Why not eat below BMR?

chooselove

I don't think it is that your body begins to "cannibalize" the organs.

OP's word was "catabolize" not "cannibalize".. Two very different things..

Mads1997

If eating below your BMR is perfectly fine, then where do you draw the line? When does a diet cease to be a diet and become an eating disorder. Personally, threads like this seem to simply be attempts at justifying and validating eating disorders. Have fun with y'all's anorexia and 500-800 net calories per day.

I'm out...gotta hit the gym and get home to finish eat my 2150 calories today.

I don't know about anyone else but I will draw the line when I get to a healthy fat percentage. Not the 35+ % fat that I am now. I am most certainly anorexic LOL

Doberdawn

And now for something completely useless:

I don't think it is that your body begins to "cannibalize" the organs.

OP's word was "catabolize" not "cannibalize".. Two very different things..

Why not contribute meaningfully to the actual discussion instead of policing people's spelling and such. *shakes head* Posting stuff like this just LOOSES the point. Yes, misspelling intentional. But, feel free to tell us that too.

Ploogy

Wow, I get home and there's a ton of responses! Guess I struck a nerve.

I'm thankful for all the responses, especially for the fact that its drawn some very intelligent folks out of the wood work, many of whom I am hoping to add as MFP friends on this journey.

The reason I had asked was because I have been eating at BMR for 4 weeks now with no movement on the scale; I've been counting calories for 6 weeks and lost about 8 pounds right off the bat in the first 3. I am 65 pounds or so overweight (245lbs, 37 years old, 5'10"). Being stuck, I wanted to make an adjustment but was being torn in both directions - afraid to go lower than BMR because of all the warnings of the dreaded starvation mode and metabolic downregulation, but not quite convinced that my "stall" was do to eating too little and, in fact, feeling overstuffed at trying to eat back all the exercise calories necessary to net my BMR.

So here I am, not much more clear on where to go - as this thread alone shows, there is no real consensus in this forum with respect to BMR. From the posts that make the most sense to me, and many of them seem to be in relative consensus with one another, I gather that the real importance of BMR is its use to derive one's TDEE, a far more useful number from which you can run a controlled deficit, and not as some magical floor. While eating 500 calories a day is obviously harmful, dipping consumption to or slightly below BMR is not the evil that people parrot here.

Thanks a bunch for your help!

If eating below your BMR is perfectly fine, then where do you draw the line? When does a diet cease to be a diet and become an eating disorder. Personally, threads like this seem to simply be attempts at justifying and validating eating disorders. Have fun with y'all's anorexia and 500-800 net calories per day.

I'm out...gotta hit the gym and get home to finish eat my 2150 calories today.

Choosing an arbitrary number for the sake of having a threshold is one thing, but at least admit to what it is. If all you've seen this thread to be is an attempt to justify an eating disorder, then you have some issues with sliding down slippery slopes too easily - not all of us do.

jzammetti

Twinketta is right about losing your Lean Body Mass too. You will lose muscle, a lot more than you want to. That is another problem with radical cutting diets.

AND, once you begin to lose LBM, your metabolism slows. Increasing LBM is the only way to increas metabolism. When it slows down enough, you will begin to GAIN weight even below your BMR

SRH7

Bump

Doberdawn

Wow, I get home and there's a ton of responses! Guess I struck a nerve.

I'm thankful for all the responses, especially for the fact that its drawn some very intelligent folks out of the wood work, many of whom I am hoping to add as MFP friends on this journey.

The reason I had asked was because I have been eating at BMR for 4 weeks now with no movement on the scale; I've been counting calories for 6 weeks and lost about 8 pounds right off the bat in the first 3. I am 65 pounds or so overweight (245lbs, 37 years old, 5'10"). Being stuck, I wanted to make an adjustment but was being torn in both directions - afraid to go lower than BMR because of all the warnings of the dreaded starvation mode and metabolic downregulation, but not quite convinced that my "stall" was do to eating too little and, in fact, feeling overstuffed at trying to eat back all the exercise calories necessary to net my BMR.

So here I am, not much more clear on where to go - as this thread alone shows, there is no real consensus in this forum with respect to BMR. From the posts that make the most sense to me, and many of them seem to be in relative consensus with one another, I gather that the real importance of BMR is its use to derive one's TDEE, a far more useful number from which you can run a controlled deficit, and not as some magical floor. While eating 500 calories a day is obviously harmful, dipping consumption to or slightly below BMR is not the evil that people parrot here.

