How (much) can a metabolism slow down?

Question

Always when you read about diets, you also read a warning saying that a too small calorie intake will slow down your metabolism. However, I can't find any sources explaining what exaclty slows down and how much engery is conserved through this slow metabolism (1%, 5%, 50% or maybe 99%?).

I know from experience that I can't reach the max. performance in sport during a diet as glycogen reserves are depleted faster and ketogenic processes take a while to kick in, but these kinds of reduced enegery consumption are in general additonal accitivies that are not required to keep up life.

When I sit on a chair and just fart around, is my energy "consumption" reduced through the following things, as compared to a normal food routine?

• Does the heartrate sink?
• Is blood renewed slower?
• Does my brain work less?
• Do hair and nails stop growing?
• Does my immune system take a holdiay?
• Do my bowls drain the food more completely?
• Do I produce less semen?
• Do injuries heal slower?
• Do my cells get older (recycled later), thus maybe even increasing the cancer risk?
• Is my temp lower?

I presume most of these things can only be reduced slightly in order to survive, so where does the body optimize energy "consumption", and how much savings are we talking about?

Even when eating massivly less than required (e.g. 5000KJ under maintainance requirements over more that 2 weeks) I do not notice massive drawbacks in bodily functioning, therefore I am questioning these warnings. Furthermore you often read about a "starvation mode" and even a "ruined metabolism" - is there any scientific indication that such things exist and are inreversible (slowly reversable)? If so, why does the body in gerneral not work in this more effiecent state if there are no drawbacks?

I'm not questioning that lifting heavy weights while eating less will conserve more muscle mass. Also please don't give answers along the lines of changing your food consumption in general to lose weight (e.g. what the word diet really means). I'm using the word "diet" in the wrong modern way, as a limited time change of food intake.

Answer 1

I don't know what bodily functions are affected by a decreased energy expenditure but I have found two studies on the subject which I think you might find interesting.

The first is on 3 days of fasting, conducted at Department of Medicine, University of Rochester School of Medicine and Dentistry, NY. Six men fasted for three days and as a result their metabolism (RMR) decreased by 8%. During this period their glucose production declined by 38%, while their proteolysis (traced by leucine) went up.

The second study is on 6 months calorie restriction and comes from Pennington Biomedical Research Center, Louisiana. It includes 48 overweight subjects over six months, divided in four groups with different diets and activity plans. The calorie deficit groups had their daily energy expenditure (TDEE) decreased by 270-430 kcal. However, the one group who were dieting but also exercising did not have their TDEE affected at all.

So starvation mode does exist, but you can completely mitigate it by working out while decreasing your calorie intake.

This perhaps only answers half of your questions and maybe my conclusions are wrong (I'm no expert in nutrition) but either way; there's two tangible studies on the subject if you want to dig in.

Answer 2

Here are my 50 cents.

I would say, by reading your question, that it is convenient to remember that the 3 following things are totally different:

• When on a diet, mathematically absorbing less minerals, vitamins and other essential oils
• Being on a reduced activity (not moving = consumming less energy, != lowering your metabolism)
• Having an actual drop of metabolism.

These 3 elements are complementary, not necessarly interconnected.

Remember that you will have body issues even on a high calories diet if you don't pay attention to what you eat.

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So, now that this is clear, let me see whether I can help about the metabolism.

Your basal metabolism is if I'm not mistaken defined by the energy your body is consumming when resting (extended on a bed, for example). This involves many different aspects that I don't control totally, but the main are:

1. Your body will consume energy to feed your brain
2. Your body will consume energy to maintain its temperature
3. Your body will consume energy to maintain your muscular mass
4. Your body will consume energy to feed the central system (mainly, the nervous and endocrinal systems).

When you lower down your energy (food) intake, you're right to say that not so many things can be adjusted. But definitely the first things to drop down will be your central system quality level (sleep of less good quality, hormonal state a bit down, less strength...) and then your muscular mass, hence reducing the energy consummed every single second. That's why when some people go on an uncontroled diet they not only loose fat but drop down their metabolism, making it then really easy afterwards to gain weight again.

I hope I helped a little bit. Cheers!