Why you should never take B9 without checking your vitamin B12

Vitamin B9, also known as folic acid (synthetic form) or folates (natural form), is the pregnancy vitamin. It is systematically recommended for women who wish to have a baby. It is almost always included in a fertility supplement. And for good reason: it plays a central role in the prevention of neural tube defects in the foetus.

Health authorities recommend starting supplementation at least three months before conception and continuing for at least the first trimester of pregnancy. B9 also contributes to cell multiplication and DNA synthesis — fundamental processes for the development of your future baby. But what is less well known is that vitamin B9 does not work alone in your body!

The inseparable duo: vitamin B9 and vitamin B12

Vitamins B9 and B12 work in close partnership in your body. For vitamin B9 to fulfil its role fully, it needs the contribution of vitamin B12 in order to be activated and correctly used by your cells.​

Think of B12 as a key that allows B9 to unlock the door to your cells and enter them to carry out its benefits. Without this precious key, B9 remains outside, unable to act.

This metabolic interdependence is central, particularly for the recycling of homocysteine, a substance whose excess can disrupt fertility and increase the risk of complications during pregnancy.​

There is a relatively little-known issue that is important not to overlook: supplementation with folic acid can have the effect of masking a vitamin B12 deficiency.​

Indeed, when you are deficient in B12, your body develops what is known as "megaloblastic" anaemia — meaning your red blood cells are too large and too few in number. By taking folic acid, you correct this anaemia at the blood level. Your blood tests may then appear normal, and you think everything is fine. But in reality, the B12 deficiency can persist and continue its damage silently, particularly at the neurological level. It is like switching off the warning light in your car without repairing the engine!

Recent scientific studies confirm this risk. A 2024 publication in the Food and Nutrition Bulletin by international experts[1] (Miller et al.) demonstrates that excess folic acid not only masks B12 deficiency, but may also exacerbate or accelerate its neuropathological progression. The identified mechanism: excess folic acid depletes serum holotranscobalamin (the active form of B12), further reducing the capacity to deliver vitamin B12 to the enzymes that depend on it, leading to accelerated neurological deterioration.

A review published in 2025 in the Journal of the Neurological Sciences by neurologist Edward Reynolds[2] confirms that "recent clinical, epidemiological and experimental studies provide substantial and consistent evidence indicating that excess folic acid can be detrimental to the nervous system", with the most serious effects occurring when vitamin B12 deficiency coexists with excess folic acid.

These findings are corroborated by a clinical case published in May 2025[3] documenting a patient who developed demyelination of the spinal cord after 30 years of supplementation with folic acid alone, without B12.

An undetected and untreated vitamin B12 deficiency can have a significant impact on your conception plans and your overall health.

Impact on fertility

Scientific research shows that a vitamin B12 deficiency could lead to temporary infertility in women, generally reversible with appropriate treatment[4]. Studies have also shown that adequate consumption of multivitamins containing B12 is associated with a lower risk of ovulation-related infertility[5].​

Low B12 concentrations are frequently observed in infertile men, with a positive correlation between B12 levels in seminal plasma and sperm concentration.​[6]

Health consequences

Some signs may be associated with a B12 deficiency:

• Unusual fatigue and low energy levels
• Tingling or numbness in the hands and feet
• Changes in mood or concentration
• Mood disorders (anxiety, depressive episodes)
• Migraines with aura

If you notice such symptoms, ask for a blood test to check for a possible deficiency. When identified early, a B12 deficiency is easily corrected, provided you choose an appropriate supplement.

In addition to older adults, certain groups are more vulnerable to vitamin B12 deficiency:

• People following a vegan or vegetarian diet (B12 is found primarily in animal products)​
• People with digestive disorders affecting absorption
• Couples experiencing difficulties conceiving​

If you identify with at least one of these categories and are considering B9 supplementation, checking your B12 status is an essential first step!

In addition to B12 deficiency, there is another issue that is often overlooked: studies show that mutations in the MTHFR gene affect a significant proportion of European women (13.5% to 31% depending on the type of mutation). This mutation can reduce by 40 to 50% the activity of the MTHFR enzyme needed to convert folic acid into its active form[7].

In these women, standard folic acid accumulates in the body in an unmetabolised form, unable to exert its beneficial effects. What is more, this accumulation can impair the proper functioning of the MTHFR enzyme and have a negative impact on fertility.​

Choosing folates in directly active methylated form

Unlike synthetic folic acid, which requires several enzymatic conversions before it can be used, Folates Mama by Jolly Mama contains L-5-methyltetrahydrofolate (5-MTHF), the already methylated and active form of vitamin B9.​

This active form offers two key advantages:

• It is directly usable by your body, with no prior conversion needed
• It is absorbed by all women, including those carrying the MTHFR mutation​

Research shows that methylated folate offers more efficient absorption than standard folic acid, which is particularly relevant for people with variations in the MTHFR gene. It may even support fertility in infertile women carrying this mutation.​

In this regard, a comparative clinical study published in 2021[8] provided particularly compelling evidence of the efficacy of methylated folates in women carrying the MTHFR mutation with a history of recurrent miscarriages.​

The results speak for themselves:

• Miscarriage rate of only 6% in patients who received methylfolate
• Miscarriage rate of 54% in those who received standard folic acid
• Full-term pregnancy rate of 60% with methylfolate, compared to only 22% with standard folic acid​

The study also demonstrated a significant reduction in pregnancy-related complications (premature labour, pre-eclampsia) in the group that received methylfolate.

Why combine vitamin B12 and vitamin B9?

As we have seen, supplementing with vitamin B9, even in its active form, offers little benefit if you are deficient in B12. Hence the value of combining both vitamins.

This combination allows you to:

• Benefit fully from the effects of B9 thanks to the presence of B12
• Prevent the masking of a potential B12 deficiency
• Protect your nervous system
• Support your fertility in a holistic way​
• Give the foetus what it needs to develop

If you choose to supplement with B12, take care not to combine Folates Mama with a multivitamin supplement that already contains B9 (even in synthetic form), to avoid any risk of overdose.

To optimise your chances of conception whilst looking after your health:

Before starting a supplement

• Have your vitamin B12 levels checked with a blood test (the recommended level for a woman in early pregnancy is above 300 ng/L[9])​
• Discuss your diet and any potential risk factors with your healthcare professional

Choose a comprehensive supplement

• If you are deficient in B12, opt for a multivitamin supplement combining B9 and B12, such as Baby Bump 
• If your B12 levels are sufficient, choose a supplement containing only methylated B9, such as Folates Mama
• Choose directly active methylated forms, particularly if you have difficulty conceiving or a family history of MTHFR mutation
• Opt for a varied diet including sources of B12 (meat, fish, eggs, dairy products)

Pay attention to the signs

If you experience unusual fatigue, memory problems, tingling, or mood changes, speak to your doctor. These may be signs of a B12 deficiency.

Taking care of your fertility also means taking care of your overall health. Supplementing with vitamins B9 and B12 is not simply a matter of ticking boxes in your pregnancy preparation. It is a carefully considered approach that must specifically take into account your individual profile and needs.

With Folates Mama, you choose a scientifically optimised B9 formula that respects the physiology of all women, including those carrying the MTHFR mutation. By also supplementing with B12, you give your body everything it needs to support your fertility and protect your neurological health.

Your baby project deserves the very best. In some cases, the causes of difficulties conceiving are hidden in those "invisible" details that targeted supplementation can address.