• Loss of muscle mass and tone, decreased muscle to fat ratio
• Loss of bone density
• Loss of flexibility, joint disorders such as arthritis
• Deterioration of lung elasticity and capacity
• Disorders of the circulatory system - decreased sensitivity of baroreceptors
• Degenerative disorders of nervous system - e.g. tremor, Parkinson's disease
• Sensory and cognitive impairment
• Psychiatric conditions - depression, anxiety, dementia
• Reduced immune function
• Reduced reserve capacity (slower recovery from exertion, injury or disease)
• Sleep disorders
• Impaired glucose tolerance and insulin sensitivity, strongly linked to abdominal obesity
• Further complications can occur as side effects of medication, or medication may mask symptoms of new diseases [2]

There are many and varied theories of aging, among them the following:

The free radical theory of aging states that over time, cells accumulate oxidative damage caused by free radicals which are the normal by products of metabolism. Aging is characterised by a decline in ability to neutralise free radicals [2].

The rate of living theory states that lifespan is inversely related to metabolic rate. It appears to apply to many species; however a notable exception is birds.

Another theory suggests that lack of protein turnover may cause aging.

Evolutionary theories of aging suggest that the previous generation age and die to make way for their offspring, maintaining genetic diversity within the population.

According to the programmed aging theory, aging is genetically programmed, and age related changes in cellular function result in increasing susceptibility to disease and eventually lead to death [2].

Theories of aging based around programmed cell death (apoptosis) imply that as people age, more of their cells start to decide to die.

The cell division limit theory states that there is a specific limitation on the number of divisions that somatic cells might undergo.

The telomeric theory of aging postulates that as telomeres (regions of repetitive DNA at the ends of chromosomes) shorten each time a cell divides; this leads damage to essential DNA. This results in cellular damage due to the inability of the cell to duplicate itself correctly. Elevated levels of oxidative stress and inflammation further increase the telomere attrition rate [12], [28]. This theory ties in with the free radical theory and the cell division limit theory.

Other theories ascribe age related problems to the accumulation of random genetic errors over time, also decline in DNA repair capability of cells.

Many of these theories are interlinked, and all appear to have some validity, but a definitive answer has not yet been found.
 

There are many known benefits to the regular practice of yoga, which would help to minimise many of the problems associated with aging.

Regular exercise (asana) can help to maintain muscle strength and tone and bone density, joint flexibility, and improve posture, balance and maintain mobility. Combined with pranayama, regular practice can help to maintain circulatory and respiratory health.

Yoga has also been shown to be beneficial in the management of stress, anxiety and depression, aiding in the maintenance of mental health.

A vegetarian diet can also aid in extending life - it has been shown that vegetarians live longer, have less heart disease and lower rates of cancer [4].
 

Caloric restriction in laboratory animals has been shown to have significant impact on that metabolism.

The biological characteristics of animals on CR diets seem to apply to longevity in people. A continuing study in Baltimore by George Roth of the National Institute of Aging concluded that the same biological markers produced in CR animals are evident in the men who are living the longest.

These markers include:

• Lower levels of blood glucose and insulin

• Reduced body temperature

• Less fat in the blood, more HDL (high-density lipoprotein - good cholesterol )

• A steady level of DHEA (dehydroepiandrosterone - a steroid hormone) [14], [29]

Plasma melatonin levels may also represent a possible biomarker of aging in primates [30].

One of the most popular proposed theories by which CR promotes lifespan extension is the rate of living theory. It is hypothesized that a lowering of the metabolic rate results in lowering of reactive oxygen species (ROS) and rate of oxidative damage to vital tissues [27].
 

Type 2 diabetes, heart disease, arteriosclerosis, liver disease, elevated cholesterol and hypertension are among the medical conditions associated with insulin insensitivity and elevated blood glucose levels [2].

Calorie restricted animals show a significantly increased sensitivity to insulin compared to freely fed animals. CR also has a significant impact on insulin sensitivity in humans [3], [16].

With normal aging, people tend to develop abdominal obesity. High levels of intra abdominal fat have been found to be predictive of heart attack risk and also linked to high cholesterol, high blood pressure, high triglycerides. There is a strong association between increased waist circumference, insulin insensitivity and type 2 diabetes. Yoga practice seems to weaken this link [9].

High levels of stress lead to increased cortisol levels, which is associated with higher levels of abdominal fat. Any form of exercise would help to reduce visceral fat and thus reduce abdominal circumference.

It has been suggested that relaxation and stress reduction may not cause overall weight loss, but may result in a healthier distribution of body fat [18]. It has been found that there were favourable metabolic changes in overweight and underactive subjects who practiced restorative yoga [10]. The effect of restorative yoga on body fat distribution would make an interesting basis for further study.

A recent study found that long term yoga practice was associated with increased insulin sensitivity, and significantly lowered fasting plasma insulin levels [9].

