Kelvin Science Essay Competition entry by Jack Teh
Life expectancies for men and women have been extended significantly over the years since cutting edge medical practices and technologies have progressed. For example in the United Kingdom, the average life expectancy at birth was 77.99 years , whereas, it has increased up to 80.29 years in 2013. We cannot overlook the effort doctors and researchers have spent to extend people’s lives; such as, inventing effective medicine, performing surgeries and even vaccinating the population so that our bodies are prepared against incoming infections. Modern surgery, unlike in the past, is performed in a sterile environment therefore have a very high success rate and a low rate of complications after the surgery.
Interestingly, it is noticeable that there is a difference in longevity between genders. In 2013, an average man can expect to live to 78.16 years, while an average woman lives 4.38 years longer . By the age of 85, there are approximately six women for every four men in the UK, and by 100 the ratio is more than two to one . In addition, amongst the 70 verified living supercentenarians (someone who has lived to or passed his/her 110th birthday) across the globe, 65 of whom are female and only 5 are male . The world’s oldest recorded person is a French woman named Jeanne Calment who lived for 122 years . The question why women live longer than men has been raised and scientists are currently trying to understand the reason why women age slower and have higher longevity.
Ageing is a multidimensional process which includes changes in physical, psychological as well as, social states and is considered progressive and irreversible . It is referred to as a stochastic process since it is impossible to predict exactly when the effect of aging starts to be visible . Tom Kirkwood proposed a theory called ‘Disposable Soma Theory of Ageing’ in 1977 explaining why living organisms age . He states that organisms have a finite amount of energy available to be used for passing the genes to the next generation (i.e. reproduction) and maintaining the non-productive aspects of the organism (i.e. digestive organs, respiratory organ, etc.). Therefore, the reason why our bodies deteriorate as we become older is because they are not able to devote sufficient resources (i.e. nutrients and repairs) to maintain themselves in the pristine state of our youth . Furthermore, Kirkwood postulated that our bodies do not completely repair themselves as well as they could, because over the evolutionary process, natural selection has favoured organisms to view reproduction as their priority, rather than ‘investing’ energy into extending their lives .
Additionally, through studies in the laboratory, he discovered that animals that naturally have longer lives have superior maintenance and repair systems than short-lived species. These phenomena provide evidence to prove the ‘Disposable Soma Theory of Ageing’ because for short-lived species, passing their genes to the next generation is their first priority. Furthermore, studies show that cells taken from female rodents repair damage better than cells taken from males . However, if the ovaries are removed surgically, the difference will be eliminated. A further strand of supporting evidence comes from studies in Japan where ‘super female’ mice were created by using genetic material from two mothers only. Surprisingly, they live 186 days on average longer than normal female mice . These studies have proved that genes in female cells are better at repairing themselves than male cells and thus, females have a lower rate of senescence than males.
Another explanation for why females repair cells better than males is that females are driven by evolutionary process for having a healthy body, since they are ‘forced’ to carry the young and provide the young nutrients during pregnancy and milk after birth. In most scenarios, the female has the responsibility of rearing and nurturing the young. For males, on the other hand, their role in reproduction is considered short and ‘less important’ (apart from producing and delivering sperm), thus less related to maintaining good health. Hence, males suffer more ‘damage’ inside their bodies than females and since males are worse at repairing their own cells, mutations in male cells will start to accumulate and not repaired in time.
Harman in 1956 first proposed the free radical theory regarding ageing . Free radicals are extremely reactive because they have one or more unpaired electrons in the outermost shell that are not used in bonding. The extra electron causes the molecule to be unstable, making it tends to lose or gain another electron in order to become stable. The free radical causes damage to cells because it often causes a chain of chemical reactions by pulling off an electron from its neighbouring molecule and turning it into a new free radical. This will subsequently lead to another electron being pulled off by the new free radical. The chain reaction ends by removing an electron from a molecule, which renders it either damaged or causing the cell itself to cease functioning.
The mitochondrial theory of ageing, extended from the free radical theory, proposes that oxidative damage (known as oxidative stress) generated during oxidative phosphorylation of mitochondrial macromolecules such as mitochondrial DNA (mtDNA), proteins, or lipids is responsible for ageing . Oxidative phosphorylation is the final stage of aerobic respiration when adenosine triphosphate (known as ATP, ‘currency’ of energy in the body) is synthesised from ADP. It is suggested that ‘free-radicals’ that leak from mitochondria and other subcellular particles cause oxidative stress in the body. Cells accumulate genetic deletions as we get older, producing more free radicals causing more damage by forming a vicious cycle. Oxidative stress is described as a state of physiological stress in the body that arises from exposure to high levels of reactive oxygen species (ROS) such as superoxide (•), hydroxyl radical (•OH), etc. relative to antioxidants . Antioxidants are useful molecules because they prevent free radical chain reactions by donating electrons without destabilising themselves . Having too much oxidative stress can lead to a much higher risk of neurodegenerative diseases such as Parkinson’s and Alzheimer’s disease . Free radicals also affect nuclear DNA and can start a cell on the road to form a tumour, which is cancer in simple terms .