Thanks a bunch for your help!

If eating below your BMR is perfectly fine, then where do you draw the line? When does a diet cease to be a diet and become an eating disorder. Personally, threads like this seem to simply be attempts at justifying and validating eating disorders. Have fun with y'all's anorexia and 500-800 net calories per day.

I'm out...gotta hit the gym and get home to finish eat my 2150 calories today.

Choosing an arbitrary number for the sake of having a threshold is one thing, but at least admit to what it is. If all you've seen this thread to be is an attempt to justify an eating disorder, then you have some issues with sliding down slippery slopes too easily - not all of us do.

Not sure you stalled long enough to be worried about a major change up. But, when I stalled for months... and I tried dropping lower and lower... it did nothing. I did the full diet break recommended by Lyle and then returned to a significantly higher calorie load and I've resumed losing. http://www.bodyrecomposition.com/fat-loss/the-full-diet-break.html

RoadsterGirlie

I've never eaten below my BMR any time, except possibly when I've been sick. The lowest point I've ever gone is 1400 to 1500 calories per day (I don't eat back exercise calories, fyi).

I now maintain on about 2000 a day.

As you can see, I lost weight just fine doing this. I'm not sure what would happen if I did eat below my BMR since I've never experienced it, but so far what I did worked.

Docmahi

My understanding is that your body goes for muscle before fat, and will eventually get used to operating at a reduced efficiency because there isn't enough new food energy. I dont have any links for you, just what I've read over the years from here, books, and websites.

I think it's a choice, do you want to lose fat or muscle? I personally want to lose fat, so I dont want to eat less than 1200 calories or go into starvation mode.

If you look at people anorexic who eat very little or nothing, they lose weight... lots, they lose all their muscle first and then can be 'skinny fat'. eek!

I dont think any of that is right

like any of it

Docmahi

My BMR is 1411 or 1441, something above 1400 anyway and MFP put me on a 1220 calorie intake so I am trying to stick to that.

With 7 Lbs to lose? FTW? MFP simply spit out a number based on what you input as your goal...which is apparently aggressive weight loss with little to lose. MFP isn't god...MFP didn't put you on ****, you put yourself on that.

cwolfman generally posts what I am thinking in most threads

BurtHuttz

Stick around a few more months and count the people who cannot lose weight despite their claim to eat under their BMR on a daily basis. Count the success stories of people who gleefully proclaim that they feel great and lost all their weight eating 1000 calories daily.

Edited to add: From a local, OP. You've stated that you've got two paths to go. I believe the phraseology in your writing indicates you believe basal metabolic rate to be irrelevant. Thus I would encourage you to go lower and report back in three months. Yours would be a nice case study to add.

Erica_theRedhead

I think it's just that people do not understand the real meaning of starvation mode. A few days of a caloric deficit doesn't "throw you into starvation mode." It could take days, weeks, or months to reach this stage, depending on the level of deficit.

When you are eating at a deficit, you body pulls energy from fat stores in your gut, liver, adipose, and muscle. Without exercise to increase your lean body mass, you are losing weight, but it is from fat AND muscle. This is why many advocate exercise, usually with strength training. Exercise is also stressing your body. You can choose to use it to calculate you daily expendature (TDEE) or not. If not, this is when many advocate eating back your earned exercise calories. If you don't count exercise, it's at your discretion. The goal is to NET negative calories from TDEE, but remain above BMR.

Now what is BMR? BMR is the loweest amount of energy that your body needs to run its biochemical processes. If you eat below BMR, you may be able to keep your metabolism running for awhile, but there is no extra energy to preserve muscle strength or promote growth. By eating at a severe deficit like this for a long time, your body will begin to slow down it's metabolic processes to conserve the energy that it has. Eventually, it will stop pulling energy from your fat all together, as it believes that it will run out eventually. (this is where the dreaded "I've plateaued even though I only net 1000 calories a day" thread pops up.)

Continuing at this rate of decreased calorie consumption, along with a decreased metabolism, your organs (specifically your brain) will begin to starve from lack of glucose. It will begin breaking down amino acids from muscle, cause acidosis in your blood, and will eventually lead to organ dysfunction. This is where starvation mode is in full effect, however your body was on the way to it prior to this event.