In a 45 day study on people with Type 2 diabetes, all patients continued to take conventional medicines. The study group practiced asana and pranayama, while the control group did not practice yoga. The yoga group showed significant improvement in blood glucose, lipid profile and insulin levels and a decrease in BMI (body mass index). The control group showed an increase in weight, and non significant improvement in the other parameters [37].

Reviews of published studies found that yoga interventions are generally effective in reducing body weight and glucose levels [41], [20], [21]. These studies suggest that yoga can have a beneficial effect on glucose tolerance and insulin sensitivity.

Blood lipid profile

HDL (high density lipoprotein - good cholesterol) helps to remove cholesterol from the blood, protecting from cardiovascular disease. Higher levels of HDL are correlated with better health outcomes.

LDL (low density lipoprotein - bad cholesterol) is thought to deposit cholesterol in artery walls, increasing the risk of heart disease. High levels of LDL are associated with atherosclerosis. This includes VLDL (very low density lipoprotein).

Along with reductions in basal metabolic rate (BMR), people on CR diets experienced large reductions in LDL cholesterol, and had very high levels of HDL cholesterol [16].

In a study on normal, healthy volunteers, after 30 days of practicing pranayama, a significant reduction in triglycerides, free fatty acids and VLDL cholesterol along with significant elevation of HDL cholesterol was observed in the men. Free fatty acids were reduced in women.

After adding asana exercises to the pranayama for another 60 days, free fatty acids increased in both men and women, and women demonstrated a significant fall in serum cholesterol, triglycerides, LDL and VLDL cholesterol [26].

It is of interest that free fatty acid levels increased after the subjects started doing asana exercises. It is possible that their bodies were breaking down fat. Further research would aid in clarifying this somewhat contradictory effect.

In another study on patients with coronary artery disease, at the end of one year of yoga training, total cholesterol was reduced by up to 23% in the yoga group of patients, compared to 4.4% in the control group. LDL cholesterol was reduced by 26% in study group patients as compared to 2.6% in the control group. A much higher proportion of the yoga group showed regression and arrest of progression of the disease than in the control group [43].

Studies indicate that pranayama and yoga asanas can be helpful in patients with lipid metabolism disorders [26], and have a positive effect on blood lipid profile, reducing cholesterol [41], [20], [21].

Melatonin

Melatonin is produced by the pineal gland, and appears to have anti aging properties [36], likely due to its antioxidant properties. It is also a natural immune enhancer and has been shown to extend longevity in some animal studies [22]. Higher melatonin levels are also associated with an increased sense of well being.

Melatonin is released mainly at night during sleep. Moderate physical activity has been shown to increase production of melatonin [35].

Calorie restriction has been shown to prevent the usual age related decline in melatonin levels in monkeys [30].

Yoga and meditation have been shown to significantly increase melatonin levels, with regular meditators found to have a higher level of melatonin than non-meditators [19], [40], [34].

In a 2004 study on normal, healthy volunteers, a yoga group practiced asana, pranayama and meditation while a control group did body flexibility exercises, slow running, and played games.

Yogic practices for 3 months resulted in an improvement in cardiorespiratory performance and psychological profile, with an improved sense of well being. The maximum night time melatonin levels in yoga group showed a significant correlation with well-being score.

The yoga group showed an increase in plasma melatonin, indicating that yoga could be used as a psychophysiologic stimulus to increase endogenous secretion of melatonin [19].

In another study, experienced meditators practising either TM-Sidhi or another form of yoga showed significantly higher plasma melatonin levels in the period immediately following meditation compared with the same period at the same time on control nights. It was concluded that meditation, at least in the forms studied here, can affect plasma melatonin levels [40].

Longer term studies are required to ascertain whether the higher melatonin levels in yoga practitioners and meditators are sustained.

Basal metabolic rate and body temperature

Studies measuring metabolic rate in CR animals indicate that it lowers the BMR. CR in animals is associated with a robust decrease in energy metabolism, including a lowering of resting metabolic rate, lowering of the thermic effect of meals and a decrease in the energy cost of physical activity [27], [15].

Some studies measuring metabolic rate in CR animals give conflicting results [24], and lowered metabolic rate does not necessarily entail a prolonged life span [15].

However, specific metabolic rate correlates highly with oxidative DNA damage. This is consistent with the theory that free radical induced DNA damage may play a central role in the aging process [1].

Body temperature, one of the biomarkers of longevity, is linked to metabolic rate; a lowered BMR would be associated with a slightly lowered core body temperature.

It seems logical to expect that because yoga asana is an energy expenditure activity, it would increase the resting metabolic rate. However, two different studies using healthy volunteers found that the BMRs of yoga groups practicing asana, pranayama and meditation were significantly lower than the BMRs of control groups [7] [8]. Asanas when practiced along with pranayama and meditation over a period of time actually significantly reduce the metabolic rate [7] [8].

The metabolic rate is an indicator of autonomic activity. The lower metabolic rates in the yoga subjects may have been due to decreased sympathetic nervous system activity and probably, a stable autonomic nervous system response achieved due to training in yoga [7] [8].

