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| Sex and Disease |
Sex is an important aspect of human identity, but it is also important in health. Women outlive men. In the United States at the start of the twenty-first century, a woman's life expectancy at birth is 79 years, and a man's is 72. While other countries have greater or lesser average life expectancies, female life expectancy is still greater in nearly all countries. In fact, for most animalsthat have been studied, females outlive males; female sperm whales outlive males by thirty years on average. Many factors contribute to this effect, including genes, hormones, and lifestyle factors. Historically, the greatest death risk for women has been childbirth. In developed countries, however, this risk has decreased markedly in the last century, significantly increasing a woman's lifespan.
Males die more often than females - even before birth. Although there are 115 male fertilized eggs for every 100 female, the ratio for live births is
104 males to 100 females. Each year after birth, more males die than females; so, by age 100 there are only 11 males for every 100 females. With improvements in health care, the gap between longevity in men and women is decreasing. However, one troubling factor contributing to the narrowing of the gap is an increase of diseases in women that have been typically considered male diseases, especially cardiovascular disease.
There are two aspects of the longevity gap: Why do men die young and why do women live so long? Hormones appear to be part of the answer to both of these questions. Testosterone may contribute to early death in males. The greatest difference in death rates between males and females occurs during the teen years, when males experience a surge in testosterone. This increase correlates to increases in death in males by accidents, homicide, and suicide; however, these behavior-related deaths continue to contribute throughout life to male mortality more than they do to female mortality.
While teenage females also die from behavioral causes, the incidence is much lower than for males. Female teenagers also experience an increase in hormones; these hormones, however, generally correlate with increased longevity in women. The strongest evidence for the protective effects of female hormones is the increased risk for several diseases after menopause, notably cardiovascular disease and osteoporosis. In males of all ages, testosterone increases the levels of undesirable LDL cholesterol and decreases the levels of the desirable HDL cholesterol, increasing the risk for cardiovascular disease. In contrast, estrogen appears to have a beneficial effect on cholesterol levels. As of 2003, there is much controversy about whether estrogen replacement after menopause gives any significant health benefits for women. In fact, some studies suggest that replacement therapy may do more harm than good.
Women may also enjoy advantages over men in physiology and metabolism, probably because of hormone differences. Women have lower metabolic rates than men, likely leading to less oxidative damage to cells. Oxidative damage results from free radicals, which alter DNA, RNA, and protein in cells. This may explain why oxidative damage is linked to diseases such as cancer, Alzheimer's, and atherosclerosis. In animal studies, lowering metabolism by decreasing calorie consumption has been shown to significantly increase lifespan. In addition, because they menstruate, women have less iron in their blood. (High levels of blood iron are associated with oxidation of LDL cholesterol, which contributes to cardiovascular disease.)
Women also enjoy a genetic advantage because they have two copies of the X chromosome. Mutations in genes on the X chromosome typically do not cause disease in females because there is a normal copy. Two X-linked diseases are hemophilia and muscular dystrophy. Because X-inactivation occurs randomly in each cell, about half of the cells of women heterozygous for these conditions would be normal. Additionally, the normally inactivate copy of the X chromosome in females (resulting from X-inactivation during development) may be at least partially restored as women age, allowing the inactive X to provide a good copy of a gene that was lost or altered by mutation in the other X chromosome.
One area in which women do not enjoy an advantage over men is in autoimmune diseases. Women are more susceptible to these diseases, such as systemic lupus erythematosus (lupus) and rheumatoid arthritis. There isn't a simple explanation for this increased risk; instead, it appears to result from a combination of genetic, environmental, and hormonal effects.
Lifestyle choices also affect longevity. Early in the twentieth century, men smoked more than women, a factor that is thought to account for much of the gender gap in longevity. As more women began to smoke, the gap decreased. Studies indicate that women smokers may have an increased risk of lung cancer because they have higher levels of an enzyme that produces carcinogens from tobacco smoke. In addition, middle-aged women smokers live no longer than do men smokers, suggesting that smoking eliminates any health advantage conferred by gender.
Despite the evidence for gender-based differences in physiology, metabolism, disease, and response to certain drugs, women were excluded from most medical studies for many years. Why? It wasn't just sexism: the difficulty in controlling the monthly cycles of hormones, and the concerns about possible pregnancy simply made it easier to leave women out of the studies. In 2001 the Institute of Medicine issued the report "Exploring the Biological Contributions to Human Health: Does Sex Matter?" The report concluded that sex was very important in health, and that women should be included in all studies of diseases that could affect them.
With improvements in health care and an understanding of the importance of nutrition and exercise, it is likely that the longevity gap will continue to decrease. We may eventually understand which components of female longevity are the result of sex and which are the result of gender.
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| ||End Notes |
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- Sinclair, A. 2001. The Scientist 15:18.
- Rozen, S., H. Skaletsky, J. D. Marszalek, P. J. Minx, H. S. Cordum, R. H. Waterson, R. K. Wilson, and D. C. Page. 2003. Abundant gene conversion between arms of palindromes in human and ape Y chromosomes. Nature 423:873-76.
- Money, J., J.G. Hampson, and J.L. Hampson, 1955. Hermaphroditism: Recommendations concerning assignment of sex, change of sex and psychological management. Bulletin of the Johns Hopkins Hospital 97:284.