I work as a pharmacist who also provides nutrition to patients in a hospital setting. The biochemistry of the body is fairly well understood. We use BMR, total daily expendature, and stress factors routinely in our practice. I chose not to factor in my exercise into TDEE. So I eat above BMR and eat back my exercise calories. Since I've upped my intake, I am more energetic, awake, and am continuing to get stronger physically, but still continue to lose weight.

Hope this helps!

Ploogy

If you don't eat back your calories, I'm willing to bet you netted below your BMR regularly.

I've never eaten below my BMR any time, except possibly when I've been sick. The lowest point I've ever gone is 1400 to 1500 calories per day (I don't eat back exercise calories, fyi).

I now maintain on about 2000 a day.

As you can see, I lost weight just fine doing this. I'm not sure what would happen if I did eat below my BMR since I've never experienced it, but so far what I did worked.

birdiecs

For me it's rather simple. My BMR is barely 1230, to eat below that would make me so unbearably *****y, I would fear for those around me. I stick to at least 1450 on sedentary days and 1800'ish on active days, so far so good and everyone is safe.

CristinaL1983

Someone bumped this earlier and I think it would answer your question pretty comprehensively.

https://www.myfitnesspal.com/topics/show/475726-very-low-calorie-diets-and-metabolic-damage

[Surprise twist: the not eating below your BMR is based in nonsense (as usually espoused on here)]

Edited to add in quote from the post:

I'm not posting this thread to restart all the flaming and trolling. I want this to shine a bit of objectivity on the subject of very low calorie diets and metabolism. I spent a decent amount of time explaining things in the thread that was deleted, so it was disheartening to see the information deleted. I think we can all agree that among other important goals, this community is about education.

As this thread in question is being discussed on my profile page, someone asked me to re-explain what I discussed in the removed thread. I'm simply going to copy and paste my response here:

Yes, very low calorie diets (VLCD) can reduce RMR and disrupt various components of the endocrine system. But correlation is not causation. Meaning... is it the VLCD or is it the effect of the VLCD that leads to the slowdown? Put differently, VLCD cause high rates of fat loss due to the massive energy deficits. Fat happens to be the home of the master hormone responsible for metabolic regulation - Leptin. Leptin lets out bodies know that it's fed. So if we have less fat, we have less Leptin. If we have less Leptin, we have less of a "fed" signal to the brain. If we have less of a fed signal, the body responds accordingly with the slowdown in RMR (and some other adaptations).

But the logical question you should be asking is, "Won't smaller or normal deficits also cause a loss in fat? And won't that loss in fat lead to the same sort of negative adaptations?" And the answer is yes. It's just that the VLCD will cause these adaptations to happen faster... but you'll also lose fat faster. Follow me?

This isn't an argument for people to start following VLCD as most will fail miserably. Once you factor in the psychology aspects of them, they're just not right for most long term fat loss plans. In addition, the lower your energy intake is, the more careful you have to be about nutritional adequacy. Meaning it becomes very easy to shortchange yourself of particular facets of nutrition, which can ultimately tap into "health."

For example, back in the day when the medical community was busy trying to fix obesity, they used VLCDs and weren't mindful of protein quality or quantity. Protein happens to support muscle mass. Sure, around here we're interested in preserving skeletal muscle, as that's what helps us get "toned," "lean," "athletic looking," "ripped," or whatever the cool buzzword is nowadays. But these experiments on the obese patients led to losses in other types of muscle... namely cardiac muscle, which plays an important role in being alive, obviously.

Point is, the more food you eat, the easier it is to cover all of your nutritional bases. You can still screw things up, but it's just harder.

In the thread that was deleted last night, I spoke of the Minnesota Starvation Experiment ran by Ancel Keys, which took already relatively lean men and locked them down in the lab where they were given 50% of their calorie needs for half of a year. They also had supervised exercise ever day if memory serves me correct. We know that lean people will react faster (in terms of metabolic slowdown) to big energy deficits than fat people will. Which makes sense.... fat bodies don't "think" they're starving as quickly since they have all of this excess energy in storage, right?

But even with the test subjects in this experiment being lean to start, after the 6 month period, they only experienced a slowdown in RMR of 15% or so. I mean total energy expenditure dropped by 40% or so, but the remaining 25% (above the 15%) was due to the loss in weight (tissue costs something to maintain and a bigger body is more expensive to move around). Everyone knows that as weight is lost, calorie needs go down.

