I have read that some people when engaging in an extended Fast sometimes lose more weight than expected from calculating TDEE. Is there science behind this?
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Source? Hard to say anything without the “facts” or how this conclusion was reached. Following is link to Fasting science, Weight loss for extended fasts discussed. Average weight loss (when several individuals used) for extended fasts closely matches the expected weight loss assuming TDEE is 2000 calories. Individual #s varied, as individual TDEEs vary.
https://www.dietdoctor.com/ancient-secret-weight-loss
There are so many different factors that could be involved… two possible ones that immediately come to mind are:
The estimation for their TDEE is wrong (there is going to be some individual variance, and activity levels adds a lot of room for over estimation); or
they have lost additional ‘water weight’. When the muscles have full glycogen stores, they suck up a lot of extra water associated with that (I’m sure there’s a lot more to it than that, but that’s the basic idea), as you fast and/or reduce carbs, those glycogen stores will be used up, releasing a lot of water with it. This can be a pretty significant amount for a larger person, it’s a short-term effect on the scales (first week or two) and should stabilise after that once you start to consume more carbohydrates. Not all weight loss is fat loss – not all weight gain is fat gain.
You can’t always rely on numbers in a spreadsheet being correct… experiment, take notice of what works best for you, be your own scientist.
I agree with Vaderz. There is a vast difference between weight loss and body fat loss.
My dietitian has explained that a woman’s body (not sure of the numbers for men) retain approximately 3kg of glycogen (which our body makes from the carbohydrates we consume) and every gram of glycogen is stored with a corresponding gram of water. Half of this is stored in the liver and the rest in our muscles. This is a total of 6kg of body weight (but it is not body fat). On any day when you eat fewer calories than your body needs it raids its glycogen stores. As soon as you break your fast and consume food that contains carbohydrate it tries to replace the lost glycogen (along with the water that is stored with it). This means that if carb intake fluctuates from day to day then so too can weight, without ever actually shifting any of your stored fat (or not shifting as much as we think we have). This is one of the reasons that people notice a massive initial weight loss when they start a low carb eating regime and a corresponding jump in their weight after any day when they break their plan and eat carbs. Just because our weight changes, doesn’t mean we have changed body fat. It’s why our weight can be a rather blunt instrument in monitoring the effect various eating patterns are having,particularly if we only look at the short term weight data.
When fasting your body will raid your glycogen stores just to keep brain function, circulation, endochrine system, nervous system… all going. As soon as you break your fast your body is designed to replace those glycogen stores as quickly as it can. The only way to see what effect fasting is truly having on your body fat is to monitor body weight over a longer term (unless you have access to reliable body fat measurement). For example take your weight at the end of each non-fasting period and monitor the trend in those weights over several months. This will give you a more realistic idea of the amount of body fat you are actually losing.
Last year I had a DEXA scan and my TDEE from that was calculated to be well over 2800 kc/day. This is done by looking at your actual muscle mass and how much fat you have. Calculators tend to give me a much lower TDEE, less than 2500 kc/day. Probably if it was actually measured in a lab it would be a different value. These types of numbers are just rough estimates.
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