February 27, 2004

How Long Will You Live?
The following discussion is based upon Dr. Roy Walford's book, "Beyond the 120-Year Lifespan". (See reviews, Review of "The Immortalist Manifesto")
My interpretation of these reviews of his book (on Amazon) is given here.

It appears to me that we have a total lifetime calorie allotment of the order of, for a male, about 80,000,000 calories (which, technically speaking, are really kilocalories). How long we live depends upon how fast we spend our calories.
Our Goals and Nature's Goals Are Diametrically Opposed
Living organisms are designed to spend their calorie bequests at a maximum rate because that maximizes growth rates and reproduction rates, thereby best optimizing species... but not individual...  survival. For this reason, if rats are allowed to choose their food ad libitum, they will choose a high-calorie diet that maximizes their growth and reproduction rates, but minimizes their personal survival times. And as anyone who frequents McDonalds is aware, humans operate the same way.
Our Minimum Calorie Requirements
The way organic metabolic systems are constructed, a certain amount of food is required in order to repair and replace cells, and to perform useful work... and, in the case of mammals, to maintain body temperatures.
Wasting Our Inherited Caloric Nest-Egg
If you eat just these calories and no more, you will operate at peak efficiency, utilizing 50% of the calories you consume to maintain a body temperature in the mid-90's1 (ºF), to carry out repairs and replacements, and to think and to act2.

1 - When you're operating at maximum efficiency (that is, in a caloric-restricted mode), your core body temperature will be a few degrees lower than the average temperature of 98.6.
It may of interest to note that people who are going to be long-lived also have body temperatures that are a couple of degrees cooler than average.
2
-  When I climb Suicide Hill three times, I'm lifting something like 60 kilograms a total of 150 meters. Now 60 kilograms of weight is about 600 newtons of force (60 kilograms X 9.8 newtons per kilogram), so the amount of mechanical work I'm doing to lift my 60 kilogram (132-pound) carcass 150 meters is about 90,000 joules, or 90 kilojoules. The calories we eat are really kilocalories, and the conversion factor from kilojoules to kilocalories is 4.155 kilojoules = 1 kilocalorie. Dividing 90 kilojoules by 4.155 gives us about 21.5 kilocalories of mechanical energy required to hoist me up Suicide Hill three times.
Since the maximum efficiency with which my body can extract chemical energy from food is 50%, it will only take 43 (kilo)calories of food to
power my monotonous clamberings up and down Suicide Hill.
Doesn't make exercise sound like a very good way to lose weight, does it?
Actually, I probably spend quite a few other calories in the whole process, since it probably takes a certain amount of energy for my lungs to breathe harder and for my heart to pump harder, and it takes me a while to cool down and slow down afterward. Still, this brings home the fact that most of our calories are spent keeping ourselves warm, thinking, digesting our food, etc., and not usually all that many calories are expended in mechanical work. (It's estimated that the maximum mechanical power output of which we humans are capable is about 1/5th horsepower, or 150 watts.)

If you eat more than the bare minimum number of calories required to maintain your weight... and most of us do... then you have a "grace zone" that allows you to up to 50%-to-70% additional calories before the body begins to store these additional calories as fat. These additional calories are spent generating waste heat and boosting your metabolic rate.  Generally, this spans the range from our minimum calorie requirement of about 1,800 calories a day for a man or about 1,500 calories a day for a woman to about 50% to 60% more calories than the minimum number we require for body maintenance (about 2,600 calories a day for a male or about 2,200 calories a day for a female). If still more calories are ingested, they tend to be stored in the form of fat reserves against a time when food may not be so plentiful. (I've said "tend to" because there are some people who can eat eat and eat and never put on weight... e. g., like yours truly until I reached my late twenties.)
The animal kingdom is designed so that hunger pushes most animals to this higher metabolic rate. As mentioned above, this higher metabolic rate maximizes their rate of growth and their ability to reproduce, which is nature's ultimate goal.