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Page 6 of 22 D’Aimmo et al. Microbiome Res Rep 2024;3:11 https://dx.doi.org/10.20517/mrr.2023.59

Figure 3. Folate-mediated one-carbon metabolism. There are two interrelated cycles in folate metabolism that compete for folate
cofactors, namely DNA biosynthesis (represented by blue) and methylation (represented by green). In addition, the trans-sulfuration
pathway (depicted in pink) breaks down homocysteine. 5,10-methenylTHF: 5,10-methenyltetrahydrofolate; 5,10-methyleneTHF:
5,10-methylenetetrahydrofolate; 5-methylTHF: 5-methyltetrahydrofolate; 10-formylTHF: 10-formyltetrahydrofolate; AHCY:
S-adenosylhomocysteine hydrolase; B : riboflavin; B : vitamin B (pyridoxine); B : vitamin B ; BHMT: betaine-homocysteine
2
6
12
12
6
methyltransferase; CBS: cystathionine β-synthase; CTH: cystathionase, DHF: dihydrofolate; DHFR: dihydrofolate reductase; MAT:
methionine adenosyltransferase; MTHFD1: methylenetetrahydrofolate dehydrogenase 1/methenyltetrahydrofolate
cyclohydrolase/formyltetrahydrofolate synthetase; MTHFR: methylenetetrahydrofolate reductase; MTR: methionine synthase; R: methyl
acceptors, such as DNA or histones; SAH: S-adenosylhomocysteine; SAM: S-adenosylmethionine; SHMT1: serine
hydroxymethyltransferase 1; THF: tetrahydrofolate; TYMS: thymidylate synthetase.
This higher resistance could be attributed to a steric hindrance action against the oxidative compounds. It is
worth noting that the stability of folates is affected by various factors such as pH, temperature, and the
presence of metal ions such as copper and iron. Different forms of folate have varying ranges of pH stability.
For instance, 5,10-CH=THF is stable at pH values below 2, whereas folic acid and 5-HCO-THF are more
stable at pH values above 5 [38,39] ; 10-HCO-THF is highly unstable at all pH values. THF is also unstable, but
its stability improves at pH values above 8; 5,10-CH2-THF is stable at pH above 9.5 , whereas
[39]
5-methyl-THF is relatively stable in the pH range 2 to 10 . All folate forms are sensitive to
[40]
photodegradation and, therefore, require protection from UV light. The length of the glutamate tail does
not interfere with the stability of the molecule [35,41] .

Food handling and preservation processes frequently involve steps with high oxygen levels and changing
pH conditions. Folates in food, therefore, easily break down even before consumption. This decline can be
attributed to factors such as exposure to oxygen, high temperatures, and light or leakage into the water used
for cooking . In addition, different cooking methods can result in varying levels of nutrient loss. Dang
[42]
et al. examined the loss of folates in chickpeas and peas due to soaking, boiling, and pressure cooking and
[42]
found that pressure cooking was the most effective method for preserving folates in peas .

Soaking legumes during cooking can also reduce folate levels. Leakage-related losses were more significant
in field peas than in chickpeas. McKillop et al. discovered that boiling spinach in water could lead to a 51%
decrease in folate levels, whereas broccoli may lose up to 56% of its folate content . In contrast, steaming
[43]