B-vitamins are inadequate. Our review indicates that a range of compensatory mechanisms exist to maintain a metabolic balance. However, these become overwhelmed if the activity of more than one enzyme is reduced due to genetic factors or insufficient folate, riboflavin, vitamin B6 and/or vitamin B12 levels. Consequences include increased Hcy levels and reduced capacity to synthetize, methylate and repair DNA, and/or modulated neurotransmission. This seems to favor the development of hallmarks of AD particularly when combined with increased oxidative stress e.g., in apolipoprotein E (ApoE) ε4 carriers. However, as these effects can be compensated at least partially by adequate intakes of B-vitamins, achieving optimal B-vitamin status for the general population should be a public health priority.

Plasma trimethylamine-N-oxide following supplementation with vitamin D or D plus B vitamins

SCOPE

We compared the effect of supplementation with vitamin D + B or vitamin D on plasma trimethylamine N-oxide (TMAO) and choline metabolites.

METHODS AND RESULTS

This is a randomized single-blinded nonplacebo-controlled study. Twenty-seven participants received 1200 IU vitamin D3 and 800 mg calcium, and 25 participants received additionally 0.5 mg folic acid, 50 mg B6, and 0.5 mg B12 for 1 year. Plasma homocysteine (Hcy), TMAO, and choline metabolites were measured at baseline and 12 months later. TMAO declined in the vitamin D arm by 0.5 versus 2.8 μmol/L in the D + B arm (p = 0.005). Hcy decreased and betaine increased in the D + B compared to the D arm. Within-subject levels of plasma choline and dimethylglycine and urine betaine increased in both arms and changes did not differ between the arms. TMAO reduction was predicted by higher baseline TMAO and lowering Hcy in stepwise regression analysis. The test-retest variations of TMAO were greater in the D + B arm compared to vitamin D arm.

CONCLUSION

B vitamins plus vitamin D lowered plasma fasting TMAO compared to vitamin D. Vitamin D caused alterations in choline metabolism, which may reflect the metabolic flexibility of C1-metabolism. The molecular mechanisms and health implications of these changes are currently unknown.

Homocysteine Induced Cerebrovascular Dysfunction: A Link to Alzheimer's Disease Etiology

A high serum level of homocysteine, known as hyperhomocystenemia (HHcy) is associated with vascular dysfunction such as altered angiogenesis and increased membrane permeability. Epidemiological studies have found associations between HHcy and Alzheimer’s disease (AD) progression that eventually leads to vascular dementia (VaD). VaD is the second most common cause of dementia in people older than 65, the first being AD. VaD affects the quality of life for those suffering by drastically decreasing their cognitive function. VaD, a cerebrovascular disease, generally occurs due to cerebral ischemic events from either decreased perfusion or hemorrhagic lesions. HHcy is associated with the hallmarks of dementia such as tau phosphorylation, Aβ aggregation, neurofibrillary tangle (NFT) formation, neuroinflammation, and neurodegeneration. Previous reports also suggest HHcy may promote AD like pathology by more than one mechanism, including cerebral microangiopathy, endothelial dysfunction, oxidative stress, neurotoxicity and apoptosis. Despite the corelations presented above, the question still exists – does homocysteine have a causal connection to AD? In this review, we highlight the role of HHcy in relation to AD by discussing its neurovascular effects and amelioration with dietary supplements. Moreover, we consider the studies using animal models to unravel the connection of Hcy to AD.

Riboflavin status modifies the effects of methylenetetrahydrofolate reductase (MTHFR) and methionine synthase reductase (MTRR) polymorphisms on homocysteine

