All myths. 1lb of fat is 3500 calories. That's science nothing I've invented. Also the metabolism will not slow down again another myth almost on a par with believing that eating after 6pm means you'll not lose weight.
Eating at a dangerously low daily intake of 1000 over a sustained period of time will damage your metabolism of course but if they read correctly its 1000 less than your body needs.
A 17 stone man, average height will need around 2800 calories per day at a guess to SUSTAIN his weight. Therefore eating 1800 per day would see him lose 2lb per week. That's simply how it works.[/QUOTE]
There are a couple of key things as to why your assumption is wrong. Much of this was learned from Lyle MacDonald, who's a bit of a bell-end but is worth reading his stuff if you are interested in a scientific approach to weight loss.
1. muscle and fat are not identical
Your advice is based off the assumption that the weight being lost is 100% fat. When burned by the body, one gram of fat provides 9 calories so 400 grams of fat contains about 3600 calories of stored energy. Now you know where the old 3,500 calories to lose a pound comes from. It's incorrect because on a diet where you just eat 1,000cals less of anything you also lose muscle and connective tissue. Muscle and connective tissue does not provide as much energy to the body as a pound of fat does (muscle provides about 600cals when broken down).
So hypothetically someone who was made up of 100% fat would lose 1lb on a 3500/week deficit, someone who was 50% fat 50% muscle would lose around 1.7lb and someone who was 100% muscle would lose 5.8lb all with the same 3,500cal/weel deficit.
2. The energy balance equation is not static (and metabolic slowdown is not a myth)
People assume that if their maintenance caloric intake is exactly 2500 calories (at calorie balance); therefore if they start eating 2000 calories they should lose exactly 1 pound per week. Or that that the 2,500 calorie/day maintenance will not change.
Aside from the muscle vs. fat thing, this is still incorrect because the equation isn’t static, this makes predicted and actual changes in body mass different.
When you lose weight, BMR goes down becasuse a smaller body burns less calories. However, there are also changes in hormones like leptin, insulin, nervous system output and thyroid hormones. Each of these factors have an influence on basal metabolic rate which lessens the actual deficit being created. The previously estimated maintenance value is no longer correct. To keep losing fat at anywhere near the same rate, calories have to be reduced further to take this reduction into account.
The Thermic effect of food and water balances are other important factor but since I've been rambling on I'll not go into them.
So, to go back to your example and your advice 'A 17 stone man, average height will need around 2800 calories per day at a guess to SUSTAIN his weight. Therefore eating 1800 per day would see him lose 2lb per week'
If he was a 17 stone man of average height with a bodyfat % of 10% eating 1800 would not see him lose 2lb per week, for the reasons above. If he was a 17 stone endomorphic man he may lose 2lb per week but eventually this would slow as his BMR becomes lower.
If we were talking about a small woman who only needs 1250 calories per day, she couldn't even survive on 250cals a day that would be left over if she went for a 1000cal deficit.
So now do you unbderstand why you can't just say to absolutely everyone 'just eat 1000 cals less than what you need everyday and you will lose 2lb per week'?
