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.

2 comments | Topics: BMR, science, weight loss tools

Weight Loss Is A Numbers Game

April 13, 2010 @ 10:26 am

A number of the bloggers I follow don't weight themselves with any regularity ... or at all. The idea is that one shouldn't let their lives and weight loss efforts be ruled by a number on a scale; it emphasizes that this journey is about overall health and well being, about how clothes fit, etc. It's totally valid, and I agree with not weighing oneself daily, but I can't avoid the scale entirely.

When I avoid the scale, bad things happen. Humans tend to have a skewed perspective on themselves; we're really good at fooling ourselves that we've not eaten that much, or that we're losing weight when nothing is further from the truth. I'm the king of this - and so I need a frequent reality check to ensure my perceptions are still grounded.

The Game

Whether you use a scale or not, weight loss is a numbers game. Number of calories in, number of calories spent. Spend more than you've eaten and you start tapping your reserves, a.k.a. fat. If you tap 3,500 reserve calories you lose a pound of fat. Then repeat the process 100 times.

The key is knowing your basal metabolic rate (BMR) - this is the number of calories your body needs to fuel its daily operations. For me, it's around 2,200 - 2,300 (all equations are only estimates). That means to maintain my weight, I need to eat that many calories. If I eat less than that, I'll start burning reserve calories, which eventually whittles away at my fat stores.

You would think staying under 2,000+ calories is an easy task, that feels like a lot of food, until you start eating a lot of breads, pastas, and pints of Ben & Jerry's in a single evening. When I think about what I've been eating this week compared to what I must have been eating before, it astounds me. Especially when you consider how much exercise I was getting.

Exercise affects the numbers game by essentially giving you wiggle room. If I eat 1,500 calories a day, that's a deficit of 3,500 a week, or 1 pound. If I also exercise and burn 500 calories a day, that's a deficit of 7,000 for the week - or 2 pounds. The key is net calories - calories eaten less calories burned. Measure that against BMR and you have your deficit (or God forbid, a surplus).

The Awesomest Weight Loss Spreadsheet Ever

All this is basic weight loss that anyone on this journey comes to understand. But it's been refreshed in my head because it's become painfully clear that unless I focus on the numbers side of weight loss - and track what I'm actually doing - I will fail. So being a business & tech nerd, the obvious tool to turn to is a spreadsheet. I may have gone overboard. Click to see a full version.

14 comments | Topics: BMR, calories, food and eating, science, weight loss tools