If person has extreme low calories diet they will experience what? Like reduce RMR or increase RMR? Or do they lose lean muscle? Does muscle metabolic rate increase? What happens with low calories intake?
Yes! Following an extremely low calorie diet will sabotage both your efforts to lose weight, as well as your lean body tissue. Lean body tissue includes not just muscle (which is highly metabolically active and can help increase your metabolism) but also your organs, such as your heart. This is why extremely low calorie diets must be physician supervised.
Eventually a low calorie diet (below 1200 calories) will reduce your resting metabolic rate (rmr). It's a way humans evolved to save ourselves in times of famine: if there is no food available we slow our metabolism to survive longer. It takes a while for this to occur, but eventually you will end up gaining weight by eating a normal amount of food if you stay on a very low calorie diet for long.
A restricted calorie diet is a poor method for losing and keeping weight off. Just take a look at long distance runners and triathletes. You'll notice that their lean muscle mass deteriorates as their caloric intake decreases and as they increase their cardiovascular stress. Apart from being a curiosity to the scientific community, I would strongly advise you not to consider caloric restriction, since your muscle mass will decrease (therefore, reducing your RMR), you will have greater difficulty in meeting your daily nutritional needs and lean muscle mass, to be grown and developed that is, requires substantial proteins and enzymes, not just the raw energy of simple carbohydrates. Keep in mind, that in order to process these nutrients, your body also needs at least 20% (for the average person) healthy fats, mono and polyunsaturated, to keep the veins and arteries pliable.
It sounds as though you are interested in increasing you RMR and it can be done safely and effectively without the dangerous side effects of caloric restriction. No doubt you've heard about the benefits of resistance training for increasing RMR, but you can increase your metabolism by what you eat as well, such as green teas, healthy green vegetables, legumes and lean meats. There are a myriad number of natural herbs that taken in moderate quantities have been shown to elevate a persons metabolic rate safely, such as yohimbe, gotu kolu and others (without the caffeine rush and stress of a well know stimulant).
You ultimately shouldn't consider a restricted food regimen, since you can obtain the same metabolism boosting effect through what and when you eat (the WEWE principle I've written about in www.anti-agingbeauty.com). It is important to spread your meals throughout the day and to keep track of what you eat and in what elemental proportion, meaning, carbs, proteins and fats. No matter what your body structure, if you can adhere to a restricted calorie regimen, you can certainly adhere to a metabolism boosting, protein oriented, time considered lifestyle.
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