The Sourdough School · Supporting resource for Proven

Dr Vanessa Kimbell on Understanding folate in food, fermentation, and the human body

In 2026, mandatory folic acid fortification of flour comes into force in the UK. For many people this was reassuring news — a public health measure designed to reduce neural tube defects in pregnancy. But for a significant proportion of the population, the picture could be considerably more complicated. Why? because not all folate is equal. Not all bodies process it in the same way. And for people with certain common genetic variants, synthetic folic acid — the form added to flour — can accumulate in the body rather than being converted into the active form that the brain and cells actually need.

I’ve been baking sourdough for over 40 years and actively reduced the folic acid in my bread for a while now. This article explains some of the reasons why. In part it has to do with how I source my flour, and how we try and keep the wholegrain in the bread we bake, but this article sets out the science of folate forms, introduces the two enzymes at the heart of the conversion process, and explains — in three parts — why bread made to BALM principles offers something that fortified flour cannot. This is the science that sits behind Proven bread, and it is science that has been building in the peer-reviewed literature for two decades.

Folate is not one thing

When we talk about folate, we are talking about a family of related compounds. Understanding the differences between them matters, because where they come from, how the body processes them, and whether they reach your cells as usable nutrients varies enormously.

Naturally occurring food folates

These are the reduced folates found in whole foods: leafy green vegetables, pulses, eggs, liver, and — crucially — traditionally fermented foods. In their natural state in food, folates exist mainly as polyglutamates — molecules with several glutamate units attached. During digestion, the gut gently converts these into absorbable monoglutamate forms. The process is graduated and overseen by the body’s own mechanisms.

Tetrahydrofolate (THF) — the core folate molecule

You do not eat THF directly. It is produced inside the body after absorption of food folate, and it is the foundation from which all other active folate forms are made. Think of it as the raw material in the folate workshop — present everywhere, essential to everything, but not the finished product.

5-Methyltetrahydrofolate (5-MTHF) — the active form

This is the main circulating form of folate in the blood, and the form the body uses for methylation — the process by which genes are switched on and off, neurotransmitters are synthesised, and homocysteine is kept in check. 5-MTHF is also the only form of folate that crosses the blood–brain barrier. It is found naturally in foods after the metabolism of food folate, and it can be taken directly as supplements labelled as methylfolate or L-methylfolate. This distinction matters, as we will come to.

5-Formyltetrahydrofolate (folinic acid)

A naturally occurring reduced folate that acts as an intermediate in folate metabolism. It is not itself a methyl donor but can be converted into other active folates. Found in small amounts in leafy greens and liver, and also used in clinical settings. It is entirely distinct from synthetic folic acid — a point of persistent confusion.

10-Formyl-tetrahydrofolate

This form is involved in DNA and RNA synthesis, particularly in rapidly dividing cells. It is generated within the body from food folate and does not appear on food labels — but it is quietly essential, especially during pregnancy and periods of rapid growth.

Folic acid — the synthetic form

Folic acid is a fully oxidised, synthetic form of folate. It does not occur naturally in any food. It was developed in the 1940s and has been used in fortification and supplementation ever since — first voluntarily, and now, as of 2025, mandatory in non-wholemeal wheat flour in the UK. It must be converted in the liver before it can enter the folate cycle at all, a process that requires a specific enzyme — dihydrofolate reductase, or DHFR — and that enzyme has limited capacity. When large amounts of folic acid are consumed regularly, unmetabolised folic acid (UMFA) is thought my functional nutritionalists to circulate in the blood. The health implications of elevated UMFA are still being actively researched, but there is growing concern that it may interfere with immune function and compete with natural food folates at cellular receptors.

Microbially synthesised folate

Certain microorganisms — including wild yeasts and specific strains of lactic acid bacteria — can synthesise folate during fermentation. This folate is in naturally reduced forms. It does not require DHFR to begin its conversion. It is not the same as synthetic folic acid, and it behaves very differently in the body.

The MTHFR and DHFR problem

Two enzymes sit at the heart of the folate conversion story, and variants in the genes that encode them are far more common than most people realise.

