Main Menu Health Library Female Hormones and Risk
The MULTIPLE CAUSES of RISK of CANCER in WOMEN
Abstract: A large number of female related factors determine a woman's risk of Breast and other Genital cancers. Some that are largely non-actionable in addition to family history are age at first period, age at menopause, age at first birth, number of children, extent of breast feeding, and use of oral contraceptives. Those that are actionable include intake of various vitamins, antioxidants and minerals, exercise and cardiofitness, smoking, body weight, diet factors as fat and fiber, mammograms, alcohol intake, and use of post menopausal hormones. All of these and other female factors are considered and valued in every Life Ahead computation of risk. Some facts about some these factors are summarized following, and others are discussed in much more detail elsewhere as noted.
Few health factors have been studied more extensively than the causes of women’s breast and other cancers. Unfortunately many of these causes are largely or entirely non-actionable. Yet a combination of these various non-actionable factors can combine with actionable factors to profoundly affect a woman’s risk of these devastating diseases. Life Ahead may be able to provide women with a clearer understanding of their overall risks and what they can do about it.
An average US woman of age 20 has a 1% change of developing breast cancer by age 40, and about a 3.4% chance of developing it by age 50. Risks of cancer from any cause are about 1.7% and 5.8% during these same years. Although these percentages appear small, breast cancer still is the disease of largest risk and concern to most women during this period of life. And a woman with family history risk of cancer and other elevated risks can have a several times higher cancer risk than this 5.8% by age 50. The following will first identify the factors that can increase this risk that are usually considered as non-actionable, and then mention those actionable factors that can help bring the risk under at least partial control.
Researchers have published hundreds of studies on women’s risk of breast cancer – and especially of risks during the childbearing years of life. The factors are too many and complex for most people to keep track of and understand. Life Ahead computes all of their individual and combined effects and shows overall risks from the simple entries made into the program. But it is useful to have a general understanding of these many factors and how they can affect risk. If non-actionable risks are high this means that added attention is needed to reduce risks by those factors that are actionable.
Non-Actionable Factors and Cancer Risk
Family History: This is by far the largest known factor that can increase the risk of Breast and other Cancers. Family History health risk is explained elsewhere in more detail. Family History related risks can multiply by several times the the above average 3.4% risk of breast cancer by age 50.
About Estrogen: The years between first period and menopause are a woman’s years of increased estrogen. Young girls and boys have near similar and very tiny risks of heart disease but these risks increase steadily with age and with cholesterol. But estrogen as a powerful antioxidant reduces the usual effect of cholesterol in producing cardiovascular diseases by about 4% each year after a girl’s first period. By age 50 women’s risk of heart disease has moved down to only a 25% of that of men. After the menopause and estrogen level is reduced, women’s risk of cardiovascular diseases gradually increases up closer toward that of men. See the Health Library item on Hormones for more on this.
During these same years when estrogen is moderating the risk of cardiovascular diseases, it acts in an opposite way to gradually and steadily increase the risk of cancer, and especially of breast cancer. This is partly revealed by the fact that cancer risk increases about 12 % for each year between a woman’s first period and menopause, and then declines to about 3% per year thereafter. And this is revealed further by the fact that risk of cancer changes both with the age of first period, and the age of menopause. On balance, women's risk of an early death is lower during this period before menopause han that of men because their reduced risk of death from heart disease is larger than their increased risk of death from cancer.
Age at first Menstrual Cycle: Eleven studies studied in the Life Ahead development showed that risk of Breast Cancer increased an average of 5.9% for each year lower of age at first period. A woman that had first period at age 10 for example had a 26% higher risk than one that had first period at age 14. Thus a longer exposure to higher estrogen increased risk.
Age at Menopause: Results of six studies analyzed showed that risk of Breast Cancer increased at an average of 3.5 % for each year that age at menopause was older. Again a longer exposure to estrogen increased risk. Each year of exposure to the estrogen of the child-bearing years increases cancer risk. A woman with first period at age 10 and menopause at 52 can have a 35% higher Breast Cancer risk than one with average ages at these events of 12 and 48 respectively.
Life Ahead assumes average cancer risks for women having first period at age 12 and menopause at age 48. Differences in risk are estimated based on differences in age at first period of 5.9% per year from age 12 and on differences of 3.5% per year from age 48 at menopause. Thus depending on ages of these events, a woman can have either a higher or lower risk of cancer than average from duration of estrogen exposure.
