What does BMI Have To Do With Running Anyway?

Twelve weeks ago I limited myself to just two energy bars a day – fourteen bars every week. This had been my routine for quite some time with no ill effect – until this year all at once it was a problem. I hit the panic button.

It has been suggested healthy runners will race about two seconds per mile faster for every pound they lose. Studies have also shown it is much more valuable for a runner to lose fat rather than muscle mass. In reality, there is a fine line between healthy and too thin – the latter causing a decrease in performance.

Dr. Vanderburgh, a runner and physiologist who ran a 3:31 marathon at 170 pounds, decided to devise a weight-age-graded calculator (the Flyer Handicap). It’s a fun little experiment actually to calculate your finish time for various races assuming you were 25 years old and an ideal weight.

The science behind the Flyer Handicap is based on the documented decline in aerobic function, specifically distance run speed, due to increasing age and fat-free mass. 

Buried deep in the report, was a detailed analysis of some “what-if” scenarios.

Consider a man, 45 years, 85 kg, 15% body fat, who runs a 25:00 5K. If he gained 3 kg of fat weight with no change in functional capacity, his 5k time would be 25:56 – 56 seconds slower (Vanderburgh and Laubach 2007). Conversely, if he gained 3 kg of only muscle mass, assuming that functional capacity remained proportionally the same with respect to lean body mass, he would show a net improvement of 13 seconds.

I have made a trip to the Solo Health station at Walmart once a week for twelve weeks and it has given me just enough data to be dangerous. I measured my blood pressure, which was anywhere from 118/69 the day after my first 20-mile run to a low of 84/63 a few weeks ago. My heart rate has been 52 and 71, but generally remains between 58 and 61.

I have virtually banished those tasty wonders of energy from my diet and now consume no more than 0-2 bars per week. You would think I would be skin and bones – most certainly I am not. During these 12 weeks, my weight has stayed relatively stable at 107 with a BMI of 19.

According to The National Institutes of Health:

BMI is an estimate of body fat and a good measure of your patients’ risk for diseases that can occur with overweight and obesity. For adults, a healthy weight is defined as the appropriate body weight in relation to height. This ratio of weight to height is known as the body mass index (BMI). People who are overweight (BMI of 25–29.9) have too much body weight for their height.

For twelve weeks I have watched the BMI number associated with my weight with razor focus. I complained to my husband, “it’s just a ratio of height and weight so the only way to lower it is to lose weight.” Turns out, I would need to weigh just 101 pounds to reach what I believed would be the ideal athletic BMI. That is ridiculous and is exactly what causes some of our young athletes to become deady thin.

Obviously BMI is one reference that can be used to determine ideal weight. A runner could also determine ideal weight based on how well they feel – a better gauge it seems.

One author suggests we run a 10k once every four weeks (95% effort) and record our time and body weight as the level of fitness improves. Ideal body weight is the weight just before time declines for the 10k effort.

Maintaining the proper weight has always been front and center in my life….not a diet so much as a way of life. Research for this post has revealed some interesting lessons:

1. in sports, as in life, there is a point of diminishing return – you can be too thin,
2. it is much more valuable to gain muscle mass than to lose random pounds,
3. better to maintain a weight that is comfortable – everyone is unique and charts are only references, not identities or labels.

My son’s major in college is Psychology and one of his classes this semester is the Psychology of Women….. makes for interesting mother/son conversation for sure. This morning we talked about this post.

He said they had just discussed female athletes who struggle with body image/weight. The experts suggest female athletes consider what their body is able to achieve, not how much their body weighs.

He said to me, “Mom, your body can run a marathon. Think how strong your body has to be to run a marathon.”

It is a powerful conclusion to the struggle of achieving the ideal body mass index. We might be ideal just the way we are.