Fortified foods are marketed as being good for us, helping to plug nutritional gaps that can lead to deficiencies and poor health.

However, the government’s recent decision to add folic acid to supermarket bread has become the centre of a heated online debate.

From December this year, a new law will see food manufacturers add the synthetic nutrient to non-wholemeal flour, meaning that certain varieties of breads, cakes and other supermarket items will all carry additional fortification.

Once the rule is in place, it will be mandatory for millers in the UK to add 0.25 mg of folic acid per 100 grams of non-wholemeal wheat flour.

But what actually is folic acid? Why are people concerned, and is there any truth to the concerns swirling on social media?

To better understand the conversation, we asked a nutritionist to answer some of the most common questions.

Folic acid is the synthetic form of the vitamin folate, also known as B9.

Folate helps the body to manufacture healthy red blood cells and plays a role in how efficiently our cells divide and grow. It’s naturally found in foods like leafy greens, chickpeas and cruciferous vegetables, with the NHS saying that most adults can get all they need from a healthy diet alone.

In early pregnancy, having good levels of folate becomes particularly important, as it helps to aid the proper development of the baby's brain, skull and spinal cord.

During these weeks and months, it can be more challenging for women to obtain enough folate from diet alone, as the body's requirements increase significantly.

This is why the NHS recommends that all pregnant women take 400 micrograms of folic acid every day – from before they fall pregnant until their 12-week scan.

"Folic acid is being added to bread in the UK because folate intakes are still too low in women of childbearing age, and that has real, preventable consequences," says Rob Hobson, nutritionist and author of Unprocess Your Life.

"Around nine in 10 neural tube defects, including spina bifida (a condition where the baby’s spine and spinal cord don’t fully develop in the womb), develop very early in pregnancy – often before many women even know that they’re pregnant.

"Despite years of advice to take supplements, uptake has been patchy in the UK, particularly among younger women and those from lower-income backgrounds."

In England, a study of nearly half a million women showed that only 31% take a folic acid supplement prior to pregnancy.

"That’s why fortification exists," says Hobson. "It follows years of evidence review, careful modelling of risks and benefits, and advice from independent scientific bodies.

"Countries that already fortify flour have consistently seen reductions in neural tube defects, without clear evidence of harm to the wider population."

One recent review that looked at data from multiple countries found a clear link between the introduction of folic acid fortification and a reduction in neural tube defects, with Chile in South America seeing incidences drop by 55% over a decade.

Much of the online controversy focuses on a gene called "MTHFR", which plays a role in how your body metabolises folate.

Some people are concerned that, if you have a mutation of this particular gene, you may be naturally less efficient at processing synthetic folic acid, leading to higher levels of the unmetabolised nutrient in the bloodstream.

Online, panic has spread around this "build-up" of folic acid, with claims being made that it could contribute to everything from immune problems and heightened cancer risk to increased depression and fatigue.

However, Hobson believes that these concerns have largely been sensationalised, and are not based on robust scientific evidence.

"It’s true that variants of the MTHFR gene are common and can slightly affect how folate is processed in the body," he clarifies. "However, the idea that people with these variants can’t safely handle folic acid, or that fortification is harmful at a population level, isn’t supported by good-quality evidence.

"This argument often comes from unregulated sources online, and it tends to overlook the scale of public health benefit."

Fears around folic acid and cancer appear to stem from early research looking at high-dose supplementation.

Some early studies suggested that very high intakes of folic acid might be linked to a possible increase in certain cancers. However, a 2013 meta-analysis of randomised trials looked into these concerns, and found that folic acid supplementation didn't significantly increase overall or site-specific cancer risk over a five-year follow-up period.

When it comes to your weekly loaf of bread, Hobson says it’s sensible to keep the fortification levels being increased in context.

"The level of folic acid being added to flour is small and tightly regulated," he reminds. "Even with fortified bread, most people will still be consuming folic acid well below the upper safe intake level. This is very different from taking high-dose supplements."

He adds: "Claims on social media that fortified bread can 'slow down bodily processes' or 'promote cancer' aren’t backed up by robust human evidence at the levels used in fortification.

"While very high supplement doses may overwhelm certain pathways in theory, there’s no good evidence that the small amounts used in flour fortification impair metabolism or cause harm in the general population."

Of course, if you’re really anxious about the new law and you’re meeting your daily folate requirements through diet alone, you can always opt for wholewheat varieties that aren’t affected by the rule, or try making your own wholemeal sourdough at home. Both are healthy and nutritious ways to increase your fibre intake.

However, if you love nothing more than tucking into a white bread sarnie, Hobson believes there’s no immediate reason to panic. "The bottom line is that fortification isn’t about optimising nutrition for people who already eat well and take supplements," he notes.

"It’s about reducing avoidable birth defects across the population. From a public health perspective, the benefits are clear, the risks have been carefully assessed, and the policy is grounded in decades of evidence."