Methylenetetrahydrofolate reductase (MTHFR) and methionine synthase reductase (MTRR), riboflavin-dependent enzymes, participate in homocysteine metabolism. Reported effects of riboflavin status on the association between the MTHFR 677C>T polymorphism and homocysteine vary, and the effects of the MTRR 66A>G or MTRR 524C>T polymorphisms on homocysteine are unclear. We tested the hypothesis that the effects of the MTHFR 677C>T, MTRR 66A>G and MTRR 524C>T polymorphisms on fasting plasma total homocysteine (tHcy) depend on riboflavin status (erythrocyte glutathionine reductase activation coefficient, optimum: <1.2; marginally deficient: 1.2-1.4; deficient: ≥1.4) in 771 adults aged 18-75 years. MTHFR 677T allele carriers with middle or low tertile plasma folate (<14.7 nmol/L) had 8.2 % higher tHcy compared to the 677CC genotype (p < 0.01). This effect was eliminated when riboflavin status was optimal (p for interaction: 0.048). In the lowest cobalamin quartile (≤273 pmol/L), riboflavin status modifies the relationship between the MTRR 66 A>G polymorphism and tHcy (p for interaction: 0.034). tHcy was 6.6 % higher in MTRR 66G allele carriers compared to the 66AA genotype with marginally deficient or optimal riboflavin status, but there was no difference when riboflavin status was deficient (p for interaction: 0.059). tHcy was 13.7 % higher in MTRR 524T allele carriers compared to the 524CC genotype when cobalamin status was low (p < 0.01), but no difference was observed when we stratified by riboflavin status. The effect of the MTHFR 677C>T polymorphism on tHcy depends on riboflavin status, that of the MTRR 66A>G polymorphism on cobalamin and riboflavin status and that of the MTRR 524C>T polymorphism on cobalamin status.

The metabolic burden of methyl donor deficiency with focus on the betaine homocysteine methyltransferase pathway

Methyl groups are important for numerous cellular functions such as DNA methylation, phosphatidylcholine synthesis, and protein synthesis. The methyl group can directly be delivered by dietary methyl donors, including methionine, folate, betaine, and choline. The liver and the muscles appear to be the major organs for methyl group metabolism. Choline can be synthesized from phosphatidylcholine via the cytidine-diphosphate (CDP) pathway. Low dietary choline loweres methionine formation and causes a marked increase in S-adenosylmethionine utilization in the liver. The link between choline, betaine, and energy metabolism in humans indicates novel functions for these nutrients. This function appears to goes beyond the role of the nutrients in gene methylation and epigenetic control. Studies that simulated methyl-deficient diets reported disturbances in energy metabolism and protein synthesis in the liver, fatty liver, or muscle disorders. Changes in plasma concentrations of total homocysteine (tHcy) reflect one aspect of the metabolic consequences of methyl group deficiency or nutrient supplementations. Folic acid supplementation spares betaine as a methyl donor. Betaine is a significant determinant of plasma tHcy, particularly in case of folate deficiency, methionine load, or alcohol consumption. Betaine supplementation has a lowering effect on post-methionine load tHcy. Hypomethylation and tHcy elevation can be attenuated when choline or betaine is available.

Choline nutrition programs brain development via DNA and histone methylation

Choline is an essential nutrient for humans. Metabolically choline is used for the synthesis of membrane phospholipids (e.g. phosphatidylcholine), as a precursor of the neurotransmitter acetylcholine, and, following oxidation to betaine, choline functions as a methyl group donor in a pathway that produces S-adenosylmethionine. As a methyl donor choline influences DNA and histone methylation--two central epigenomic processes that regulate gene expression. Because the fetus and neonate have high demands for choline, its dietary intake during pregnancy and lactation is particularly important for normal development of the offspring. Studies in rodents have shown that high choline intake during gestation improves cognitive function in adulthood and prevents memory decline associated with old age. These behavioral changes are accompanied by electrophysiological, neuroanatomical, and neurochemical changes and by altered patterns of expression of multiple cortical and hippocampal genes including those encoding key proteins that contribute to the biochemical mechanisms of learning and memory. These actions of choline are observed long after the exposure to the nutrient ended (months) and correlate with fetal hepatic and cerebral cortical choline-evoked changes in global- and gene-specific DNA cytosine methylation and with dramatic changes of the methylation pattern of lysine residues 4, 9 and 27 of histone H3. Moreover, gestational choline modulates the expression of DNA (Dnmt1, Dnmt3a) and histone (G9a/Ehmt2/Kmt1c, Suv39h1/Kmt1a) methyltransferases. In addition to the central role of DNA and histone methylation in brain development, these processes are highly dynamic in adult brain, modulate the expression of genes critical for synaptic plasticity, and are involved in mechanisms of learning and memory. A recent study documented that in a cohort of normal elderly people, verbal and visual memory function correlated positively with the amount of dietary choline consumption. It will be important to determine if these actions of choline on human cognition are mediated by epigenomic mechanisms or by its influence on acetylcholine or phospholipid synthesis.