Number of Children: Interestingly, childbearing reduces risk of breast and related cancers. An average result from 6 different studies showed that each child born - at least up to five - reduces risk of breast cancer by 10% or by a 0.90 factor. This suggests that the bearing of a child interrupts the estrogen-related increase in cancer for nearly two years, and that a woman with 5 children will have only half the risk of breast cancer that will a woman without children. However, this factor for number of children is refined in Life Ahead somewhat based on the result of the research cited following on the effect of breast feeding.
Age at First Birth: Another finding from the average result of another 10 studies is that a delay of each year in the birth of a first child increases risk of Breast Cancer by about 3.8%. A perplexing inconsistency here – but that seems to be verified – is that a woman that has a late birth of a first child can have a higher risk than that of a woman with no children. This puzzling observation was specifically noted on some of the studies, but the differences in risk involved are not large.
Extent of Breast Feeding: The more and longer women breast feed their children, the lower is their risk of Breast Cancer. This conclusion was reached from an analysis of results of 47 studies from 30 countries on 147,000 women. (Lancet 2002, 360:187). This massive study suggested a reduction in risk of 7% (0.93 factor) for each child born, plus a credit of 0.4% (0.996 factor) for each month a child was breast fed. These factors now are used in Life Ahead. The study authors did note that these factors explain how the risk of breast cancer formerly in developing countries could be only half of that in developed countries. This large difference in risk is due to the larger number of children born per mother and larger extent of breast feeding usual in the less developed countries.
Effect of Oral Contraceptives: An enormous amount of research has been devoted to studying the effect of oral contraceptives on women’s risk of cancer. Reassuringly, the results show that in aggregate the use of these contraceptives probably do increase the risk of breast cancer but only modestly. And offsetting this they also reduce the risk of ovary, uterine and colorectal cancer. More on this follows. Although an actionable factor, contraceptives are used for other than health purposes per se.
A major analysis of risk of contraceptives on breast cancer was by the Collaborative Group on Hormonal Factors in Breast Cancer (Lancet 1996, 347:1713). This analysis of results from 54 studies in 25 countries on 153,000 women found a 24% increase in risk (1.24 factor) after taking the pills for extended time. This risk dropped gradually to near zero 10 years after taking them was stopped.
In contrast, risk of both ovary cancer and uterine (endometrial) cancer is reduced about 40% for use of oral contraceptives, and is reduced further reduced by years of their use. Results from 10 studies of contraceptives of ovary cancer and another 10 studies of uterine cancer showed an average risk factor of 0.62 that was further reduced by 0.014 times years of contraceptive use. This factor is used in Life Ahead with a minimum accepted risk of 0.50. An average risk of colorectal cancer associated with contraceptive use from 4 studies found was 0.80 of average. .
The lower risks for ovary and uterine cancer moved back up toward unity after contraceptive use was stopped at about 5% per year. Breast cancer risk moved down toward unity at about 20% per year. Life Ahead computes a net gain of about 30 Well-Days for use of birth control pills between ages 20 and 50 by an average US woman. This result is essentially a zero effect within the accuracy of the various risk estimates included. This lifetime net effect of oral contraceptives could differ for other included risk factors but probably still will be very small.
Other non-actionable factors include risks from pollution and passive smoking. These usually will be in the range of 10-20% and are smaller than risks of the above factors. Body height affects risk of all cancer with taller people having somewhat higher risk
Actionable Factors and Cancer Risk
The important actionable factors that can affect risk of Cancer are discussed separately in the many different analyses in the Life Ahead Health Library. A brief recap here of these many factors that can help reduce Cancer risk up to several fold and others that can increase risk substantially follows as. Every Life Ahead analysis values risks not only from all of the above Non-actionable factors but for all of the following Actionable factors:
Reduces Cancer Risk:
Antioxidants: Vitamins E, Vitamin C, Beta Carotene, Selenium
Physical Exams
Increases Cancer Risks:
Female Hormones – Post Menopausal
In addition to Life Style Habits some medicines can reduce risk of cancer. As example Tamoxifen has reduced risks of cancer significantly for those with high cancer risks. This is a medical factor to be discussed with the doctor and its effect as for other medications is not included in Life Ahead.
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