Weight Loss Science

Basal Metabolic Rate Equations

April 22, 2010 @ 11:33 am

I've been thinking a lot lately about calories and the basic numbers game that makes up weight loss. Essential to a weight loss effort is understanding Basal Metabolic Rate or Resting Metabolic Rate. In a nutshell, BMR is the number of calories your body needs to burn for basic operations every day (breathing, heart, moving, walking, cellular regeneration, etc). Eat less than your BMR and you start burning your reserve energy ... your fat.

Calculating BMR is a squishy science. It's pretty tough to know exactly how many calories our bodies use, and everyone is different. The BMR for a fat 250-lb guy with 32% will be different than that of a 250-lb guy who's all muscle and 4% body fat. But still, we can get rough estimates using one of three equations.

Harris-Benedict

Harris-Benedict is the oldest equation for calculating BMR, developed in 1919, and is the most widely used. But it's also the least reliable, having the potential to overestimate by at least 5%. The discrepancy was highlighted by Mifflin & St Jeor who came up with the second equation below. But since this is a squishy science, it's better to look at all the equations for a rough estimate of where your BMR actually is.

Mifflin-St Jeor

In the 1990s, Mifflin & St Jeor came up with a new equation for calculating BMR. They found that the Harris-Benedict equation tended to overestimate by up to 5% (more for heavier people), which would mean people could think they have a higher BMR than they do, eat more than they should, and not lose the weight they would expect to lose.

Katch-McArdle

So the first two equations only take into account total body weight, BMI and age. But the reality is that muscle and fat have different energy requirements, so it's important to distinguish between them for the most accurate estimate. Unfortunately, the downside of this equation is that most people don't have a reliable method for calculating their Body Fat percentage, leading to a garbage-in-garbage-out problem with Katch-McArdle. At-home devices are not always accurate, or consistent. Nevertheless, as part of well-rounded view of your BMR, if you have a scale that measures Body Fat %, try this one too.

Handy Tools

So this is a lot of math, and it's in the metric system, which just adds more complexity for those of us in America. For me, I created a behemoth spreadsheet system in iWork Numbers that calculates all sorts of things and predicts my weight loss journey. But I've simplified that a bit for an easy download that calculates your BMR based on all three equations. I also threw in a "Weight Loss Predictor" that mathematically predicts your weight based on your calorie intake and calories burned*

Download the LWD Basal Metabolic Rate Tool (Excel)

* Note the predictor is just an estimate based on mathematical extrapolation. Individual actual results vary from person to person based on a lot of factors, so this tool does not guarantee any particular weight loss at any particular rate.

1 Comment So Far

Reinaldo — Apr 23, 2010 @ 12:18 am