Longevity is defined as long life or the length of a person's life (life expectancy). Reflections on longevity have usually gone beyond acknowledging the basic shortness of human life and have included thinking about methods to extend life. Longevity has been a topic not only for the scientific community but also for writers of travel, science fiction and utopian novels. The longest human lifespan on record that has been authenticated is the 122 years 164 days of Jeanne Calment, though fiction, legend, and mythology have proposed or claimed vastly longer lifespans in the past or future and longevity myths frequently allege them to exist in the present.
From Matt Ridley's Genome:
"Each species, it seems, comes equipped with a program of planned obsolescence chosen to suit its expected life-span and the age at which it is likely to have finished breeding. Natural selection carefully weeds out all genes that might allow damage to the body before or during reproduction. It does so by killing or lowering the reproductive success of all individuals that express such gene?s in youth. All the rest reproduce. But natural selection cannot weed out genes that damage the body in post-reproductive old age, because there is no reproduction of the successful in old age. Take Dunnet's fulmar, for instance. The reason it lives far longer than a mouse is because in the life of the fulmar there is no equivalent of the cat and the owl: no natural predators. A mouse is unlikely to make it past three years of age, so genes that damage four-year-old mouse bodies are under virtually no selection to die out. Fulmars are very likely to be around to breed at twenty, so genes that damage twentyyear-old fulmar bodies are still being ruthlessly weeded out." ()
Various factors contribute to an individual's longevity. Significant factors in life expectancy include genetics, access to health care, hygiene, diet, exercise and lifestyle. Below is a list of life expectancies in different types of countries
|77-83 years (eg. Canada: 80.1 years, 2005 est)
|65-77 years (eg. Russia: 67.1 years, 2005 est)
|35-60 years (eg. Mozambique: 40.3 years, 2005 est)
Tobacco smoking is generally accepted to significantly reduce longevity, and is one of the main statistical factors explaining differences in life expectancy between advanced nations. This may be offset by other factors; Japan, a country with a high rate of tobacco consumption, has one of the highest life expectancies in the world (81.15 years, 2005 estimate). Hong Kong, a dense 7 million people city with constant stress, recently reported a higher life expectancy than Japan (81.39 years, 2005 estimate)
Population longevities can be seen as increasing due to increases in life expectancies around the world
The mainstream view on the future of longevity, such as the US Census Bureau, is that life expectancy in the USA will be in the mid-80s by the year 2050 (up from 77 today) and will top out eventually in the low 90s, barring major scientific advances that can change the rate of human aging itself, as opposed to merely treating the effects of aging as is done today. The Census Bureau also predicted that the USA would have 5.3 million people aged over 100 in 2100.
Recent increases in the rates of obesity-related diseases, such as diabetes, hypertension, and heart disease, may however drastically slow or reverse this trend toward increasing life expectancy in the developed world.
Caloric restriction and longevity
Calorie restriction or Caloric restriction (CR) is the practice of limiting dietary energy intake to improve health and retard aging. In human subjects, CR has been shown to lower cholesterol and blood pressure. Some consider these to be biomarkers of aging, as related diseases are more frequent with increasing age. Except for houseflies (below), animal species tested with CR so far, including primates, rats, spiders and rotifers, have shown lifespan extension. CR is the only known dietary measure capable of extending maximum lifespan, as opposed to average lifespan. Energy intake must be minimized, but sufficient quantities of vitamins, minerals and other important nutrients must still be ingested. To emphasize this, CR is often referred to by a plethora of other names such as CRON or CRAN (calorie restriction with optimal/adequate nutrition), or the "high-low diet" (high in all nutrients aside from calories, in which it is "low").
Caloric rstriction and Sir2 (SIRT1)
Recent discoveries have suggested that the gene? Sir2 might underlie the effect of CR. In baker's yeast the Sir2 enzyme is activated by CR, which leads to a 30% lifespan extension in test subjects. David Sinclair showed that in test mammals the Sir2 equivalent gene known as SIRT1? is turned on by a CR diet, and this protects cells from dying under stress. Leonard Guarente showed that SIRT1 releases fat from storage cells. Sinclair's lab reported that they have found small molecules (e.g. resveratrol?) that activate Sir2 and can extend the lifespan of yeast.
