The fitness world is full of mathematical equations. We use math to try to reach our goals all the time, sometimes without even realizing what we are using is nothing but another mathematical equation. We use math to count our calories and our macros. We use math to track how many calories we burn while wearing things like heart rate monitors. We use math to get an approximate 1 rep max when we don’t want to actually go for that max or use a percentage of our 1 rep max to get an idea of what kind of weight to use for a certain rep scheme. We use math to calculate body fat, BMI (don’t even get me started), BMR, heart rate, max heart rates, target heart rates, and the list goes on and on.
With all this math available I wonder if we sometimes forget one very important detail. Our bodies aren’t a math formula. We all know what works for one person doesn’t necessarily work for another. Even two people with the exact same gender, height, weight and body fat will still have big variations in how their bodies work and respond to different training and nutrition stimulus. With that I have to strongly question where this whole 3500 calories equals 1 pound of fat came from and how much truth there is to it.
I suppose for simplicity sake it makes sense to have a number for people to refer to. Burn 500 calories more than you consume each day and lose a pound a week. Sounds pretty simple doesn’t it? It does, but as you have probably come to realize it is never that easy. Even as a major supporter of the IIFYM diet I realize that all calories are not created equal and certainly genetics play a huge role. 3500 calories to a true hard gainer and 3500 calories to a naturally heavy-set person are very different.
This doesn’t even go into things such as metabolic adaptation. We know that metabolism slows down when there is a caloric restriction, there is no way around it, we simply try to minimize it as much as possible. (On that topic if a coach or trainer ever tells you they can speed up your metabolism while putting you in a caloric deficit you need to run in the other direction as fast as possible, they only care about taking your money.) Obviously you can’t just keep losing a pound a week by consuming 500 calories less than you consume forever otherwise there would be nothing left and our bodies inner survival mechanisms fight fat loss the best it can.
There is a different type of formula that can be used for fat loss that’s called the dynamic model. A study published in the International Journal of Obesity in 2013 gathered 7 studies and analyzed them to compare the dynamic model of weight loss with this 3500 calorie rule. Subjects lost 20.1 ± 11.3 lost, which was 7.4 lbs less than the 27.6 ± 16 lbs predicted by the 3500 calorie rule. When it came to the 5 studies that were analyzed using the dynamic model subjects lost 17 lbs ± 11 lbs in comparison to the dynamic model prediction of 18.4 ± 13.8 lbs, showing a much more accurate formula for weight loss. If you see model 2 from the study over 400 days the actual weight loss is much less than the predicted weight loss using the 3500 calorie rule. (1)
So there you have it, as expected the 3500 calorie rule is not very accurate, and gets less accurate the longer time goes on. Really no surprise knowing the way the human body works. Thankfully there is a more complex formula out there that can give you a better idea if you really feel like you need some sort of starting point. If you are interested in dynamic model you can check it out here. Of course, even the dynamic model is still nothing but a math formula, but at least it factors in things like body composition, age, height, gender and degree of caloric restriction. That will certainly be more accurate than assuming everyone responds the same to a 500 calorie deficit.
All of that said, to me there is nothing more important than paying attention to your body and how it responds to what you do. There is no better indicator for results than listening to and learning from your own body by tracking your progress. There is no substitute for real first hand experience, but you have to pay attention and be consistent. Anything else is just a starting point to hopefully at least get you close. I think the important thing to remember out of all of this is that fat loss is not a linear process and expecting it to be will set you up for disappointment.