DHFR — the first bottleneck

Dihydrofolate reductase (DHFR) is the enzyme responsible for the first step in converting synthetic folic acid into a usable form. Variants in the DHFR gene reduce this enzyme’s efficiency. When DHFR is sluggish — through genetic variation, age, or medication such as methotrexate, which works by blocking DHFR — functional medics are concerned that folic acid backs up. Variants in the DHFR gene reduce this enzyme’s efficiency and this may put the liver under strain. UMFA accumulates. For these individuals, the mandatory addition of folic acid to flour is not a straightforward benefit.

MTHFR — the second bottleneck

Methylenetetrahydrofolate reductase (MTHFR) converts 5,10-methyleneTHF into 5-MTHF — the active, circulating form. Variants in the MTHFR gene (most commonly C677T and A1298C) reduce this enzyme’s efficiency by anywhere from 30 to 70 percent. Estimates suggest that around 10–15% of the population carry two copies of the C677T variant, meaning both copies of their MTHFR gene are affected. A much larger proportion carry one copy.

For these individuals, synthetic folic acid is doubly problematic: DHFR struggles to convert it into the folate cycle at all, and MTHFR then struggles to complete the conversion to 5-MTHF even once it enters. The result is a person who may have adequate folic acid intake on paper but functionally low levels of active folate in their blood and brain.

The fortification of flour with folic acid is a genuine public health intervention — designed for a population average. But it was not designed for individuals with MTHFR or DHFR variants, for whom the synthetic form may offer limited benefit and potential complications.

Why Proven bread is different: three sources of folate

A loaf made to BALM principles using Botanical Blend flour does not simply contain folate. It contains folate from three distinct and cumulative sources, each with its own biochemical significance. Together they represent a fundamentally different nutritional profile to fortified white bread.

Folate synthesised by the plant itself

Botanical Blend flour was created specifically with folate in mind. Wheat, lentils, and peas all synthesise folate as part of their own biology. Folate is essential for DNA replication and cell division — no living organism can grow without it. The grain does not contain folate because we put it there. It contains folate because it needed folate to become what it is.

Plants synthesise folate through conserved biosynthetic pathways: precursor molecules are made in the cytoplasm and chloroplast, then transported to the mitochondria where folate is assembled [Blancquaert et al., 2014; DellaPenna, 2007]. Legumes are particularly rich in this intrinsic folate. Peer-reviewed analysis of lentil cultivars published in the Journal of Agricultural and Food Chemistry found folate concentrations ranging from 216 to 290 ?g per 100g, with a mean of 255 ?g — enough to provide 54–73% of the adult recommended daily intake in a single 100g serving, significantly outperforming chickpeas and field peas [Sen Gupta et al., 2013].

The critical caveat — and this is where milling matters enormously — is that in wheat, this folate is concentrated in the bran, aleurone layer, and germ, not the starchy endosperm. A peer-reviewed review in npj Science of Food documents that refined wheat flour contains between 21 and 90% less folate than whole grain flour, depending on milling intensity [Gawlik-Dziki et al., 2025]. Standard industrial milling strips out the very fractions where the plant stored its own folate, then adds synthetic folic acid back to the naked endosperm. Heritage whole grain flour, stone-milled to retain all three fractions, begins from a completely different baseline — before fermentation has even begun.

Bioavailability unlocked by fermentation

Folate in whole grains and legumes is partially bound up with phytic acid — an antinutrient that prevents vitamins and minerals from being absorbed. The folate may be present, but without the right conditions, much of it passes through the gut unused.

Long sourdough fermentation changes this decisively. The acidic environment created by lactic acid bacteria activates phytase enzymes that break down phytic acid, releasing the folate and other bound minerals for absorption. A foundational study by Lopez et al. showed that prolonged fermentation of whole wheat sourdough significantly reduced phytate levels and increased soluble mineral availability [Lopez et al., 2001, J Agric Food Chem]. Poutanen et al. confirmed that lactic acid bacteria fermentation can achieve up to 90% phytate breakdown in bran [Poutanen et al., 2009, Food Microbiology].

The same principle applies to the legume flours in the botanical blend. Research on fermentation of lentil and pea flours with Lactobacillus plantarum and Pediococcus acidilactici confirmed significant reductions in phytic acid and enzyme inhibitors, improving overall nutrient bioavailability [Adebo et al., 2021, J Sci Food Agric]. Fermentation does not add folate to the legume — it makes the folate that was always there actually available to the person eating it.

bread bakes by nutritional expert Dr Vanessa Kimbell

New folate synthesised during fermentation

Beyond releasing what was already present, fermentation actively creates new folate. The wild yeasts in a mature sourdough starter synthesise folate de novo during the bulk ferment — and the forms they produce are naturally reduced vitamers, including 5-MTHF itself, the active circulating form that does not require DHFR or MTHFR conversion.