Free radicals and glycation
Two very prominent theories of aging are the free radical theory and the glycation? theory, both of which can explain how CR could work. With high amounts of energy available, mitochondria? do not operate very efficiently and generate more superoxide. With CR, energy is conserved and there is less free radical generation. A CR organism will be less fat and require less energy to support the weight, which also means that there does not need to be so much glucose in the bloodstream. Less blood glucose means less glycation of adjacent proteins and less fat to oxidize in the bloodstream to cause sticky blocks resulting in atherosclerosis. Type II Diabetics are people with insulin insensitivity caused by long-term exposure to high blood glucose. Obesity leads to type 2 diabetes. Type 2 diabetes and uncontrolled type 1 diabetes are much like "accelerated aging", due to the above effects. There may even be a continuum between CR and the metabolic syndrome.
In examining Calorie Restriction with Optimal Nutrition, it is observed that with less food, and equal nutritional value, there is a higher ratio of nutrients to calories. This may lead to more ideal essential and beneficial nutrient levels in the body. Many nutrients can exist in excess to their need, without side effects as long as they are in balance and not beyond the body's ability to store and circulate them. Many nutrients serve protective effects as antioxidants, and will be at higher levels in the body as there will be lower levels of free radicals due to the lower food intake.
Glycated proteins produce 50-fold more toxic free radicals than nonglycated proteins. As a result of this, AGE exert multiple detrimental effects in the body. For example, AGE induced free radicals activate the proinflammatory [Cytokines? cytokine] TNF-a [Tumor Necrosis Factor alpha (TNFa)? (tumor necrosis factor alpha)], known to be elevated in the elderly. TNF-a has been shown to be particularly high in inflammatory diseases of the joints (like rheumatoid arthritis), central nervous system (Alzheimer's disease, multiple sclerosis and ischemia) and is considered to promote neurodegeneration.
Objections to longevity as a result of caloric restriction
A major conflict with calorie restriction is that a calorie excess is needed to prevent catabolizing the body's tissues. A body in a catabolic state promotes the degeneration of muscle tissue, including the heart. It also makes gaining muscle tissue difficult. Loss of muscle tissue is a strong indicator of aging.
Physical activity testing biases
While some tests of calorie restriction have shown increased muscle tissue in the calorie-restricted test subjects, how this has occurred is unknown. Muscle tissue grows when stimulated, so it is possible that the calorie-restricted test animals exercised more than their companions on higher calories. The reasons behind this may be irrelevant, as in any case it would be a bias in testing. Such tests need to be monitored to make sure that levels of physical activity are equal between groups.
Insufficient calories and amino acids for exercise
Exercise has also been shown to increase health and lifespan and lower the incidence of several diseases. Calorie restriction comes into conflict with the high calorie needs of athletes, and may not provide them adequate levels of energy or sufficient amino acids for repair.
Intermittent fasting as an alternative approach
Studies by Mark P. Mattson, PhD, chief of the National Institute on Aging's (NIA) Laboratory of Neurosciences, and colleagues have found that intermittent fasting and calorie restriction affect the progression of diseases similar to Huntington's disease, Parkinson's disease, and Alzheimer's disease in mice () In one study, rats and mice ate a low-calorie diet or were deprived of food for 24 hours every other day (). Both methods improved glucose metabolism, increased insulin sensitivity, and increased stress resistance. Researchers have long been aware that calorie restriction extends lifespan, but this study showed that improved glucose metabolism also protects neurons in experimental models of Parkinson's and stroke.
Another NIA study found that intermittent fasting and calorie restriction delays the onset of Huntington's disease-like symptoms in mice and prolongs their lives (PMID 12589027). Huntington's disease (HD), a genetic disorder, results from neuronal degeneration in the striatum. This neurodegeneration results in difficulties with movements that include walking, speaking, eating, and swallowing. People with Huntington's also exhibit an abnormal, diabetes-like metabolism that causes them to lose weight progressively.
Does blood type influence lifespan?
As perhaps expected the results are conflicting. A study of Italian physicians showed a higher percetage of those over the age of 75 were type O () while another study showed that type B was associetd with a higher lifespan. ()
Group B individuals on average appear a bit healthier than their counterparts. Since they tend to fall almost invariably between A and O with regard to disease susceptibilities, this 'tempering effect' might be expected to translate into a higher percentage of type B individuals attaining a more advanced age.
The NN subtype of the [MNS Blood Group]?ing system may be associated with a slight increase in longevity (especially in women) ()