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Despite the aforementioned studies, some reports have discussed an association of elevated folate intake with an increased risk of cancer progression. A literature review indicated that excessive intake of folate can increase the likelihood of precancerous cell growth.¹⁷ A randomized clinical trial (RCT) indicated a correlation between folic acid supplementation and an increased risk in pre-existing lesions and adenoma multiplicity.⁷⁷ There has been substantial interest in this topic. The interaction between folic acid and CRC is more complex. Several studies have revealed that individuals older than 50 years of age who take B vitamins, like B9 and B12, have a high risk of cancer, particularly CRC.^78–80 A nested case–control study performed by Van Guelpen et al. over a 4-year follow-up period showed that participants with elevated plasma folate levels had an approximately four-fold higher risk of CRC than those with lower levels.⁸¹ A large multicenter trial was conducted on a resected colorectal adenoma population who were randomly given either 1 mg of folic acid or a placebo over a 3- to 5-year follow-up period. Follow-up colonoscopy showed no protective impact of folic acid supplementation.⁷⁷ In fact, additional work found a notable risk in the multiplicity of recurrent adenomas with supplementation, as well as a marginally significant increase in “high-risk” adenomas.⁸² In controlled studies in a variety of colon cancer animal models, a setting in which neoplastic tumors are already present, intake of folic acid was no more protective. With established foci, additional microscopic foci and macroscopic tumors rapidly arose along with supplementation.^83,84 A potential link between lung cancer and folic acid intake has become a subject of interest. Stanisławska-Sachadyn and colleagues reported that folate levels above the median value (>17.5 nmol/L in the healthy controls) were associated with a high risk of lung cancer among smokers. It was also believed that smoking carcinogenicity was increased with the addition of folic acid intake in the heavy smoker population. Additionally, women with the solute carrier family 19 member 1N (SLC19A1) genotype and folic acid supplementation are at high risk of lung cancer.⁸⁵ Treatment with folic acid plus vitamin B12 was found to be related to increased cases of cancer and all-cause mortality in patients with ischemic heart disease in Norway, mainly driven by increased lung cancer incidence in participants.⁸⁶ The connection between folic acid and prostate cancer has also been a research focus. In a study conducted by Rycyna et al., increased blood folate levels were believed to play a role in prostate cancer development.⁸⁷ The results of a meta-analysis also supported this finding,⁸⁸ along with those of the Aspirin/Folate Polyp Prevention Study (AFPPS) trial, which followed patients with folic acid supplementation over 10 years.⁷⁷ Moreover, a study on cardiovascular patients who took folic acid to reduce homocysteine levels showed a high incidence of prostate cancer among men with a mean age of 60 years.⁸⁸ In a randomized trial, men who received folic acid as a preventative measure for colon polyp recurrence had increased prostate cancer diagnoses.⁸⁹ Likewise, Zhang et al. reported that routine intake of folate >400 µg/day could enhance the risk of BC.⁹⁰ A leading study observed a considerable positive association between total folate and supplemental folic acid intake amounts with postmenopausal BC.⁹¹ A trial was conducted among a population of pregnant women, with those who were randomly given the highest folic acid dose (5 mg/day) having a 70% greater risk of total cancer than the placebo group.⁹² In the 1940s, clinical investigators gave huge doses of folic acid to individuals with acute leukemia. Subsequently, the proliferation rate of the leukemic clone increased enormously,^93,94 corresponding to what Sidney Farber politely termed ‘‘the acceleration phenomenon’’.⁸² A cohort study was performed to explore the interrelationship between childhood cancer risk and high-dose folic acid consumption in mothers with epilepsy. The researchers found that prenatal exposure to a high-dose of folic acid could increase the risk of cancer by three-fold in the children of mothers with epilepsy who had filled the prescriptions for anti-seizure medication. However, the children of mothers with epilepsy who filled the prescriptions for anti-seizure medication, but not for high-dose folic acid, did not have an increased risk of cancer. In the folic acid-exposed groups, leukemia was the most common cancer type, followed by lymphoma and central nervous system tumors.⁹⁵

The roles of folate in nucleotide synthesis and as a cofactor in the rate-limiting step of DNA synthesis,⁹⁶ makes it a probable growth factor for neoplastic cells.⁸² Aggressively dividing cells, such as those present in solid tumors, have high consumption rates of particular vitamins. As a result, there is overexpression of the receptors responsible for the uptake of those vitamins.⁹⁷ Additionally, many cancer cells can upregulate the expression of membrane receptors that mediate folate uptake, as well as the expression of certain critical folate-dependent enzymes that are necessary for DNA synthesis.^98,99 There are two folate receptors, folate receptor α (FR-α) and FR-β, which are both expressed in malignant tissues of epithelial and non-epithelial origin.⁹⁷ Furthermore, they have been detected in ovarian carcinoma (>90%), lung cancer (50%), BC (25%), endometrial cancer, renal cancer (50%), CRC (including Caco-2 cells), and cancers of myeloid hematopoietic cells, the brain (66%), and placental cells.⁹⁷ FR expression levels in tumors have also been directly associated with the tumor stage.⁹⁷