Should BMR decline with Weight Loss?

StatChicBayes

I started exercising and using MFP back in Feb after my wt climbed above pregnancy weight 12 years back and worsening lipid profile. Since this is not my first rodeo with weight loss, I decided to have my resting metabolic rate tested to help with gauging caloric intake (a month after starting exercising/diet). Surprisingly to me and the nutritionist doing the test, the result came back at 1510 calories (BodyGem). This was actually in closer agreement with my Apple Watch Resting Calories for that period - while one of the formulas for RMR that yielded 1290. (closer to what I thought was correct given my height of 4'11'')

Prior to starting exercise/diet/tracking, my Apple Watch readings for RMR were around 1490; after starting to exercise they increased to around 1600+. I am 57 yo, Female, and 4'11'' so my current estimated body fat is around 41% (still a ways to go)! My nutritionist initially suggested a target of 1200-1400 calories a day and lipid specialist suggested 38% carb, with the rest split between protein/fat, with carbs coming primarily from low GI vegetables, high fiber/complex carbs (whole grains), low GI fruits. Protein is fish, chicken, duck, plant sources with trying to have primarily healthy fats. So far lipids have responded splendidly and I have lost 25 lbs since Feb 4; initially I lost about 12 lbs the first month but it has been around 1-2 lb a week since increasing calories to the 1200-1500 level (well except for this week which has been net 0 :-()

I am trying to exercise 6-7 times a week with a mix of cardio (step, spin), core work (pilates), strength training (free weights ~ 5 times a week & machines with heavier wts twice a week). Since May, I joined an Apple Watch challenge, and have increased calories burned (based on the watch) to around 1000 a day and exercise minutes.

What has concerned my is that since starting the challenge my Apple Watch measurements of RMR dropped to around 1550

Questions:

1) all of the formulas suggest that RMR should decrease with weight. so is what I am seeing normal due to weight loss? Or is this an ecological fallacy based on statistical models (individual trajectory versus group averages)?) Can my RMB stay constant with a calorie deficit and exercise to increase muscle mass?

2) From my understanding, exercise calories on the apple watch are based on detecting an increase in heart rate equivalent to a brisk walk. Is it possible that by having more time spent in exercise mode that the estimated RMR is now lower since I am resting less? (if so do I adjust intake or increase it if I am going into "starvation mode"?)

My concern is that I am not eating enough calories given expenditures and that is causing a drop in RMR, but well aware that trackers are not perfect so worried about making adjustments (increase in calories) on inputs that are off due to various errors.

Thoughts?

JeromeBarry1

It does.

I answered with a simple sentence to illustrate that you're overcomplicating things.

Stay in a calorie deficit to lose weight. Weigh your food. Log your food. Be honest with it. Be patient.

About the Apple Watch. It can measure your heart rate. As you perform vast amounts of cardiovascular exercise, you optimize your heart to perform efficiently during your exercise. As you lose weight, you reduce the workload on your heart. The happy result of exercising and losing weight is that your resting heart rate declines. Apple Watch simply doesn't know the whole story.

230137isntmyweight

If you lose weight you have less mass to support, so you need less calories.

AnnPT77

clyde37 wrote: »

I started exercising and using MFP back in Feb after my wt climbed above pregnancy weight 12 years back and worsening lipid profile. Since this is not my first rodeo with weight loss, I decided to have my resting metabolic rate tested to help with gauging caloric intake (a month after starting exercising/diet). Surprisingly to me and the nutritionist doing the test, the result came back at 1510 calories (BodyGem). This was actually in closer agreement with my Apple Watch Resting Calories for that period - while one of the formulas for RMR that yielded 1290. (closer to what I thought was correct given my height of 4'11'')

