The Harris Benedict formula is often recommended to calculate basal metabolic rate (BMR) in order to determine how many calories we should eat to lose weight. Basal metabolic rate is the energy the body burns in total inactivity and it is not recommended to go below this number. In addition, an “activity factor” is used to provide a number that is representative of daily calories burned. The factor typically ranges from 1.2 (sedentary) to 2.0 (strenuous).
The formula given is:
BMR = 655.09055 + 9.5634w + 1.8498s – 4.6756a
w = weight in kg (divide pounds by 2.2)
s = height in cm (multiply inches by 2.54)
a = age
Using my stats, I end up with a BMR = 1,258.4597 (Love the 4 decimals! It makes it look very precise.)
How accurate is this number? Developed by modern science? Based on a large number of subjects?
Hardly. The Harris Benedict formula was published in 1918. It was based on 103 women (plus 136 men and a number of infants, but I disregard these in my discussion). Basal metabolic rate was measured for these individuals and the results are presented in the graph below.
The horizontal axis shows the weight (in kg) and the vertical axis shows the BMR. Notice how the dots (individual results) are all over the place? I am impressed that they managed to develop a formula out of this huge individual variation. The first two terms of the formula are represented by the line in the graph.
I took my weight as an example to see what the variations were in their subjects. Reading off the graph at 56 kg, I find a low BMR of 1085 and a high BMR of 1635! The high BMR is 50% higher than the lower! The theoretical value (on the line) is around 1300. The individual variation was -20% and +25%.
[The line in the diagram doesn’t include the two last terms that adjust for height and age, but even assuming that the “high” woman is 6’6”, 20 years old and the “low” woman is 4′, 100 years old doesn’t account for the difference.]
So what does this tell me about my calculated BMR of 1,258? That it could be 25% higher (1,573) or 20% lower (1,006)? Perhaps it can deviate even more from the “norm”? Using an activity factor of 1.5 gives me a range of 1,509 – 2,360 as daily calories that my body will use. Not very useful, in my opinion. The only way to find out MY critical calorie level for weight loss is by trial and error.
While the BMR is a good starting point (what else do we have?) be prepared to adjust the target number.
Why doesn’t the medical community spend some money to study these types of questions? To provide tools to people that want to lose weight? The answer is pretty obvious. There is no money in it. Who would fund a study that didn’t lead to needing a medical procedure, pills or products?