Why do I need to match my BMR in calories?

It is because your BMR is the amount of calories you need even if you just layed in bed all day. It's the calories your body uses to keep your organs functioning. If you do something other than laying in bed all day you need more calories than your BMR to be able to do these things.

It is because your BMR is the amount of calories you need even if you just layed in bed all day. It's the calories your body uses to keep your organs functioning. If you do something other than laying in bed all day you need more calories than your BMR to be able to do these things.

This ^^

It is because your BMR is the amount of calories you need even if you just layed in bed all day. It's the calories your body uses to keep your organs functioning. If you do something other than laying in bed all day you need more calories than your BMR to be able to do these things.

Exactly! If you don't maintain your BMR consistently, you will die.

I guess in the very, very long term for most people, that may happen. However, there's a good chance that many on this site have enough spare fat to lose that being concerned about death by starvation or malnutrition isn't a very significant worry.

As someone else noted, the OP appears to be confusing TDEE with BMR. At any rate, can someone point me to peer-reviewed medical research that supports the notion that one MUST eat their BMR to function/live? If you eat below your BMR, you're body will just continue to hack away at fat (and, presumably, lean muscle mass) reserves to get the energy it needs to function . It's not like all of your vital organs shut down immediately if one doesn't consume at his/her BMR. Heck, that's part of the entire reason why humans can store extra body fat and why there are pathways to allow the body to breakdown muscle mass for energy use.

Again, in the long term, there is a metabolic hit that occurs (see the Minnesota - Key study on this from decades ago). I just haven't seen any research that supports the extreme fear some people seem to have about eating below their BMR. If you happen to be very lean and have very little additional fat, it could be a significant issue. I don't think that characterizes most of the people on this site, however.

Never eating below one's BMR usually requires that one take a "slow and steady" approach to dieting, which in turn usually requires a longer-lasting lifestyle change. After all, if one wants to crash diet, one can lose weight quickly, although it's much easier to go back to one's old (less healthy) lifestyle since habits can take a long time to change. In this light, the "stay above BMR" is good advice, since it's akin to "don't crash diet".

I guess in the very, very long term for most people, that may happen. However, there's a good chance that many on this site have enough spare fat to lose that being concerned about death by starvation or malnutrition isn't a very significant worry.

As someone else noted, the OP appears to be confusing TDEE with BMR. At any rate, can someone point me to peer-reviewed medical research that supports the notion that one MUST eat their BMR to function/live? If you eat below your BMR, you're body will just continue to hack away at fat (and, presumably, lean muscle mass) reserves to get the energy it needs to function . It's not like all of your vital organs shut down immediately if one doesn't consume at his/her BMR. Heck, that's part of the entire reason why humans can store extra body fat and why there are pathways to allow the body to breakdown muscle mass for energy use.

Again, in the long term, there is a metabolic hit that occurs (see the Minnesota - Key study on this from decades ago). I just haven't seen any research that supports the extreme fear some people seem to have about eating below their BMR. If you happen to be very lean and have very little additional fat, it could be a significant issue. I don't think that characterizes most of the people on this site, however.

Never eating below one's BMR usually requires that one take a "slow and steady" approach to dieting, which in turn usually requires a longer-lasting lifestyle change. After all, if one wants to crash diet, one can lose weight quickly, although it's much easier to go back to one's old (less healthy) lifestyle since habits can take a long time to change. In this light, the "stay above BMR" is good advice, since it's akin to "don't crash diet".

As you point out, there are studies that show the metabolism does slow down without a bunch of stuff being very correct, and even those in the studies have terrible maintenance on continuing their loss or keeping the weight off, when no longer under strict Dr or study care and observation.

So obviously there is a line in the sand for everyone, and unless you can have some major testing done, you'll never know that exact line.
Let alone the fact that line can change week to week depending on your other stress, variety of diet being good or bad with more stress on body, exercise getting way to intense and stressful, ect.

So where do you draw the line in the sand without this ability to be in a constantly monitored study and watched over?

Take the best things studies have found to maintain muscle mass all by themselves - reasonable deficit, enough protein, resistance training.
Combined, should mean no loss of muscle mass unless you try really hard.

And draw that line for reasonable deficit to be based on TDEE, the high end, not BMR, the line in the sand not knowing better details, the low end.

Of course, as you point out, 1 or 2 days below BMR, not deathly. Shoot, everyone could fine some 24 hr period in the week where they don't consume their BMR calories, the body can adjust.
And adjust it does seem to do when too steep a deficit is constant.

But, many on the Spike day diet, where daily you are eating probably a tad below BMR, show they can keep going in weight loss. Then again, there are plenty failing on that program too because their metabolism is already shot, or other stresses in life, are too much.