Is it bad to cut below BMR?

What is your height, weight, age and activity level? There is usually a much larger gap between BMR and TDEE. You may be mistaking BMR (coma calories) and NEAT (normal daily calories without exercise).

You risk losing a lot more muscle eating below BMR. Not to mention a host of other problems such as losing your libido, hair, bone density problems, etc etc.

ieroen wrote: »

My BMR has been determined at 2078. The past month, I've started my first cut at 1800. My TDEE is about 2350 right now, because I've been pretty sedentary. I've seen good results with this, as I've lost about 11.2lbs (5.1kg) over the past 4 weeks. However I'm wondering if it's unhealthy to systematically stay below BMR during a cut. I'm guessing it would probably be better for health / fitness in general, to keep calorie intake at BMR and then raise my TDEE with workouts. But speaking purely from a nutritional standpoint, is it considered bad or harmful to go below the BMR?

To have a BMR of 2078 and a TDEE of only 2350, you would have to be less than sedentary (that's only a 1.13 factor). I agree with @samanthaluangphixay that you're likely confusing something, although I'm not sure what.

And DEXA scans don't determine BMR. DEXA scans look at body mass and bone density. Are you saying the person or algorithm that calculated your BMR took your DEXA results into account to determine the amount of muscle you have, which affects BMR along with a lot of other factors?

Assuming that you've been sticking to your 1800 daily calorie goal, your actual results say that your TDEE is more in the range of 3200 (11.2 lbs/4 weeks X 3500 calories per lb/7 days in a week = 1400 calorie daily deficit; 1800 calories consumed + 1400 cal deficit = 3200 TDEE), although obviously some of the early weight could be water loss. You'd get a better estimate if you keep at it another couple of weeks, drop the data from the first two weeks, and repeat the calculation based on actual results from weeks 3 - 6.

As for your question about staying below BMR, @lemurcat12 is spot on. Your body doesn't tap fat reserves for running laps or lifting weights but then suddenly say, "no, no way, not touching the fat reserves to keep the heart beating or carry at cellular level activities."

However, as @lemurcat12 also pointed out, it seems your overall deficit is too steep. Given that you're already a healthy weight for your height (6'5" and 200 lbs.), averaging more than 2 lbs a week weight loss -- actually, close to 3 lbs a week -- is counterproductive. I'm assuming since you're using the "cut" terminology, your goal is to maximize fat loss and minimize muscle loss. Losing weight this rapidly leads to more muscle loss than is necessary. Muscle retention is a three-legged stool: resistance training, adequate protein, and a small calorie deficit.