This is robustly evidenced. Kariluoto et al. showed that total folate content increased considerably during sourdough fermentation, with the increase attributable primarily to yeast activity, and that the new folate appeared as naturally reduced forms including 5-CH?-H?folate (5-MTHF) [Kariluoto et al., 2004, Cereal Chemistry]. A second study by the same group confirmed that wild sourdough yeasts were the primary folate synthesisers, with fermentation of non-sterilised flour producing up to threefold increases in folate content [Kariluoto et al., 2006, Int J Food Microbiology]. Lactic acid bacteria contribute further folate synthesis in some fermented contexts — particular strains of Lactobacillus plantarum produce extracellular folate in the form of THF and MTHF [Sybesma et al., PMC 2021], though in grain fermentation the dominant synthesis signal comes from yeast.

This third fold is the one that most directly addresses the MTHFR and DHFR problem. Naturally reduced folate produced by fermentation bypasses both conversion bottlenecks entirely. It enters the folate cycle at a later stage, available to the body regardless of an individual’s enzyme variants.

Why this matters

For the significant minority with MTHFR or DHFR variants — and for anyone who wants their bread to actively support rather than merely not harm their health — the source and form of folate in their food matters. Naturally reduced folates from fermented whole grain, synthesised by wild yeasts during a long, slow ferment, enter the body’s folate cycle on fundamentally different terms to synthetic folic acid from fortified white flour.

The mandatory fortification of flour with folic acid is well-intentioned, and for a significant proportion of the population it will prevent harm. But it rests on an assumption that all folate is equivalent and that all bodies process it identically. Neither is true.

Understanding this is not alarmist. It is simply precise — and it is what BALM-based baking has been building towards for more than a decade. The three folds described here are not marketing language. They are the mechanistic basis on which Proven bread is formulated.

If you want to go further into the genetics, our nutrigenetics glossary entry is a good place to start.

Full list of references

All titles link directly to the study or abstract.

Sen Gupta D, Thavarajah D, Knutson P et al. (2013). “Lentils (Lens culinaris L.), a Rich Source of Folates.” Journal of Agricultural and Food Chemistry, 61(32): 7794–7799.
Kariluoto S, Vahteristo L, Salovaara H et al. (2004). “Effect of Baking Method and Fermentation on Folate Content of Rye and Wheat Breads.” Cereal Chemistry, 81(1): 134–139.
Kariluoto S, Vahteristo L, Piironen V et al. (2006). “Effects of Yeasts and Bacteria on the Levels of Folates in Rye Sourdoughs.” International Journal of Food Microbiology, 106(2): 137–143.
Poutanen K, Flander L, Katina K. (2009). “Sourdough and Cereal Fermentation in a Nutritional Perspective.” Food Microbiology, 26(7): 693–699.
Lopez HW, Ouvry A, Bervas E et al. (2001). “Prolonged Fermentation of Whole Wheat Sourdough Reduces Phytate Level and Increases Soluble Magnesium.” Journal of Agricultural and Food Chemistry, 49(5): 2657–2662.
Adebo OA, Njobeh PB, Adebiyi JA et al. (2021). “Fermentation Performance and Nutritional Assessment of Physically Processed Lentil and Green Pea Flour.” Journal of the Science of Food and Agriculture, 71(13).
Gawlik-Dziki U et al. (2025). “Biological, Dietetic and Pharmacological Properties of Vitamin B9.” npj Science of Food.
Blancquaert D, Van Daele J, Van Der Straeten D. (2014). “Exploration of Folate and Its Derivatives in Grains of Wheat with Different Colors.” PMC / Frontiers in Plant Science.
Sybesma W et al. (2021). “Folate in Milk Fermented by Lactic Acid Bacteria from Different Food Sources.” Frontiers in Nutrition / PMC.
Quinlivan EP, Gregory JF. (2009). “The extremely slow and variable activity of dihydrofolate reductase in human liver and its implications for high folic acid intake.” Proceedings of the National Academy of Sciences (PNAS), 106(36): 15424–15429. https://www.pnas.org/doi/10.1073/pnas.0902072106

Folate, Folic Acid & Why They Are Not the Same Thing