The total content of folic acid in the human body is about 5-6 mg. it is a substance that cannot be synthesized by the human body, and can only be taken and used from food. Studies show that supplementing folic acid during preparing for pregnancy and the period of pregnancy is not only important for the neurodevelopment of the fetus, but also can prevent various diseases such as cleft lip/palate, cardiovascular abnormalities.
In addition to the benefits to the fetus, folic acid is also beneficial to the expectant mother. Previous studies have shown that folic acid supplementation can prevent the occurrence of color cancer and rectal cancer. But as research continues, this view has been questioned because scientists have found that the deterioration of color cancer and rectal cancer is also related to folic acid supplementation.
Double Effects of Folic Acid on Colorectal and Rectal Cancer 
The effect of folic acid on color cancer and rectal cancer is mainly at the DNA level. It is an important cornerstone of DNA methylation. DNA methylation is a form of chemical modification of DNA that refers to changing genetic expression without changing the DNA sequence. As an important way of gene regulation, about 90% of the 5 cytosine bases of the CpG sequence are methylated in the DNA of human cells, and the DNA sub-chain of cell division and replication must also undergo appropriate methylation modification to ensure its stability.
Folic acid can provide methyl groups for DNA methylation. Previous studies have confirmed that folic acid deficiency may cause DNA methylation instability, which can induce mutations in genes, especially for the colon and rectal mucosa, which have strong metabolism and rapid increase in value. Therefore, moderate intake of folic acid may prevent color cancer and rectal cancer.
While people are excited about finding a way to prevent cancer, another mathematical model predicting the relationship between folic acid and the incidence of color cancer and rectal cancer has reached the opposite conclusion. The model found that, unless people take folic acid before the age of 20, the incidence of color cancer and rectal cancer after taking folic acid did not decrease, but increased. Animal experiments have confirmed that before tumor forms, folic acid supplementation may reduce the risk of carcinogenesis, but after the formation of precancerous lesions, folic acid can promote tumor proliferation. In addition to verifying the above conclusions, the results of the epidemiological investigation also found that excessive doses of folic acid supplementation may also increase the risk of cancer.
From this point of view, folic acid can be described as a “double-edged sword” for the colon cancer and rectal cancer. It may prevent tumors by improving the stability of DNA methylation before the tumors have formed, and may promote tumor development after small tumor lesions have formed in the intestine.
Taking Folic Acid, Do Intestinal Preparation First
Color cancer and rectal cancer is the third largest malignant tumor in the world. According to the statistics of the 2018 “Consensus of Experts in the Early Diagnosis and Screening of Colorectal Tumors in China” , the number of new cases of the disease is 429,200 and the number of deaths is 281,400 annually. The incidence of precancerous lesions (polyps or adenomas) of color cancer and rectal cancer is very high, but the early symptoms are not obvious. Diarrhea, constipation and other symptoms that appear with the growth of the tumor are also easy to be ignored. Therefore, it is necessary to do some preparations for pregnant women before supplementing with folic acid.
Firstly, before taking folic acid, enteroscopy should be routinely screened to exclude early microadenomas. For women who are affordable in economy, it is recommended to measure the concentration of folic acid in red blood cells and sigmoid colonic epithelium to assess the current level of folic acid in the body and avoid excessive folic acid supplementation.
Secondly, choose proper folic acid supplement. At present, there are two options of folic acid tablets for pregnancy: 400μg and 800μg. Since folic acid is 1.7 times more effectively absorbed into body through tablets than that through food. Therefore, taking a 400μg tablet is equivalent to taking 680μg of folic acid, and 800 μg tablet of folic acid is beyond the recommended dosage for pregnancy. Therefore, it is recommended to choose a 400 μg tablet for supplementation to avoid excessive folic acid.
Finally, if gastritis and gastric ulcer and other mild digestive diseases during pregnancy is found, you may continue to take folic acid. However, it is best to treat with medicine before preparing for pregnancy. Probiotic preparations can be appropriately supplemented during medication to maintain the microecological balance of the intestinal flora. Due to the variety of drugs used to treat digestive diseases, it is recommended to continue preparing for pregnancy for more than 1 week after the drug is stopped to avoid the effects of the drug on embryo development.
 Chen Jian, Liu Jie. Double Effects of Folic Acid on Occurrence of Color Cancer And Rectal Cancer [J]. Shanghai Medical and Pharmaceutical Journal, 2014 (01): 20-23
 China Anti-Cancer Association Special Committee for Colorectal Cancer, Expert Panel on Formulating Early Screening Strategies for Colorectal Tumors in China. Expert Consensus on Screening Strategies for Early Colorectal Tumors in China [J]. Chinese Journal of Gastrointestinal Surgery, 2018, 21 (10): 1081-1086