A study on alternate nostril breathing found that breathing selectively through either nostril could have a marked activating effect or a relaxing effect on the sympathetic nervous system; it is possible to alter metabolism by changing the breathing pattern [39].

Hypometabolic states have been reported in yogic studies, and meditation has been described as a wakeful hypometabolic state of parasympathetic dominance [44], [8]. Reports exist of yogis being buried underground in pits for many hours, and emerging unscathed. This may be achieved by consciously and voluntarily entering a hypometabolic state [6], [44].

This suggests not just a general lowering of BMR as a result of practice, but in advanced practitioners, eventually a learned ability to control normally involuntary bodily processes [44]. Krishnamacharya was apparently able to stop his own heartbeat and breath for several minutes with no ill effects - he demonstrated this before a panel of doctors at the age of 76 [13].

Dehydroepiandrosterone (DHEA)

Higher levels of DHEA are associated with greater feelings of wellbeing, higher muscle to fat ratios, and enhanced immune function.

DHEA also seems to increase sensitivity to insulin. Low DHEA levels correlate with lower bone mineral density and higher risk of osteoporosis, and also increased risk of heart disease [23]. A significant deficiency in DHEA in patients with several major diseases including cancer, inflammatory diseases, type 2 diabetes, atherosclerosis, Alzheimer's disease and cardiovascular disorders has been described [32].

Levels of DHEA that occur naturally in the body decline with age.

Calorie restriction has also been shown to increase DHEA levels in animals. DHEA was found to be a very good marker to measure the rates of aging in control versus calorie restricted monkeys [25].

Most forms of exercise will raise DHEA levels [35], so practicing yoga asana would play a role in maintaining DHEA levels.

Studies have shown that meditation is associated with increased levels of DHEA, as well as melatonin and GABA (gamma aminobutyric acid, which reduces anxiety) [33], [17].

Older individuals practicing meditation had higher levels of DHEA than an age-matched control group of non-meditators. [33], [17]
 

Results of further long term naturalistic studies would be informative as to whether the effects described above are sustained over time.

Many of these effects could also result from other forms of exercise. Comparative studies measuring these markers in yogis and athletes of a similar age would be required to find out whether yoga practice has a greater or different effect than other physical activity.

Larger studies are needed - at present only relatively few exist, involving relatively small numbers of people. More studies of diverse populations are required.

Several studies found that yoga interventions are generally effective in reducing body weight, blood pressure, glucose level and high cholesterol, but only a few studies examined long-term adherence.

Animal studies cannot always be directly correlated with humans; the results of animal experiments are unreliable when attempting to extrapolate to humans. Although I have referred to animal studies, personally I believe experimentation on animals to be unethical and often very cruel. What we really need is more human studies.

Furthermore, in her blog [31], Sandy Szwarc points out that in a recent CR study on rhesus monkeys [11], if deaths from factors other than age related diseases are taken into account, there is in fact no significant difference in lifespan between CR and non CR animals. Also, the control group were actually overfed by 20%.

Reviews of other animal CR studies looking at causes of mortality other than old age would be enlightening.

Finally, it may be found that the hypothesised biomarkers of longevity actually have little or no significance in extending longevity. For example, as we find out more about the SIR2 gene, many currently widely accepted theories may be discarded.
 

It is the side effects of CR, the so called biomarkers of longevity , that appear to delay the onset of age related chronic disease and extend longevity.

Insulin resistance seems to be a major factor contributing to age related disease, possibly the most important factor. However the lowered BMR of yoga practitioners is particularly interesting, as it is a curious and rather anomalous effect.

There is a high incidence of obesity related disease in westerners; the average western diet is over caloric. However the more extreme CR has associated health risks.

The risks and possible negative side effects of calorie restriction include hunger, malnutrition, eating disorders, reproductive issues in women, osteoporosis, sensitivity to cold, and slower healing [5]. On top of this there may be as yet unknown long term side effects.

It has been found that some yoga practitioners do in fact practice caloric restriction [6]. The yogis interviewed by Dr Bushell at the Kumbha Mela festival in 2001 followed a classic Indian form of CR based on 1-2 small meals a day consisting of legumes, milk, and augmented with fresh vegetables and fruit.

Simon Borg-Olivier lives on a diet of mostly raw fruit and vegetables, and seems extremely strong and healthy. He would make a particularly interesting case study; it seems possible that he could live to a very old age.

All of the physiological factors - a lowered BMR, with increased insulin sensitivity and lower insulin levels, increased melatonin and DHEA, and better cholesterol levels - appear likely contribute to longevity and health into old age with consistent yoga practice.

The combined metabolic effect of the practices of asana, pranayama and meditation may work in synergy with other physical effects, possibly setting yoga practice apart from other forms of exercise in delaying or preventing the onset of age related disease.

Yoga practice is a natural and healthy way to potentially achieve many of the benefits of CR, without the associated risks, and in this manner to aid in slowing the aging process.