It has been suggested that women live longer because women suffer less oxidative stress than men . The reason for this is that women are relatively iron-deficient compared to men. It is said that iron deficiency affects about 10% of pre-menopausal women, 6% of post-menopausal women, and fewer than 2% of men . During women’s reproductive age, menstruation (loss of blood) and pregnancy (providing extra iron to the foetus) are the common cause of iron deficiency, whereas for post-menopause women, chronic internal bleeding is a possible reason that causes iron deficiency. Iron is a redox-active metal (having two oxidation states of iron ions), which generates damaging free radicals in cells.
In addition, researchers have found that faults in mitochondria have a greater effect on the men than women . As we know, mitochondria are like the “engine rooms” in the cells that undergo aerobic respiration and generate ATP. Mitochondria contain their own DNA, which is inherited only from the maternal side because even though the sperm’s mitochondria enter the egg, they are almost always destroyed by autophagy (cell degradation) and, therefore, do not contribute their genes to the embryo  . A study has found that harmful mutations in the genes of the vital mitochondria tend to be weeded out if they are detrimental to females but are retained in males . Therefore, in the natural selection process, the quality of mitochondria in females has been controlled well in order that they are not passed onto the next generation. Lead scientist Dr Dowling from Monash University in Australia asserts that if a harmful mutation arises in the mitochondrial genes of females, natural selection will identify this and eliminate it, leaving females ‘unscathed’ . By having this type of mechanism, the accumulation of harmful mutations in mitochondrial genes in males will cause males to age faster, and therefore have a lower life expectancy than females. A study between 13 groups of male and female fruit flies (Drosophila melanogaster) showed that mutations in mitochondrial genes would generally cause males to age faster across the animal kingdom . This is an astonishing result possibly explaining the differences in longevity between men and women.
Women may also have longer longevity than men because their immune systems age more slowly . Scientists analysed blood samples of 356 men and women aged between 20 and 90 years and studied the levels of white blood cells and cytokines, which are important in cell signalling in the immune system. Results show a negative correlation between age and the number of white blood cells per person as expected, but interestingly the rate of decline of most T-cell and B-cell lymphocytes and two cytokines was faster in men. T-cells are associated with cell-mediated immunity, i.e. body cells), whilst B-cells are associated with humoral immunity, i.e. immunity involving antibodies . However, on the opposite side, two specific types of T-cells; CD4 T-cells (known as T-helper cells) and natural killer cells (NK cells) increased in number with age, but the rate of increase is higher in women than men. NK cells have a very crucial role in being one of the body’s first lines of defence against cancer because they provide rapid responses to virally infected cells and respond to tumour formation, acting at around 3 days after infection . Moreover, the faster rate of decline in cytokine IL-10 (Interleukin 10) in men suggests that they are more rapidly affected by inflammatory conditions since Interleukin 10 an important anti-inflammatory cytokine that keeps inflammation in the body under control .
One contributor to the gender difference in life span is that females have an extra X chromosome . In mammals, the female is the homogametic sex, with two X chromosomes (XX), while the male is heterogametic, with one X and one Y chromosome (XY). It is the XX and XY chromosome pairing that causes males and females to possess different reproductive organs, although having the same body parts. Genes on the X or Y chromosome are called sex-linked. The sex-determining chromosomes can carry genetic mutations that cause sex-linked genetic diseases. Haemophilia, colour blindness and Duchenne muscular dystrophy are famous examples of X-linked recessive inheritance . People who are homozygous for a particular recessive allele will suffer from the disease. For women, having two X chromosomes means that the second X chromosomes can provide a backup if something goes wrong with the first one; i.e. an abnormal gene on one of the female X chromosomes can use the normal gene on the other and thereby avoid the progression of the disease (by being heterozygous), although still being a carrier of the defect . By contrast, for all males, one recessive mutation on the X chromosome will subsequently acquire the syndrome because there is no other alternative X chromosome to ‘protect’ the recessive X chromosome.
Since the Y chromosome is relatively small compared to the X chromosome, it carries very few genes and therefore there are relatively few Y linked genetic disorders. In most cases, the common symptom associated with mutations in the Y chromosome is a reduction in sperm count. Female offspring of affected fathers are never affected because they receive one X chromosome each from paternal and maternal sides. Male offspring, however, will be affected because they inherit a single Y chromosome from their father. By looking at the differences in the composition of sex chromosomes between male and female, it is seems obvious that the female has a slight advantage over the male because of her two X chromosomes. Interestingly, in birds, they have an opposite pattern of ageing because the female is the heterogametic sex (ZW) and the male is the homogametic sex (ZZ) .
The influences of sex hormones may therefore also contribute to the differences in longevity between men and women. During the early development of the embryo, the sex chromosome determines the gender of the embryo. In the case where they chromosome is present, the foetus will develop testes, which are important in triggering the production of male sex hormone testosterone. It is said that testosterone has a negative role, by increasing the level of low-density lipoprotein (LDL) and decreasing the level of high-density lipoprotein (HDL) in the blood .
