 Why ultimately do we age? What is the reason behind it? That is given by theories of aging which are known as evolutionary theories. But why do we age is kind of a philosophical question that what is the reason we are aging? But physiological mechanisms are studied in terms of how do we age and there are theories explaining this also and these are error damage theories. So when we are considering the theories of aging we have to look into two different types of theories evolutionary theories and error damage theories and by the way there are hundreds of theories of aging and obviously we cannot cover all of them and there is no point also because if we are talking about covering all 300 theories of aging that is simply means that none of them is explaining aging to the fullest. So we have not yet got the ultimate theory of aging but we will look into some crucial theories of aging. So let us first discuss about evolutionary theory of aging. Evolutionary theory says that in evolution it is such that aging will occur. So what scientists have tried to explain that there is a shadow of natural selection what is that I will just discuss shadow of natural selection in which what they say we will try to understand it by means of this graph suppose x axis it is showing the life span so here it is birth this one is age at maturity and later on it is death and on y axis we are telling the force of natural selection. So what it says is that any mutation which will affect life span later on the end of the reproductive stage so this is probably the end of reproductive stage this will not be carried on with generations understanding any mutation which is occurring here because it is in the reproductive life span it will be carried down with generation but if mutation is occurring here because the reproduction is already stopped then it cannot be carried down with generation so if this mutation is harmful okay if this mutation is harmful or maybe even neutral mutation which has occurred somewhere here okay later on harmful mutation or neutral mutation somewhere early in life but with deleterious late late life effects that will accumulate and maybe these are the mutations which are harmful and they are causing the aging so maybe during evolution such kind of mutations have accumulated and they are we are the beings which are carrying all these kind of mutations so this theory was named as mutation accumulation theory by Peter Medaver but this theory was improved upon by another theory given by John C Williams known as antagonistic pleiotropy and what is this antagonistic pleiotropy it says that mutations which are beneficial early in life and having late life negative effects they are the ones which are being happening because by the virtue of being beneficial in early life or being beneficial in reproductive age same virtue makes them harmful later in life so that is why the term antagonistic let us see some examples of these antagonistic pleiotropy example being higher level of testosterone in males if males have higher level of testosterone then the chances of reproductive success are high in reproductive life but later on same high levels of testosterone there is more risk of testosterone carcinoma later in life isn't it so it is kind of a trade which is okay beneficial in the beginning good we will be there but later on that is harmful similarly maybe high levels of IGF levels if high levels of IGF are there then in the beginning of life it is very good for growth but it has been shown that IGF levels have a higher risk of cancer and premature death so their high levels later in life is not good so these are examples of antagonistic pleiotropy there are other many examples are there coming to next theory that is the nutrient sensing theory what is this nutrient sensing theory it is said that there are pathways in the bodies which sense the nutrients presence of the nutrients so presence of the oxygen presence of nutrients like glucose amino acids fatty acids and presence of the hormones which are active and all these are presented so whenever there is high levels of glucose high levels of insulin is present so there are pathways which sense the presence of the nutrient and this is important for the regulation of cell proliferation and cell metabolism now shortage of these factors decreases the activity of torque kinase and what happens that it decreases the cell metabolism and decreases the cell proliferation and it kind of helps us sustain the bad times because when these nutrients are not available these are bad times and survival is difficult but by shutting off the cell metabolism and cell proliferation this mechanism is helping us survive harsh life threatening environments it is kind of decreasing the bodily functions so that will help us prolong the life span so this is a nutrient sensing theory and in fact based on this theory it has been found that calorie restriction and diet restriction is one of the most effective way of slowing down the aging process and extending the life expectancies so by overconsumption of the calories we are just not becoming obese and accumulating lot of predisposition to lot of diseases rather we are also accelerating our aging process in fact calorie restriction and exercising are the two main ways which have been shown to slow down the aging process so for exercise we will discuss later on let us come to another theory that is the disposable soma theory we are not giving much importance to maintenance and repair that is the soma soma is body so maintenance and repair of body is given less importance compared to the growth and reproduction so suppose these yellow balls are showing the cellular energy generated from the nutrients you see that this energy is more invested in growth and reproduction compared to that to maintenance and repair there are only five balls in this case so what it says that by investing more energy in growth and reproduction it is basically causing the proliferation of the species however it is less in maintenance of our own body and because of this the life span is a smaller here this balance is showing this is the long life span and here this is the short life span so more and more we invest in growth and reproduction you see that this part is going to go up and the life span will be shorter one example being the animals which reproduce very fast have a shorter life span so that is what is said by disposable soma theory now let us come to talk about the theories of how do we age so one theory is rate of living theory which says that increase in the rate of living causes increase in the rate of auto utilization and ultimately it causes premature death so any animal who is having very high metabolism is going to have premature death so their life span will be much smaller and how this is explained is by two ways one is the error damage theory so this error damage theory that is the free radical theory states that the more oxygen we consume in the mitochondria 1 to 2 percent of this oxygen how much ever oxygen we are consuming 1 to 2 percent of it is converted to free radicals so you can understand if metabolism is higher and more oxygen is being consumed then more free radicals will form and these free radicals in turn cause the damage to the different cellular aspects so they damage the mitochondria as well and if the mitochondria is damaged then it will initiate a positive feedback with generation of more and more free radicals normally whatever free radicals are being produced they are countered by antioxidants which are present in our body but if the generation of free radicals is more as we are telling with damage to the mitochondria or with more and more oxygen consumption then these antioxidant systems will be overwhelmed and this free radical damage will be very high so these free radicals cause oxidative damage in cells tissues oxidize the proteins DNA forms lipids and DNA is the most critical target however most of the damage which is causing the DNA most of them is repaired but you see the more the damage there is increased probability that it may not be repaired okay so this explains the one method of rate of living theory another is Havel Flick's theory of limited number of cell divisions what it says is that the number of divisions which the cell can have are limited so say suppose there is a cell which can undergo cell division hundred times okay now these hundred cell division can occur in less time maybe in five years or these cell divisions can occur in seven years depending on how fast the cells are proliferating now why is it limited that it can divide only hundred times that is because there is a cap on genome known as telomere and this cap doesn't duplicate that means every time the cell is proliferating you see DNA is going under going duplication but this end end part of this cap small part doesn't duplicate and what happens that maybe after hundred times there will be no cap on the genome and the genome will wither out so this is kind of maintaining the structure of the genome so after a critical length there can't be any further cell division and when such cell dies we cannot further increase the number of the cells so this Havel Flick's theory of limited number of cell divisions is explained by the presence of a telomere and remember that oxidative damage also shortens the telomere so one is that it already has a fixed number of divisions but this telomere is further shortened by oxidative damage fine so before we said that exercise decreases aging let us see how exercise decreases aging well when we exercise there is increase in energy expenditure yes but you see body balances everything whenever there is increase in energy expenditure what it does is decreases the metabolism of other cells okay so by decreasing the metabolism of other cells energy expenditure is brought to normal that is why they say that it is very difficult to lose weight just by exercising because the increase in energy expenditure is balanced by decrease in energy expenditure in other cells and when that happens there is decrease in the stress response there is decrease in the reproductive hormones and there is decrease in the immune activity also but with moderate amount of exercise these and all are not harmful rather the decrease in metabolic activity in the cell increases the life span but when there is extreme physical activity it has negative effect on body because of too much decreased function of other cells and it is seen in one phenomenon known as female athlete triad where there is a low energy availability it leads to actually eating disorders there is the oligomenorrhea due to decrease in reproductive hormones and there is decrease in bone density so too much exercise is also not good that is having negative effect on body