Increased consumption levels of the nutrient were found to increase the risk of colorectal polyps, which can sometimes - but not always - lead to colorectal cancer. The findings come contrary to the researchers' expectations that choline would reduce the risk of colorectal polyps, in the same way that the related nutrient folate does. However, the study, published online in the August 8 2007 issue of JNCI, is the first to examine the link between choline and colorectal polyps, and the researchers cautioned that more studies need to be conducted as other components of diets high in choline may be responsible for the association. Choline has until now been considered an essential nutrient that plays a role in normal cell function and transportation of nutrients through the body. Dietary sources include eggs, beef liver, chicken liver and wheat germ. The nutrient is involved in a biochemical process known as one-carbon metabolism. Studies have shown that people with increased intake of other nutrients required for one-carbon metabolism, such as folate, are at a decreased risk for colorectal polyps. "Although our results were contrary to expectation based on choline's role [in one-carbon metabolism], there is a potential biologic basis for the positive association that we observed," wrote the authors. "Once a tumor is initiated, growth into a detectable [polyp] depends in part on choline availability because choline is needed to make membranes in all rapidly growing cells." Led by Eunyoung Cho of Brigham and Women's Hospital in Boston, the researchers sent food-frequency questionnaires to women enrolled in the Nurses' Health Study every two to four years from 1984 to 2002. They then estimated the choline content in their diets by multiplying the frequency of consumption of each food item by its choline content and summing the nutrient contributions of all foods. Among 39,246 women who were initially free of cancer or polyps and who had at least one endoscopy from 1984 to 2002, colorectal adenoma cases were detected in 2,408 women. Increasing choline intake was associated with an elevated risk of colorectal adenoma, wrote the researchers. In an accompanying editorial, Regina Ziegler, and Unhee Lim, of the National Cancer Institute in Bethesda, Maryland, commented on the complexity of the relationship between one-carbon metabolism and the development of cancer. "Clearly, one-carbon metabolism and its role in [cancer development] is more complicated than originally anticipated, and our understanding of the underlying mechanisms is probably incomplete. More research, and caution in developing public health policy and guidance, is warranted." However, although the need for further investigation is clear, the preliminary findings are sure to shake scientific opinion of the nutrient, which has so far been considered an essential part of the human diet. Just two months ago, a study conducted at Penn State University and published in the American Journal of Clinical Nutrition, set out to evaluate healthy men and women's dietary requirements for choline, and look at the clinical consequences of choline deficiency. Consuming lower levels of the nutrient was found to be linked to fatty liver or muscle damage, prompting the researchers to suggest that current choline recommendations may not be enough for some people.
The current Adequate Intake level for choline currently recommends daily consumption of 550mg for men and 425mg for women. However, no Estimated Average Requirement has been set because of insufficient human data.