The principles of thermodynamics are not confined to Bénard cells and ecosystems. They also shed light on the functioning of that most familiar complex system, one's self. Here we show how thermodynamics underlies aging, and how exercise—a form of energy flow—leads to healthier lives. Like other complex systems, human living is an energetic process. As such it is illuminated by our gradient-based thermodynamics.

Medical researchers consider VO2 max to be an excellent gauge of cardiovascular health. After the age of thirty-five, athletes lose their performance capacity as measured by this gauge at a rate of 0.5% a year Non-exercisers by contrast lose their performance capacity at a greater rate, 2% per year. And although the difference between couch potatoes and exercisers may seem trivial, it is compounded! "[T]he effect of intervention may be small . . . yet, when the 1.5% per year difference is multiplied by decades, the difference becomes profound." What the Bortzes are saying is that for each decade of exercise that the runner ages five years (as measured by performance), the non-exerciser ages twenty years. These results are astounding, and their implications should be obvious to a global health-conscious society annually spending trillions of dollars on health care. The first thing that these data reveal is that the average active person will physiologically age 1% per year after the age of thirty-five. This is normal aging. The super-athletes among us can slow that rate to 0.5%, and the unfit of our society will physiologically age at 2% per year. After twenty years the unfit will have physiologically aged forty years and the fit just ten years. If this is true, a fountain of youth has been discovered—and its thermodynamic basis is energy flow. By pushing the body, using it energetically after the manner of our ancestors, the effects of aging can be forestalled. Thermodynamic systems require energy flow; inactivity, disuse of the cardiovascular system decreases the vitality of human life.

Yates prefers the word senescence to describe damage and harm associated with aging. "It is senescence, not aging that is the prelude to death from old age.” Yates believes that health is a synonym for stability. Poor health is a sign of instability, and the ultimate instability is the collapse of the dynamics of the system that we know as death. Yates sees senescence and death as the result of either component failure or a total system failure. Component failure can result from the disruption of any of a number of processes, such as healing, protecting DNA, guaranteeing fidelity in the replication of DNA, eliminating wastes, protection against free radicals, and the deterioration of immune abilities, as seen for example in AIDS. "System death occurs when a constellation of interrelated parts and processes experiences a shrinking dynamic range beyond some critical minimum [needed] for stability in a fluctuating environment." System death arises from changes in multiple constraints, so that stability suddenly gives way.

Yates agrees with the Bortz use-it-or-lose-it argument. Without adequate energy flow, biological systems atrophy. Also Yates elaborates on an issue that all super-athletes know; one can also use it and lose it. Yates notes that, above a range of some 2,000–3,000 kilocalories per week of exercise, anabolic yield decreases, with oxidative damage and wear and tear playing prominent roles. The figure is Yates's attempt to graph these ideas. The coordinates show increasing activity rates of the organism—basic metabolic rate, sedentary, active, and superactive—and energy throughput. Note that the energy flow increases with activity. The anabolic yield curve is most interesting. Anabolic yield measures the constructive part of metabolism, the buildup of muscle tissue, for instance. Yates depicts it reaching an optimum at about 2,500 kilocalories per week. Below that one is in the "use it or lose it" range; above 2,500 kcal, one is in the "use it and lose it" stage of hyperactivity.

Yates likens the anabolic-yield curve to the increasing efficiency and torque in a combustion engine. Increasing revolutions per minute helps generate more power, but only up to a point. Above that value, increasing revolutions per minute will decrease power and efficiency. Here is another example of a biological system operating optimally not at a maximum or minimum but within a (narrow) range. Maximum energy flow harms and degrades the organism and facilitates senescence. It burns one out. Minimum energy flow leads to atrophy and stagnation, fading away.

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Part IV: The Human

18. Health, Vigor, and Longevity

19. Economics

20. Purpose in Life






Human vitality and frailty scaled by maximum oxygen consumption (VO2 max) versus age. After the age of thirty-five years, humans begin to age as measured by loss of cardiovascular capacity, VO2 max. This is normal aging. However, the rate of loss of cardiovascular health depends on the amount of exercise one gets. Highly fit people lose VO2 max at 0.5% a year, the moderately fit age at 1% a year, and the lethargic age at 2% a year. Over twenty years the least fit will metabolically age about forty years, while the most fit will age only ten years. Disease and injury can accelerate this aging process. (Data from Bortz and Bortz 1996.)











The abscissa of this graph is human activity level, exemplified by sedentary, active, and highly active people. The ordinate is the energy throughput or energy use of the subjects. As expected, energy throughput or use increases with activity level. Of interest is the anabolic yield, or the constructive cell building of metabolism, which shows that there is an optimal level of exercise or activity level. Gene Yates estimates this optimal level to be about 2,500 kilocalories of exercise a week. Below that level the body loses metabolic capacity, and above that level the body starts to lose capacity through wear and tear and accelerated metabolic breakdown. (Adapted from Yates and Benton 1995.)