The "starvation mode" totaled 15% after half a year of low calorie dieting. And that's the primary point... life requires energy expenditure. And metabolism can be thought of as our total energy expenditure in this case. Even if there is negative adaptation to low calorie dieting, metabolism can only drop so far... there's a minimum threshold that's required to keep your heart beating, to fuel respiration, power the brain, transport nutrients, digest food, etc, etc.

I also posted a few links to more current research.... one paper compared a 25% deficit to a 890 calorie intake. The low calorie intake lost more weight than the 25% group. Yes, they had a metabolic slowdown... but so did the 25% group, which corresponds to what I said above about big and small deficits.

There's a lot more that I could say on the topic. For example, NEAT (non-exercise activity thermogenesis) is turning out to play a substantial role in the reduction in total daily energy expenditure experienced on prolonged diets... big or small deficits. Which only stands to solidify the importance of exercise while dieting. NEAT drops unconsciously... so we counteract that by doing more activity consciously via structured exercise. But we won't dive into that end of things as this is already far too long.

The bottom line is this... VLCDs are not as destructive as people around here are making them out to be. People see VLCD and immediately think of anorexia. Anorexics lose weight past the point of healthy thresholds. They also aren't mindful of nutrition quality, more often than not. Not on low calorie diets are "unhealthy."

There's a time and a place where they may even make sense for some. I've used them. I've used them with some of my clients.

It's just that those times and places don't match beginners who obviously need to learn nutrition fundamentals before they go experimenting with advanced dieting techniques. If they jump right into very strict and limiting diets, there's a good chance they're going to wind up gaining weight, not because of some crazy adaptation that winds up creating stored energy (fat) out of thin air... but because they're not going to stick with it, and when they fall off the wagon, they fall hard and typically eat their faces off.

I'm not out there advocating VLCDs. Not at all... heck, I wrote the Nutrition 101 article, which everyone should have read by now, and nowhere in it did I advocate VLCD. I'm simply trying to maintain the integrity of information while steering people in an optimal direction. It seems like too many people around here are stuck on absolutes. They believe there is 100% right ways of doing things and 100% wrong ways of doing things. There's no in between. In reality, there are very few absolutes in the game of fat loss.

Ploogy

Thanks Christina!

laurenkfj

Bump

blu_meanie_ca

Here's the thing: this website is not, in fact, a magic genie. It can not take your resting heart rate, dust off the calipers, and watch you do the hokey pokey to see what wiggles when you wobble. It can't do it for the other guy, that person, or him either. This is all just estimates. So, when estimated, it's better to play it on the safe side, keep recomendations within generally accepted levels, and hope that we all come out with the desired results.

mgsimon

I actually started "medically supervised" weight loss last week with a doctor.
As part of my pre-planning my program, she set me up to have a RMR test -- that's where they determine your resting metabolism rate -- or the energy burned by your body just sitting down quietly. I'll be happy to share the findings:

My weight: 238 pounds
My height: 5'4"
My RMR: 1800 calories.... that's what my body needs to function.
An additional 540 calories was added to the total as "lifestyle & activity" -- getting up to go the copy machine, climbing stairs to the 2nd floor, etc. That's a total RMR of 2340 calories.

The determination is:
1800-2300 calorie daily intake will maintain my current weight.
1440-1800 calorie daily intake is the "weight loss zone."
Anything under 1440 daily calorie intake should be medically supervised.

I'm going back this Saturday to the gym, to have a VO2 test -- that's where they determine your optimal heart rate for weight loss. Most people tend to exercise in the cardio zone -- which is certainly great. But for someone like me who still needs to focus on weight loss - I need to work out at a different heart rate.

I think the key is "medically supervised." I also underwent a complete blood workup and EKG... which will all be repeated in a couple of weeks. As part of my plan I meet with a nutritionist every week and either the doctor or the PA. I am NOT doing the VLC diet, frankly because that kind of scares me... but knowing this other information is pretty helpful in helping me with my day.

Yeah, what she said. I've gone through the EXACT same process as she describes above except for the nutritionist. I've been told by a doctor to eat BELOW my RMR by 15-20%. I can't seem to get a good answer on "eating back" exercise calories from him.

roiLEI1554

by eating below your BMR, you're not giving your body the nutrients it needs (vitamin and mineral supplements don't count).
You'd be surprised how much a person can eat by eating back exercise calories and STILL lose weight

bpotts44

Most people should completely ignore BMR and focus entirely on TDEE and set a deficit off TDEE either -20% or -500 calories while bottoming out at 1200. Our bodies metabolic activity is extremely complex and many people get distracted by the definition of BMR as the minimum energy our bodies will burn with a supposed minimum intake or all XYZ bad things will happen.

