250, 500, 2,000? Is there a definite limit? Or is it worthless to go more than a 500 calorie deficit?
There is no answer because of the way that you have asked the question. If you were running a marathon, you could burn a lot of body fat. If you are sitting on your ass, eating ice cream, you will gain body fat. If you are sitting still all day, and not eating, you will burn around 1,500kcal, plus or minus, depending on body size.
It’s about 31.4 kcal per pound of body fat. The keto calculator factors this and warns about a too high deficit when calculating your deficit.
I posted this in another thread:
It might have been a bit hyperbole, but the amount of fat you can lose per day depends on how big your fat stores are. You can get around 31.4 kcal per pound of fat per day from your fat stores, so more fat means you get more energy, and less fat means less energy.
Say you’re 240 lbs (162 lbs lean mass, 78 lbs fat, 31.2% body fat) and want to get down to 180 lbs (162 lb lean, 18 lbs fat, 10% BF):
At 240 lbs (162 lean, 78 lbs fat) you can get ~2449 kcal from fat stores.
At 220 lbs (162 lean, 58 lbs fat) you can get ~1821 kcal from fat stores.
At 200 lbs (162 lean, 38 lbs fat) you can get ~1193 kcal from fat stores.
At 180 lbs (162 lean, 18 lbs fat) you can get ~565 kcal from fat stores.
So as you lose weight you can lose less fat per day. Going from 240 lbs to 200 lbs would take around 80 days, but going from 200 lbs to 180 lbs would take 83 days. In this case the person loses weight around five times slower at 180 lbs than he did at 240 lbs.
There’s some variability to this due to genetics, environment, diet, etc. as an example /u/necile/ got closer to 44.7 kcal/lb/d on keto (discussion).
This article cites a paper which claims 31 kcal / lb / day.
I think I’ve seen suggestions that limits like these are usually measured for for people who aren’t fat adapted.
http://www.burnthefatinnercircle.com/members/Rapid_Fat_Loss_What_is_the_Maximum_Rate_of_Fat_loss.cfm
It varies per person, but too high a deficit is counter productive/unhealthy. So if your TDEE is 2800, a 500-1000 calorie deficit wouldn’t hurt. If your TDEE is 1400, a 500 calorie deficit would be too high.
> On the basis of our model, we propose an approximate rule of thumb for an average overweight adult: every change of energy intake of 100 kJ per day will lead to an eventual bodyweight change of about 1 kg (equivalently, 10 kcal per day per pound of weight change) with half of the weight change being achieved in about 1 year and 95% of the weight change in about 3 years.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3880593/
They also provide a calorie calculator app, here:http://www.niddk.nih.gov/health-information/health-topics/weight-control/body-weight-planner/Pages/bwp.aspx
You would burn the amount of energy you used and didn’t consume. The energy you use has to come from somewhere. If you fast you will use your body’s energy recourses to make up for it. There is no definite limit as it depends on your energy output (base metabolic rate + activity) and your energy input (food).
Btw the distribution of calories made up from body fat or lean muscle mass also varies.