Diets that create a large calorie deficit can be a very big problem for fat loss!
Most short term, fast weight loss efforts end up being temporary because:
a) They usually center around deprivation which can be difficult to adhere to.
b) They create hunger.
c) Will power eventually runs out.
2) When the body is in an extreme calorie deficit it will burn both body fat and muscle resulting in:
a) A slower metabolism.
b) Less strength and energy.
3) When the dieter runs out of will power from the extreme diet:
a) They will likely gain their previous weight back.
b) The weight they gain back will be primarily body fat.
c) Their body fat percentage will be higher and muscle percentage will be lower.
d) Their metabolism will be lower.
e) There will be less demand for calories.
f) They will feel weaker, have less energy and be more likely to gain more body fat than before the initial short term diet.
The above example can apply to someone who:
a) Creates a large calorie deficit from diet alone.
b) Creates a large calorie deficit from a combination of diet and cardio.
It is my opinion that prolonged, large calorie deficits can be a very big problem for muscle loss and fat gain which should be the exact opposite goal of any health oriented individual.
How should one try to lose weight?
1) Think Fat Loss, Not Weight Loss. How?
a) Putting nutrient requirements aside, eat in any way that an individual consistently has a small calorie deficit on a regular basis
b) Lift weights to maintain or gain muscle mass.
c) Eat a sufficient amount of protein to assist in maintaining and gaining muscle mass.
2) When the body is in a small calorie deficit it will.
a) Create less hunger and be more sustainable long term.
b) Be less likely to burn muscle.
c) Maintain a better metabolism than when muscle is being burned.
3) When the individual is also lifting weights it creates a demand for calories eaten.
a) Takes calories to work the muscle.
b) Takes calories to grow the muscle.
c) Take calories to maintain the muscle.
d) Helps keep the metabolism higher than when muscle is being burned.
e) Positively affects youthful hormones.
f) Provides shape, tone, strength and energy.
Extreme Calorie Deficit:
Burn Fat & Muscle = Difficult & Short term = Slower Metabolism and More Body Fat On Rebound
Small Calorie Deficit & Weights:
Burn More Fat Than Muscle = Easier To Maintain = Fat Loss Not Weight Loss
All of the above is based on my 26+ years of personal experience, research and knowledge. I have been in shape for 26+ years following principles like this and as a 41 year old man I continue to stay in beach body shape year round. If I can do it, others can do it too. Good luck!