Your body will burn fat to make up for any moderate caloric deficit regardless of the relationship between your actual intake and your BMR. You may lose some LBM through glycogen stores, water, or even muscle but your body is NOT going to go after your muscle first! For optimal results you need to maintain proper exercise and macro nutrient intake, but BMR is not some magical cutoff.

jonnythan

BMR is completely irrelevant when you know a day's TDEE.

myofibril

There's nothing wrong with eating under your BMR or netting under it for that matter.

I think the advice is given as a general guideline to ensure sufficient nutrient and / or energy availability for diets which are going to extend passed the short term (say 6-12 weeks) where the dieter is generally inexperienced.

Obviously prolonged severe caloric restriction can cause some metabolic issues but that is long periods of low calorie dieting (in general and usually combined with a high level of activity.)

Trilby16

My understanding is that your body goes for muscle before fat, and will eventually get used to operating at a reduced efficiency because there isn't enough new food energy. I dont have any links for you, just what I've read over the years from here, books, and websites.

I think it's a choice, do you want to lose fat or muscle? I personally want to lose fat, so I dont want to eat less than 1200 calories or go into starvation mode.

If you look at people anorexic who eat very little or nothing, they lose weight... lots, they lose all their muscle first and then can be 'skinny fat'. eek!

It is hard to believe that the body could be so stupid as to use muscle instead of fat for energy when that's what fat is stored for. Stupid body!

jonnythan

My understanding is that your body goes for muscle before fat, and will eventually get used to operating at a reduced efficiency because there isn't enough new food energy. I dont have any links for you, just what I've read over the years from here, books, and websites.

I think it's a choice, do you want to lose fat or muscle? I personally want to lose fat, so I dont want to eat less than 1200 calories or go into starvation mode.

If you look at people anorexic who eat very little or nothing, they lose weight... lots, they lose all their muscle first and then can be 'skinny fat'. eek!

It is hard to believe that the body could be so stupid as to use muscle instead of fat for energy when that's what fat is stored for. Stupid body!

It's not stupid when you realize that, if it uses muscle preferentially to fat, it can survive starvation for a longer period of time.

Lt_Starbuck

A lot of people like to think of weightloss and getting fit and getting a good body as a 'journey', I like to think of it as a road trip.

Now let's say you want to drive from California to NYC (for funsies). You calculate exactly how much gas it will take you to get from here to there. Let's say, for funsies, that it is 2800 miles, and you know to the drop how much gas that will take.

This is what I would refer to as your BMR. This is how much gas you need just to turn on the engine and drive there. In other words, cardiovascular system, central nervous system, basically just enough fuel (or calories) to drive straight there.

Now - you decide to only bring enough money for 1800 miles and just force your car to work with that. This is considered 'eating below your BMR' and it is very very dangerous to the engine. It will affect your miles per gallon, how often you have to change oil, tough on the gears, crap on everything. Just bad bad bad for the car.

Now - on your way from LA to NYC - you decide to do a few other things - like stop and sightsee here and there. Maybe go off course and find some fun things to do, stop and see friends, go to the bathroom, buy a souvenir or two, stretch your legs, get off the highway, splurge on a hotel, use a phone....

This would be stuff like exercise, cleaning your house, having sex, visiting friends, walking all over the place, playing with kids outside - other things that burn calories ON TOP OF what you burn just 'running' your body's vital systems.

But you still only brought 1800 miles worth of gas money and nothing else...

So now you're stuck in the Midwest. With no gas.

In other words... now you're stuck at this weight.. and you can't do more til you get more fuel.

Give yourself enough gas to get where you're going and do stuff you need to / want to along the way. LIKE EXERCISE.

At first you'll gain a little - and then your body will adjust and it will go away again and take extra pounds with it.

True story.

Lt_Starbuck

My understanding is that your body goes for muscle before fat, and will eventually get used to operating at a reduced efficiency because there isn't enough new food energy. I dont have any links for you, just what I've read over the years from here, books, and websites.

I think it's a choice, do you want to lose fat or muscle? I personally want to lose fat, so I dont want to eat less than 1200 calories or go into starvation mode.