Homocysteine in pregnancy

The aim of this review is to evaluate the evidence for and against fasting plasma total homocysteine (tHcy) as a biomarker/risk factor of impaired reproductive function before and during pregnancy. Apart from nutritional and lifestyle factors, tHcy is also influenced by physiological factors specific to pregnancy such as hemodilution, increased glomerular filtration rate, and endocrinological changes. These lead to a considerable reduction under normal circumstances in tHcy by midpregnancy. Stimulating excess endogenous homocysteine production before and during pregnancy in animal experiments and adding exogenous homocysteine to cell cultures result in the impairment of reproductive and developmental processes from preconception throughout pregnancy and during subsequent development of the offspring. Different studies have confirmed that elevated tHcy is a risk factor for subfertility, congenital developmental defects, preeclampsia, and intrauterine growth retardation. There is conflicting evidence that elevated tHcy is a risk factor for miscarriage, gestational diabetes, premature rupture of the membranes, placental abruption, and offspring with Down syndrome. Prospective, sufficiently powered, studies from preconception/early pregnancy are required to determine whether tHcy is a risk factor for these pregnancy complications.

Folate bioavailability: implications for establishing dietary recommendations and optimizing status

The addition of folic acid to the US food supply, along with the critical role of folate in certain health outcomes, has intensified worldwide interest in the bioavailability of folate. Bioavailability is a function of absorptive and postabsorptive processes, which in turn are influenced by diet, individuality, and complex diet-host interactions. As such, it is unlikely that a single bioavailability figure will accurately reflect food folate bioavailability from every diet for every person. Although there is broad agreement that naturally occurring food folate is not as bioavailable as folic acid, questions remain as to the extent of these differences, particularly within the context of a whole diet. This article 1) summarizes and integrates bioavailability estimates derived from studies that use whole-diet approaches; 2) highlights the influences of genetics, ethnicity-race, and sex as postabsorptive bioavailability modifiers; and 3) discusses the adequacy of the US folate Recommended Dietary Allowance in achieving folate sufficiency in select subpopulations.

Choline intake exceeding current dietary recommendations preserves markers of cellular methylation in a genetic subgroup of folate-compromised men

Severe choline deficiency adversely affects cellular methylation and DNA integrity, with potentially serious implications for disease risk. As part of a 12-wk controlled choline intervention study conducted in folate-compromised Mexican-American men (n = 60; 18-55 y) differing in the methylenetetrahydrofolate reductase (MTHFR) C677T genotype (21 677CC, 29 677TT), this study evaluated the effects of varied choline intakes (300, 550, 1100, and 2200 mg/d) on the change (i.e. wk 12-0) in markers of cellular methylation and DNA integrity. Choline intake affected the change in plasma S-adenosylmethionine (P = 0.044), with decreases tending to be greater (P < or = 0.08) in the 300 and 550 mg/d groups than in the 2200 mg/d group. Choline intake also interacted with the MTHFR C677T genotype to affect the change in genomic DNA methylation and DNA damage. In men with the MTHFR 677CC genotype, choline intake affected (P = 0.007) the change in DNA methylation, with a greater decrease (P < 0.02) in the 300 mg/d group than in the 1100 and 2200 mg/d groups. In men with the MTHFR 677CC genotype, choline intake also affected (P = 0.047) the change in DNA damage, with the increase tending to be greater (P = 0.07) in the 550 mg/d group than in the 2200 mg/d group. Choline intake did not affect these variables in men with the MTHFR 677TT genotype. Overall, these data suggest that choline intake exceeding current dietary recommendations preserves markers of cellular methylation and attenuates DNA damage in a genetic subgroup of folate-compromised men.

Homocysteine metabolism and its relation to health and disease

Homocysteine is a metabolic intermediate in methyl group metabolism that is dependent on a number of nutritional B-vitamin cofactors. An emerging aspect of homocysteine metabolism is its relation to health and disease. Perturbations of homocysteine metabolism, particularly intracellular and subsequently circulating accumulation of homocysteine (i.e., hyperhomocysteinemia), are associated with vascular disease risk, as well as other pathologies. However, intervention with B-vitamin supplementation has been shown to successfully restore normal homocysteine concentrations, but without concomitant reductions in disease risk. Thus, the mechanistic relation between homocysteine balance and disease states, as well as the value of homocysteine management, remains an area of intense investigation.