The most accurate means for estimating BMR and RMR appears to be through direct or indirect calorimetry which measures carbondioxide production. Since we can't do this on our own, I won't bother discussing the related formulas. I would very much like to know where you can go to have your RMR measured this way.
There are numerous formulas for estimating BMR based on your weight, height, age, gender, and physical composition (ex: lean body mass). All of these formulas appear to be based on statistics which are most accurate for the most average type of body. Which means that none are extremely accurate with people who's bodies are at extremes. And, even when physical attributes are the same, studies have shown a good deal of variance in actual vs. estimated BMR from person to person. So any of the formulas of calculating BMR will be imperfect. Because of this, I'm taking the most common formulas for calculating BMR and comparing the results.
Wikipedia has a nice article on BMR that shows the following formulas. I will plug in my own stats to see how they compare. Please note that these formulas are based on average body types and any extreme may throw off the results.
First, the Harris-Benedict equation:
For Men:
For Women:
where P is total heat production at complete rest, m is the weight, h is the height, and a is the age, and with the difference in BMR for men and women being mainly due to differences in body weight
For me, my P = 2801 calories / day.
Then came the Mifflin Equation around 1990 (some studies show it to be 5% more accurate):
where P is total heat production at complete rest, m is the weight, h is the height, and a is the age, and s = +5 for males & -161 for females.
For me, my P = 2460 calories / day.
The Katch-McArdle Formula for BMR:
where LBM = Lean Body Mass in kg
For me, my P = 2133 calories / day.
The Cunningham Formula (RMR):
where LBM = LEan Body Mass in kg
For me, my P = 2295 calories / day.
These last two are thought to be more accurate because lean body mass burns calories.
Adjustments for activity level:
To adjust for activity level, multiple any of these by 1.2 to 1.9 (1.2 being the least active and 1.9 being highly active).
| Little to no exercise | Daily calories needed = BMR x 1.2 |
| Light exercise (1–3 days per week) | Daily calories needed = BMR x 1.375 |
| Moderate exercise (3–5 days per week) | Daily calories needed = BMR x 1.55 |
| Heavy exercise (6–7 days per week) | Daily calories needed = BMR x 1.725 |
| Very heavy exercise (twice per day, extra heavy workouts) | Daily calories needed = BMR x 1.9 |
Based on these formulas, and factoring in little to no exercise (1.2 multiplier), my body needs between 2560 and 3361 calories per day to maintain my current weight. That is a huge range. My first thought is that any formula that doesn't take into account lean body mass probably is not very accurate unless you are pretty close to the average physical make-up. This is because muscle burns more calories than fat. But let's put it to the test.
I have been keeping a food journal for the last few days but not on any special diet. My reason for doing this was to understand my typical calorie intake so that I'd understand how I got where I am and also the degree of adjustment I'll be making when I begin a diet. Based on just a few days in my journal, I'm estimating that I consume on average between 3000 and 3200 calories per day. Let's just say 3100 per day. About this time 2 years ago, I weighted 307 pounds. I now weight between 335 and 340 pounds. Let's say I gained 33 pounds in 24 months. Generally it is thought that a pound of fat contains 3500 calories. So, to gain 33 pounds, I'd have to have had a surplus of 115,500 calories over these two years which is 158 calories per day. If I am consuming on average 3100 calories per day, that means to have maintained my original weight of 307 pounds, I would have had to have stayed under 2942, which is pretty darn close to Mifflin equation with a 1.2 multiplier for activity. Therefore, for me personally, it would appear that the Mifflin equation is the right calculation for my body type. My BMR according to Mifflin is 2460. Multiply this by 1.2 and you get 2952 which is the amount of calories I can eat without gaining or losing weight.
Aha! So, now, I have something I can work with. I will form my weight loss program around the fact that using the Mifflin formula my BMR w/ activity factor is 2952. I can now decide how much I want to lose over a given period of time and estimate how many calories under 2952 I need to consume to meet my goal. And, of course, I'll need to re-calculate my BMR occasionally and adjust if and when I begin an exercise program.
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