Resting Metabolic Rate--Why Losing Weight after 45 is a Bitch

Following an exchange I had with a commentator in a previous post, I thought I should blog a little bit about Resting Metabolic Rate  (RMR), and the closely related Basal Metabolic Rate or BMR.   In a nutshell, it's the calories you burn at rest.   Even if you stay in bed all day doing nothing, your body still burns calories. 

Essentially, RMR and BMR are the base of calories you can eat in a day and not gain weight.   They are the bottom line, if we do absolutely no moving in a day, amount of calories we can consume without gaining any weight.

Now, here's a sad fact, as we get older, RMR and BMR drops, i.e., the minimum calories we burn in a day decreases.   

Let me explain.  There are four factors that go into BMR and RMR calculations, your weight, your age, your gender and your height.     You can find quite a few calculators on the web including here and here.

For example, I am a 5'2" 50 year-old, woman who weighs 130 pounds.  My BMR is 1,276 calories and my RMR is 1,163 calories.  

Now, take the same height and weight and sex but pretend I'm 20 years old-- my BMR is 1,416 calories a day and my RMR is 1,313 calories a day.   That's approximately 150 calories fewer calories I can consume in a day, at rest and not gain weight.

What does this mean?  It means if I do nothing to my regular diet and exercise program, I will gain weight as I get older for no other reason then I got older and my body just burns approximately 10% fewer calories on it's own.

Remember that it only takes 3,500 calories to gain a pound, so burning 150 fewer calories a day means a new pound gained every 24 days.

So, lesson number one in losing weight is that as we age we have to constantly adjust our daily calorie consumption lower JUST TO NOT GAIN WEIGHT.  

Metabolism slow down is just one of those natural things about aging, you can't do anything about it (although we can take steps to not make it worse).   The commentor is indeed correct that not moving as much as we age is also a factor in weight gain, but slowing metabolism as we age is a fact that we have to also deal with in confronting weight gain.  

Additionally, while we're at it, I thought I should bring up the impact that weight loss has on RMR and BMR.   Remember, WEIGHT is also factored into calculating both.

So, assume I weighed 160 pounds, not 130.    At my current height and age, my BMR would be 1,406 and my RMR would be 1,299.   

By dropping 30 pounds, my RMR goes down by 136 calories per day.

Lesson number two for the day--this is why you can never go back to your old eating habits.   Once you drop weight, you actually burn fewer calories in a day, so you have to adjust your overall calorie consumption down to fit with your new weight.   If you go back to your old eating habits, you'll go back to your old weight because that's where you'll reach caloric stasis again.

Third lesson, if you've hit a plateau in your weight loss, it means that because your calories in now equal calories out, you are not going to lose further weight until you further reduce calories and/or find new ways to burn more calories in a day.