Prior to starting exercise/diet/tracking, my Apple Watch readings for RMR were around 1490; after starting to exercise they increased to around 1600+. I am 57 yo, Female, and 4'11'' so my current estimated body fat is around 41% (still a ways to go)! My nutritionist initially suggested a target of 1200-1400 calories a day and lipid specialist suggested 38% carb, with the rest split between protein/fat, with carbs coming primarily from low GI vegetables, high fiber/complex carbs (whole grains), low GI fruits. Protein is fish, chicken, duck, plant sources with trying to have primarily healthy fats. So far lipids have responded splendidly and I have lost 25 lbs since Feb 4; initially I lost about 12 lbs the first month but it has been around 1-2 lb a week since increasing calories to the 1200-1500 level (well except for this week which has been net 0 :-()

I am trying to exercise 6-7 times a week with a mix of cardio (step, spin), core work (pilates), strength training (free weights ~ 5 times a week & machines with heavier wts twice a week). Since May, I joined an Apple Watch challenge, and have increased calories burned (based on the watch) to around 1000 a day and exercise minutes.

What has concerned my is that since starting the challenge my Apple Watch measurements of RMR dropped to around 1550

Questions:

1) all of the formulas suggest that RMR should decrease with weight. so is what I am seeing normal due to weight loss? Or is this an ecological fallacy based on statistical models (individual trajectory versus group averages)?) Can my RMB stay constant with a calorie deficit and exercise to increase muscle mass?

One would expect RMR to decline with weight loss. You have less blood, less tissue, less everything, so it takes fewer calories just to sustain it while sitting still. Theoretically, your RMR could decline less than expected due to something like increased muscle mass, but muscle gain is so freakin' slow that any rational rate of fat loss will dramatically outpace it, so this seems unlikely to result in an equal RMR at a lighter weight in actual practice, in the short run (weeks to months).

(For a woman, under perfect conditions, 1/4 pound muscle weight gain per week would be a very good result, and perfect conditions definitely don't include a calorie deficit. OTOH, 1/4 pound of fat loss would be about the slowest observable rate of fat loss, and you'd only see it over weeks to months, as over shorter timespans it's hidden in amongst daily water weight fluctuations. While muscle mass is more metabolically active than fat mass, the difference is only something like 2-4 calories per pound per day - I always forget the exact number - so it isn't very meaningful in the short-term picture.)

Keep in mind that your Apple watch is giving you an estimate for RMR, not a measurement. The fact that it was right once doesn't mean it's right now (stopped clock is right twice a day sort of thing).

There's some other stuff that could affect RMR, but effects are small.

2) From my understanding, exercise calories on the apple watch are based on detecting an increase in heart rate equivalent to a brisk walk. Is it possible that by having more time spent in exercise mode that the estimated RMR is now lower since I am resting less? (if so do I adjust intake or increase it if I am going into "starvation mode"?)

If more exercise makes your Apple watch think your RMR is lower, that would be unlikely to reflect reality, IMO. I guess it's possible that your exercise lowers your resting heart rate, making your apple watch think your RMR is lower, but I dunno. Stupid algorithms are always a possibility, but if they're stupid, they don't reflect reality. By definition, your RMR is the number of calories you burn when resting. Your actual RMR shouldn't be less because you're resting less.

"Starvation mode", in the sense of weight loss stopping because of undereating, is not a real thing. If it were, no one would ever starve to death. Sadly, thousands of people starve to death worldwide every year, and they aren't fat when they do so. Oversimplifying only slightly, weight loss can slow if we undereat to the point of fatigue, but that's about it.

My concern is that I am not eating enough calories given expenditures and that is causing a drop in RMR, but well aware that trackers are not perfect so worried about making adjustments (increase in calories) on inputs that are off due to various errors.

Thoughts?

I'm concerned that you're overthinking.

Are you losing weight? If so, how rapidly are you losing weight? That's the key question. All of the estimates from various devices are secondary to that.

Take a deep breath, relax. You can sort out where you are, and what to do.

Best wishes!

CSARdiver

BMR is simply an output of energy it takes to sustain a cell, then multiplied over the number of cells. There is a small variation between specific cells, but this is minimal. So yes, BMR will trend along with weight loss.