Thread: Metabolism Q's

I have a few separate metabolism Q's which stem from conversation with my pre-med (read: knowitall) friend:

She says skipping a meal/waiting too long to eat is ONLY bad for you if you feel hungry. This kinda makes sense in that, if the body is full and it does not want any more food it will not send a hunger signal. So true or false?

She says that it takes 72 hrs for something called biosynthesis or start or stop I forget which, and basically you can lower your calories to starvation levels for 2-3 days with no negative effects on your metabolism. True/false?

Another friend keeps telling me I have F'd up my BMR due to excessive dieting/bad past. How can I find out if that is true or not? What can I do to raise my metabolism besides lift weights, cardio, and eat every 3 hours?

Originally Posted by greekblondechic

She says skipping a meal/waiting too long to eat is ONLY bad for you if you feel hungry. This kinda makes sense in that, if the body is full and it does not want any more food it will not send a hunger signal. So true or false?

Hunger signals are really complex - there are a multitude of hormones/receptors/physiological and psychological parameters that are involved in making your body hungry.

For this reason it is not really a good indication of when you should and should not eat.

Also, what you are trying to do when you eat frequently is prevent many of the things that send out 'hunger' signals - things such as severe lowering of your blood sugar, high glucogon secretion (one of the hormones responsible for the utilisation of amino acids for fuel), high cortisol secretion (stress hormones) etc - all of which make dieting harder (you are more likely to 'over-eat' if you wait until you are starving!!) and lead to increased muscle loss and decreased metabolism.

She says that it takes 72 hrs for something called bio-synthesis or start or stop I forget which, and basically you can lower your calories to starvation levels for 2-3 days with no negative effects on your metabolism. True/false?

This is partially true and partially false. The human body is exquisitely adaptable - it is 'built' for survival and any alteration in incoming energy will result in changes in hormonal status and the engaging of the bodies survival mechanisms.

For example, although your bodies fuel stores (such as liver glycogen and amino acid pools) will last for up to 48-72 hours without your body switching to more long-term survival strategies, your body will start to down-regulate it's metabolism in as little as 12 hours without food!

Just like when you do not eat frequently, when you do not eat your bodies hormonal response is to increase cortisol, increase glucagon, decrease insulin and decrease thyroid hormone. All these things result in a decrease in your immediate metabolic rate...

So, although you will not get 'long term' consequences from short term calorie restriction, your metabolism will not be 'maximised' and the anabolic potential and muscle-sparing properties of frequent meals is lost.

Another friend keeps telling me I have F'd up my BMR due to excessive dieting/bad past. How can I find out if that is true or not? What can I do to raise my metabolism besides lift weights, cardio, and eat every 3 hours?

It is true that excessive dieting and exercise (and other things) can result in long term disruption of your metabolism. The responses in your body to these things (such as alterations in a plethora of neurochemicals/hormones such as decrease leptin, decreased thyroid hormone, increased cortisol, decreased sympathetic tone and decreased growth hormone) are all geared towards lowering your metabolism so your body can survive through the apparent starvation.

As to how you can find out if yours has been effected - well, that is relatively hard... Having to eat much less than you used to be able to eat would be a good indication (taking into consideration any healthy changes you may have made in terms of lowering yourself from being markedly obese or lowering any gross calorie over-consumption)... Things like obsessive thoughts about food, low libido, binging, tiredness, menstrual abnormalities, frequent illness etc are all signs of chronic negative energy balance/low leptin/low thyroid hormone and are therefore signs of decreased metabolic rate.

You could also (if you had the money) get your resting metabolic rate calculated (most large hospitals and some universities have this available) and compare it with generally accepted metabolic rates for people with similar statistics to you - however RMR is so variable between individuals that this may prove useless.

Anyway - I would just take it as a 'given' that if you have been severely restricting your calories, or if you have lost substantial amounts of weight, that your metabolic rate has suffered.

What you can do is just try to do to increase it again is what you have suggested
1. Exercise (resistance training as well as cardiovascular training) will help with your nutrient partitioning - both via increasing your muscle mass (which means you have a greater 'metabolic sink' or 'energy drain' - and this means more incoming energy is needed and used by your muscles) and also via an increase in insulin sensitivity and cell receptor expression of your muscles.

The exercise does not have to just be planned. NEAT (non-exercise associated thermogenesis) is actually the most substantial contributor to a persons metabolism - this is things such as walking, sitting, fidgeting, shopping, gardening, talking and any movement you make during the day. So the more active you are the better (there is a limit however - as EXCESSIVE exercise will eventually cause a stress response which will decrease metabolism - so it is all about finding a balance).

2. Make sure you eat sufficient calories - if you know you are eating too few calories then start by slowly increasing your intake.

3. Making sure you eat a healthy balance of carbohydrates, proteins and fats. All three of these macronutrients (in my opinion anyway) are essential for maximal metabolism. Also, carbohydrates and proteins have a little extra thermogenic potential, meaning your body has to work a little harder to try to get the energy it needs from them (as apposed to fats, which have very little thermogenic effect).

Also, a higher fibre intake will also make your body work harder and omega-3 fatty acids are helpful in maximising your bodies metabolic potential (they assist in hormone production and allow your cells to metabolise fats more readily).

Trying to get most (if not all) of your calories from real foods (and not powders and supplements) is also a good idea and you should make sure you are getting your required vitamins, minerals, water etc. (eg: The B vitamins are really helpful in metabolism).

4. Make sure you eat frequent, smaller meals - this increases the energy needed to digest/absorb and utilise the calories from your food, meaning your body has to work harder and this will increase your calorie needs as a result. Eating frequently will also prevent those negative hormonal responses that I talked about above (increased cortisol, increased glucogon etc).


It may take some time but you can restore your metabolism...


(however, if you remain under your natural set-point or if there are on-going issues in terms of leptin/ negative energy balance/ stress then, unfortunately, it is possible that your metabolism will not return to its pre-dieting state).