I’m engaged in an argument with someone who thinks that hypothyroidism is no big deal–that it lowers the BMR only a couple hundred points. I’ve Googled this to no avail. What’s the lowest BMR measured for someone who wasn’t in a coma or otherwise bedridden?
Since you didn’t exclude “dead” I’m going with zero.
Okay, dead people, paralyzed people are excluded. 
Any really low numbers will do. He’s arguing that just about everyone who claims to eat less than 1500 or so and not be able to lose weight is lying/defying the laws of physics. I’ve seen some posts claiming to have a measured metabolism of 600kcal/day but I can’t find medical cites, there’s too much noise in search results.
Actually, this may be an interesting open-ended discussion… at what point does it become “defying the laws of physics”? It’s not as simple as measuring how much energy is required to keep something at 95 degrees F vs 70, due to insulation from clothes/fat, that the body temperature isn’t consistent throughout, and such.
If you are average height and overweight, your BMR is most likely above 1500. Freak cases might have 600kcal a day, but I would imagine these people would not be overweight. Your BMR is highly dependant on your body weight, because every cell needs some minimum baseline amount of energy, and the more you weigh, the more cells you have (generally, unless you have a lot of bone mass).
I too would be interested in seeing the lowest BMR recorded on a walking human being.
This is actually mostly wrong. Adiposity is for the most part a matter of stuffing more fat into the same cells which aren’t particularly metabolically active. The more accurate BMR equations consider “lean mass” and ignore fat mass. Here you can see it graphically - a huge increase in adipose tissue produces only a slight increase in bmr. Granted, what does increase with adiposity is the amount of work needed to move that tissue around, so energy expenditure from activity will increase substantially with increased fat. Also, people who weigh more will generally also have more muscle mass to facilitate carrying the load, and that muscle tissue will be metabolically active - but you can imagine a scenario of an extremely sedentary obese individual with maybe muscle atrophy who has a very low bmr despite their weight.
As for the OP, I don’t know what the absolute minimum BMR is, but I do remember one obesity researcher saying it’s entirely possible for someone to be eating 700 kCal a day and gaining weight. His scenario was of someone in a metabolic starvation mode from having lost very large quantities of weight, where the body essentially shuts down energy expenditure as much as possible. From that I assume rock bottom bmr must be around 600 to 700 Calories for an adult - but that’s not exactly the best evidence in the world.
By the way, since BMR refers to your metabolic rate minus all activity, there’s really no need to exclude the paralyzed and comatose for a strict answer to your OP. BMR is supposed to be essentially what you’re burning when you’re sleeping or otherwise immobile. Here’s a very good web page that discusses BMR and several equations to calculate it. According to it, to estimate daily calories required that includes physical activity, you should multiply the BMR by 1.2 for the sedentary up to 1.9 for the absurdly physically active. So if we take our hypothetical metabolic incompetent with a BMR of 700 or so, physical activity would bring them to 840 calories if they were sedentary, or 1330 if they were training for a marathon (though I’d guess any condition that dropped your BMR to 700 would make marathon training quite a challenge).
OK, the chain of evidence here is a little thin, but I’m at least convinced that your friend is dead wrong in drawing the line at 1500 calories. You’ll notice from the second link that their hypothetical normal 5’6", 120 lb young woman will have a BMR of only around 1300 or so calories, and 5’6" is already taller than average for women. So if we take a 4’10", 100 lb, 70 year old, the BMR is going to be around 1000. Add starvation mode, hypothyroidism, and a thrifty gene or two, and I don’t have any problem buying a potential minimum BMR well under a 1000. Excess weight, of course, would bring up that number, but as I pointed out earlier - not very much if it’s mostly fat.
Is the OP implying that the argument involves people who blame their obesity on hypothyroidism? I notice that the replies don’t attempt to score the impact of hypothyroidism. I have a mild case of it and take daily medication but my weight only came down by a few pounds, which cannot be directly attributed to the treatment anyway so it anecdotal.
Overall, being fatter would probably lower BMR, I agree; don’t forget that in addition to not burning many calories, fat would provide insulation that makes heat exertion different.
I’m one of those defiers of the law of physics, but I also fall into the “very small” category. His premise was that all fatties had to do was stop stuffing soooo much food in their faces–you know the drill. Searching around, I found one reference to someone with a BMR of 652 kcal but it didn’t say if he were a dwarf, in a coma, or otherwise quite abnormal circumstances.
I have to wonder if I’m actually the norm rather than the exception, worldwide. I’ve read that African-Americans & Asians have a lower metabolism than Caucasians. On a diet of fish, rice, and cabbage, how can Asian people eat more than a few hundred calories a day?
The argument involved people who claim to eat few calories and still have difficulty losing weight, period. In his view, they all lie.
If he were a typical man, I wouldn’t blame him, actually… it must be dumbfounding that anyone can have a difficult time losing weight. Guys have it so lucky with their height & size & muscular mass. My father is over 60, but he eats 2 full bags of popcorn (with half a stick of butter added) and a big bowl of ice cream every night in addition to a Hungry-man dinner. -Every- night and he still keeps slim.
(BTW, I’m 120 lbs 5’4", which is about 25 pounds overweight here in the West Coast–so I’m always dieting.)
Uglybeech gave a geat answer. On that page she references used the Harris Benedict formula for BMR/Active calories, everything else is much less accurate.
As a real world example I have been counting calories for over 20 years and I’m very good at it. I lift, walk and get a decent amount of exercise. I have a large amount of muscle and I’m in reasonably good shape. My daily calorie requirements are around 10 calories daily per lb of body weight and that my active calories. That’s about 20-25% less than “normal” baselines.
If I was a 5’-6" female with the same metabolic tendencies I could easily be maintaining an active lifestyle on 1200-1300 calories per day and weighing 120-130 lbs.
I just got my BMR tested, mine is 1100 because I worked a desk job for many years and live a sedentary lifestyle. If I want to lose 1 pound a week without exercise I need to eat 600 calories a day. Not recommended by my trainer. If I exercise 30 minutes a day I can eat 800 calories a day to lose that 1 pound a week. We’ve decided weight loss will just be really slow for me until my BMR increases and that ill stop gaining weight as long as I stay under 1100 calories day. Thats just to stop gaining.
Since this is a zombie, I will mention that for a couple years, my wife was beginning to feel like a zombie. Then she was tested for thyroid and it was very low. Once she started taking synthroid, she perked up immediately. Don’t say hypothyroidism is no big deal.