If you look at people anorexic who eat very little or nothing, they lose weight... lots, they lose all their muscle first and then can be 'skinny fat'. eek!

It is hard to believe that the body could be so stupid as to use muscle instead of fat for energy when that's what fat is stored for. Stupid body!

It's not stupid when you realize that, if it uses muscle preferentially to fat, it can survive starvation for a longer period of time.

Quoted for Truth

Capt_Apollo

because

jonnythan

A lot of people like to think of weightloss and getting fit and getting a good body as a 'journey', I like to think of it as a road trip.

Now let's say you want to drive from California to NYC (for funsies). You calculate exactly how much gas it will take you to get from here to there. Let's say, for funsies, that it is 2800 miles, and you know to the drop how much gas that will take.

This is what I would refer to as your BMR. This is how much gas you need just to turn on the engine and drive there. In other words, cardiovascular system, central nervous system, basically just enough fuel (or calories) to drive straight there.

Now - you decide to only bring enough money for 1800 miles and just force your car to work with that. This is considered 'eating below your BMR' and it is very very dangerous to the engine. It will affect your miles per gallon, how often you have to change oil, tough on the gears, crap on everything. Just bad bad bad for the car.

Now - on your way from LA to NYC - you decide to do a few other things - like stop and sightsee here and there. Maybe go off course and find some fun things to do, stop and see friends, go to the bathroom, buy a souvenir or two, stretch your legs, get off the highway, splurge on a hotel, use a phone....

This would be stuff like exercise, cleaning your house, having sex, visiting friends, walking all over the place, playing with kids outside - other things that burn calories ON TOP OF what you burn just 'running' your body's vital systems.

But you still only brought 1800 miles worth of gas money and nothing else...

So now you're stuck in the Midwest. With no gas.

In other words... now you're stuck at this weight.. and you can't do more til you get more fuel.

Give yourself enough gas to get where you're going and do stuff you need to / want to along the way. LIKE EXERCISE.

At first you'll gain a little - and then your body will adjust and it will go away again and take extra pounds with it.

True story.

This is an incredibly terrible analogy. Completely nonsensical.

Here's a better car analogy:

You have a huge tank, one large enough to provide fuel for 40 days, in the back of the bus. You want to use it all. You can't use it all if, at the beginning of every day, you fill the small tank up with exactly enough fuel to make it through that one day. This would be eating TDEE.

So instead you fill the small tank up with enough fuel to make it through the day, minus a couple of gallons. So for the last few miles of the day you're using from the huge tank (your fat stores).

Now ask yourself: if you're driving half the day, does it matter how much fuel you would have used if you just sat around letting the bus idle every day? No. If you want to plan how much fuel to put in the small tank, you look at how much you plan to drive that day. How much you would have used if you didn't drive is irrelevant.

You want to plan so that you can use 5 gallons from the huge tank in the back every day, so you have to look at how much actual fuel you will use that day. The amount of fuel you would have used if you didn't drive makes no difference to anything at all.

JoanB5

BMR is a medical term, not something someone on MFP made up. Your estimated BMR is in fact what they would feed you in the hospital in a coma...which is why the coma comment comes up. Generally speaking, health care professionals, dietitians, and nutritionists do not recommend having a net calorie intake below your BMR unless you have a substantial amount of weight to lose as it can be dangerous...particularly when you're talking about people with 5 or 10 cosmetic Lbs to lose and completely crashing their diets. Having a VLCD prescribed by a Dr. and monitored by a Dr. is a far cry different thing than some random person with 20 Lbs to lose netting 500 calories per day. Really, there's no reason to net below your BMR on any consistent basis without being under the supervision of a health care professional.

Individuals with a substantial amount of weight to lose can get by eating below BMR with few side effects for longer because they have substantial fat stores to burn. Your body is a smart machine...it knows the difference between having 30-40 Lbs of fat reserve to say 200-300 Lbs of fat storage reserve. Your body is going to treat that deficit differently depending on how much you have to lose.

Personally, I think it's really only an issue with people with very drastically low calorie intakes...like having a 1,200 calorie goal and then exercising and netting to 500 or 600 calories. That's just plain unhealthy and completely unsustainable. I'm not so worried about myself occasionally dipping a couple of calories below my BMR or something though...it is an estimate. It's the folks with a BMR of 1,300 or 1,400, but netting 500 that scare the **** out of me. Basically all they are doing is developing an eating disorder (anorexia) at that point.

Well said.