Colorectal cancer is the third most common cancer in men and the second in women worldwide and it accounts for 8% of all cancer deaths, making it the fourth most common cause of death from cancer (WHO, 2008). In the Philippines, colorectal cancer is the third most common cancer and, next to lung cancer and breast cancer, the third leading cause of cancer deaths (DOH, 2008). According to the World Health Organization (2008), the Philippines ranks low in terms of age standardized survival rates for colorectal Cancer in both sexes.
It ranks 27% for women, 29% for men, falling behind countries like the United States, West Europe, Japan, Thailand and India. The risk of having colorectal cancer increases sharply after age 40 with 90% of cases occurring in people over age 50 (Fauci, 2008). Of all cancers, colorectal cancer seems most connected to diet ,as a result, the past decade has produced many studies looking at colorectal cancer in relation to single nutrients or foods, including dietary fats, red meat, ? ber, vegetables and fruits, calcium, vitamin D, and antioxidants (Kornaroff, 2005).
Vitamin B6, because of its role in the synthesis of nucleic acid and DNA, it has also been proposed to affect colorectal carcinogenesis (Kim, 2004). Vitamin B6 is a water-soluble vitamin that is naturally present in many foods, added to others, and available as a dietary supplement. It is the generic name for six compounds (vitamers) with vitamin B6 activity: pyridoxine, an alcohol; pyridoxal, an aldehyde; and pyridoxamine, which contains an amino group; and their respective 5′-phosphate esters. Pyridoxal 5′ phosphate (PLP) and pyridoxamine 5′ phosphate (PMP) are the active coenzyme forms of vitamin B6 (McCormick, 2006).
Vitamin B6 in coenzyme forms performs a wide variety of functions in the body and is extremely versatile, with involvement in more than 100 enzyme reactions, mostly concerned with protein metabolism (McCormick, 2006). Both PLP and PMP are involved in amino acid metabolism, and PLP is also involved in the metabolism of one-carbon units, carbohydrates, and lipids (Macky et. al. , 2005). One-carbongroups are important for DNA synthesis and DNA methylation; hence, low vitamin B6 may increase colorectal cancer risk through aberrations in DNA synthesis, repair, and methylation (Larsson, et. al. , 2010).
The richest dietary sources of vitamin B6 include fish, beef, liver and other organ meats, potatoes and other starchy vegetables, and non-citrus fruits (Mackey et. al. , 2005) While previous studies have supported the inverse association between vitamin B6 and colorectal cancer, the findings of a cohort of studies involving both men and women by de Vogel et. al. in 2008 suggested that vitamin B6 has a protective property for men but not on women against colorectal cancer. Thus, the aim of this meta-analysis is to examine further the effect of dietary intake of vitamin B6 in women regarding the risk of colorectal cancer.