When it comes to something as life and death important as to what to feed ourselves and our families,

we should rely not on anecdotes, but on facts.

Welcome to the Nutrition Facts podcast.

I'm your host, Dr. Michael Greger.

Today we continue with our series on exercise as medicine, and I have to say I was surprised by how much

controversy there was in the medical literature over whether the apparent longevity benefits

of exercise are even real.

Whether 6% of premature mortality is related to physical inactivity, 9% or even 15%?

These estimates are all predicated on the presumption that the associations found linking

inactivity to death rates and observational studies constitute cause and effect.

A classic study from the 1950s entitled, Coronary Heart Disease and Physical Activity of Work,

illustrates how difficult it can be to tease out causality

between physical activity and longevity.

London bus drivers appeared to have twice the risk of dying from heart disease

compared to bus conductors,

who were presumed to be protected by their climbing up and down 500 to 700 steps a day in the famous double-deckers.

However, it was later revealed in a follow-up paper physique of London busmen that, based on the measurements of their starting uniforms,

the bus driver started out significantly heavier. Similar issues surrounding reverse causality continue to haunt observational

exercise studies to this day. Is exercising enabling good health in seniors, or is good

health enabling exercise in the first place?

Is inactivity leading to chronic disease or is chronic disease leading to inactivity?

Then there are the confounding factors, the archetypal one being less smoking among active individuals.

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