THE BLOG

How Long Will You Live? Body Size, Heart Rate and Metabolism

05/04/2015 03:38 pm ET | Updated May 04, 2016

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We all run out of time. Some have much longer than others. We don't give it much thought. The young don't think about it at all. The middle-aged don't dwell on it, they act out. But as we grow older, like checking our watch, we anticipate an end.

How do we make sense of the lifespan lottery?

The Arctica Islandica, an ocean quahog, can live 400 years. The Bowhead Whale swims for 200 years. Some humans may live for a century.

Why?

Why does a queen bee live one year and a worker bee live one month?

Why does a catfish live 60 years and a cat live 20?

Believe it or not, a considerable body of research addresses these questions.

The observation that large animals tend to live longer than smaller ones dates back to Aristotle and kicked off speculation about why. This question had to wait until the 1800s to be expanded upon. The industrial revolution gave rise to models of man as machine. Things fall apart. If machines wear out faster the more they are used, why not people?

Metabolic rate was easily likened to running a machine. In exploring this idea, early 20th century experiments revealed a connection between size and metabolic rate. As animal size increased, metabolic rate decreased.

This may seem counterintuitive but makes sense from the perspective of maintaining body temperature, a mammalian necessity. As body volume increases, surface area increases more slowly. An elephant radiates and loses less energy per pound than a mouse. Therefore the elephant needs to generate less energy per pound. Hence elephants have lower metabolic rates than mice.

These observations were then applied to the association of size and longevity. The larger the animal, the lower its metabolism and the longer it lived. Smaller animals have higher metabolic rates and shorter lifespans.

In other words, there seems to be a fixed amount of life that may be consumed at different rates. These ideas coalesced as the Rate of Living Theory.

Resting heart rate provides an easily measured variable to demonstrate these principles. Because the heart is generally a fixed percent of total body mass, the heart rate must correspond to the total metabolic demands of the body.

Indeed mammals that have lower resting HRs generally live longer than those with faster HRs. Despite dramatic differences in the life span of species, there appears to be ~1 billion heartbeats in a lifetime, whether man or mouse.

Great. We have a slower heart rate than mice and we outlive them. But what about within a given species? Do people with slower heart rates outlive their contemporaries whose hearts beat faster?

The answer appears to be yes.

The data suggest that this is true independent of cardiovascular disease. Of course low heart rate is associated with physical fitness, a condition that confers longer life. A recent Danish study however indicates that heart rate is not merely a marker of physical fitness but an independent risk factor for mortality.

In fact the predictive power of heart rate for mortality is higher than that of cholesterol or blood pressure.

What might this tell us about extending lifespan?

If we slow our heart rate do we buy time?

In attempting to answer this question researchers did what researchers do. They tried it with mice. A medication-induced reduction in heart rate of 14 percent yielded a 6.2 percent increase in median lifespan. Not bad.

Many studies have demonstrated the life-extending effect of drugs that lower heart rate in cardiovascular patients. Although no studies have been conducted to show that heart rate reduction prolongs life in healthy humans, there is a shedload of data pointing in that direction.

Studies have however documented a continuous increase in risk with heart rates above 60 beats/min. This suggests that a substantial lowering of the upper limit of the normal range of resting heart rate (60-100) would be desirable.

So what's the best way to slow heart rate?

Make it go faster.

Yup, physical activity. During endurance-type activity heart rate increases. But it allows for a lower heart rate the remainder of the day. This slowing stems mainly from two changes. Exercise strengthens the heart allowing more blood to be pumped per beat and therefore a reduction in beats per day. Secondly, regular exercise mutes the fight or flight signal coming from the autonomic nervous system. As we grow older sympathetic tone increases causing much of the damage associated with aging.

Sure you could use drugs (beta blockers and others). Or stop using drugs (caffeine, alcohol, tobacco, cocaine, amphetamines, and others). But we are made to move. Science will never outsmart evolution.