Is eating 800 calories okay?

You should be eating over your BMR and under your TDEE, anyway.
The only time I eat 1200 cals is when I'm sick in bed

This isn't always true though.

IE: A person weighs 200lbs. "Normal" weight would be 130lbs. BMR due to being 200lbs is 2000. No exercise and very sedentary. It would be fine to eat 1500. The whole "don't eat less than your BMR" is overused. Now if someone is within 10-15lbs or their goal, then obviously their BMR has dropped and eating under it wouldn't be the best advice.

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I'm 5'6 and currently 140 pounds. So I'm just barely at a healthy weight. My general goal is 120 pounds. And my calorie intake is not constant, as sometimes I eat 100-300 less or 100-300 more. And I'm beginning to try to incorporate exercise, mainly running and bicycling and some home strength workouts. The problem is I'm not fit, which is one of my main goals of weight loss. Anything else?

This post is so evidence of how everybody's body is different. I'm the same height and weight as you just working to lose the last few pounds. Any more and I'll start to look anorexic. I've always been more muscular.

To get fit and be fit your body needs to have exercise to run off of, which you're not going to get if you don't feed it. I'd say as long as every day is not an 800 calorie day you're probably fine, especially if you are over a couple of days a week and it all averages out. I personally can eat 800 calories just for lunch after a 5-600 calorie breakfast. Add a small piece of fruit after school and I'm pretty much done except for on workout days.

You'll be okay. You probably eat a little more in the following/preceding days anyway. If not, you'll still be okay as long as this isn't a crazy purposeful habit. And as long as you aren't the guy from Ripleys who has to eat like 10000 calories a day to live.

Never mind

Your BMR of 1500 sounds in the right ballpark. I'm near your stats, but older, and mine is in the 1400's. You must have chosen totally sedentary to get a TDEE of 1800 at age 23.

Since you are within 20 pounds of your goal weight, you shouldn't eat less than BMR.

Eat 1500, get some more exercise. (It doesn't have to be any certain kind of exercise, even 30 minutes of walking every day). You will lose weight.

800 really is not okay for someone your size, even once in awhile, unless you are so ill you cannot keep anything down. It's not going to kill you, but it is totally not necessary, and not a habit you want to get in.

I think even 1200 is too low for most people (except medical issues, very old, dwarfism etc.)

Have a look at this http://www.myfitnesspal.com/topics/show/310489-the-reason-why-starving-yourself-will-not-work

I am eating 1400 to 1500 calories a day and losing a lot more than a 1lb a week , but it will depend how much weight you need to lose if any at all. Just eat sensibly and eat the type of food you like. I think eating only 800 is unhealthy even short term. I hope you get on ok . Good luck

I've just been eating the amount MFP suggested for one pound weight loss a week, and I'm also not sure how accurate my BMR is, considering I did this online.

And, I'd let to clarify a bit on my question. I want to know if eating 100-300 calories less than 1200 every so often is harmful. I don't do this on purpose, but sometimes it happens. In fact, I'd like to eat more, but I'm trying to only eat when I'm hungry, otherwise I tend to graze all day, or eat too much.

I used to do metabolic testing in a hospital, and the online calculators are very good. That's why an equation is out there for people to use--it's been tested and tested and found to be on track for 98+% of the population. So you need to eat, at the very least, your BMR. Otherwise, stuff like hair loss starts to occur. Are you trying to be skinny AND bald?

Is eating 800 calories okay?

In one meal, yes. In a day, no.

NO