LDL is used to transport cholesterol from the liver to the tissues. LDL is often known as the ‘bad’ cholesterol because when it is circulated in the bloodstream, some will tend to infiltrate and deposit within the arterial walls . Over time, when LDL is prone to oxidation, white blood cells are attracted to digest the LDL and form specialised foam cells. These cells will die if they cannot further process LDL and will deposit even more oxidised LDL as a result. This cycle will cause the artery to become inflamed and will create an irregular patch in the arterial walls called an atheromatous plaque . The plaque could further narrow the artery and hinder the blood flow, which would increase the risk of developing thrombosis, aneurysm and coronary heart disease. This would result in men having a greater risk to develop heart disease and stroke than women in their later life.
On the other hand, the hormone oestrogen in women does exactly the opposite as testosterone does in men. It lowers the level of LDL and increases the level of ‘good’ cholesterol (HDL). In short, high-density lipoproteins are used to remove LDL from tissues and transport it to the liver. Moreover, it is believed by scientists that oestrogen has a role in screening pre-menopausal women against heart disease, osteoporosis, and even Alzheimer’s disease .However, the cease in production of oestrogen after menopause does not give women an advantage over men anymore. It is speculated that when hormone-replacement therapy (HRT) steps in as a treatment option of post-menopausal women, the injected oestrogen has more benefits other than just treating menopause-related symptoms, such as reduction in cardiovascular disease and osteoporosis. However, there are some side effects of HRT that should not be neglected. It is said that HRT can increase the risk of getting multiple types of cancer (e.g. breast cancer) , but since cancer is a multifactorial disease, it is not solely dependent on the surplus level of oestrogen. There are many other variables influencing cancer such as eating habits, exercise, lifestyle etc.
To further understand the relationship between sex hormones and patterns in mortality, research on eunuchs in South Korea reveals suggestions as to why women live longer than men . Researchers analysed the genealogical record of these “eunuch families” between 1556 and 1861 and they discovered that the average lifespan of castrated males before puberty is seventy years old, while for other men, they normally only lived into their early 50s. Moreover, three of the eunuchs lived to be over 100 years old. This evidence suggests that testosterone in men could be potentially damaging and considered as ‘toxic’. Apart from the negative effects of testosterone that were explained earlier, scientists believe that testosterone could weaken the immune system or damage the heart .
Putting the biological differences between men and women aside, it is important to analyse the social differences between men and women. Studies have shown that the effects of testosterone on the central nervous system are related to an increase in aggressive and risk-taking behaviours, and increased libido observed in teenage boys and men .It is recorded in history that soldiers were fed the testicles of animals before they went into battle because testosterone will help increase their aggression and ferociousness against their enemies . Having a testosterone ‘surge’ during puberty increases the likelihood of undergoing risky and perilous activities such as motor racing, fighting, smoking cigarettes and taking drugs. In the US, the death rate for males at the age of twelve is 46 % higher than the rate for females. At the age of nineteen, the death rate for males increases by more than six times, while for females, the mortality rate only tripled . Besides, it is known that the proportion of males that smoke and abuse alcohol and drugs is much higher than females. Without doubt, smoking is directly linked to lung cancer, which is the largest cause of cancer death, with 17% of all cancer deaths in the world (1.2 million people) . Men enjoy themselves so much that they never realise that their lives are shortened ‘silently’.
Traditionally speaking, men are regarded as the dominant member of the family, both financially and spiritually in most cultures. Men usually work outside the home, while women have multiple roles such as mother, caregiver, homemaker, breadwinner and wife . It is claimed that men cannot handle stress as well as women do because they have different ‘friends’ that women have . Women think of family members as friends because they are the closest companion and will turn to them first when they have any trouble. They have a superior social support system and develop mutuality and interrelationships in their friendships. Men, however, viewed colleagues at work as friends and are less likely to turn into them in times of stress and troubles because of their ‘masculine pride’. Men considered themselves to be tough, independent and have high self-esteem and dignity. Moreover, studies by UCLA neuroscientists show that men usually approach stress with a ‘fight or flight’ reaction, whereas women cope stress more socially . Handling stress properly is an important factor in mental health and this also suggests why women may live longer than men.
Another possible reason accounting why women have higher longevity is that women know their bodies better than men do and receive more formal health treatment than men . It is said that women recognise their symptoms better and earlier than men and, therefore, will be treated earlier. Men tend to overlook their symptoms until they are significant. Consequently, women manage to endure their younger years into old age better than men because they receive treatment sooner and they take more care of their health.
Taking everything into consideration, in terms of biological and social factors, the benefits women gain considerably outweigh their male counterparts. Despite men enjoying certain physical advantages such as being faster, stronger and taller than women, men age faster and have a shorter life expectancy. It is plausible that over evolutionary history women have adapted biologically to live longer than men because they are responsible for raising children. Nowadays, social and environmental factors have increased the differences between the longevity of men and women. However, if we ignore hereditary, social and environmental factors; the two things that are paramount to prolonging lives are a healthy diet and an active lifestyle.
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