Fat Burning and Weight Loss Myth #2
Won’t Fasting Slow Your Metabolism?
Why Your Fat Burning Metabolism “Speeds Up” When You Fast
Once again this question from one of our readers …
Just read your letter “Energy That Is Stored -Must Be Utilized.” Won’t fasting slow your metabolic rate? Doesn’t your body think you’re starving when you fast and slow down to conserve energy? If not, why are so many of the latest diets centered around several small meals a day instead of 3 square meals?
Which “metabolic rate” are we talking about?
The human body has many “metabolic rates.” In Stryer “Biochemistry” there are 344 pages on metabolism including twenty-eight pages devoted to Biosynthesis of Membrane Lipids and Steroids, twenty-six pages devoted to the “metabolism” of amino acids, twenty-four pages devoted to the biosynthesis of nucleotides and another twenty-three pages devoted to integration and regulation of metabolism.
A single bacterial cell such as Escherichia Coli has well over one-thousand metabolic reactions. While there are many “metabolisms” or “metabolic rates” we could study, for the purpose of this article we will limit the discussion to the first job that our metabolism must accomplish -provide energy to keep us alive and mobile. This also happens to be the area of interest in the question at the start of this article.
Energy Metabolism
The human body has evolved because of a rather elegant biochemistry of energy metabolism. There are times when we must utilize energy and there are times when we need to store energy. Without the ability to do both of these, the human would not have evolved. Man needed to utilize energy to obtain food that would eventually be stored. This stored energy would later be called upon to provide energy to obtain more food. As man evolved it was not uncommon to have several days between meals. Human metabolism had the amazing ability to provide energy to keep man alive during periods of starvation and it has not changed since then. The only real change is that man now seldom if ever goes two or three hours between meals, much less two or three days.
the biochemical pathways of energy storage and utilization are distinct
“An important general principle of metabolism is that biosynthetic and degradative pathways are almost always distinct.” (p456 Stryer Biochemistry)
Therefore we have the metabolism of energy storage (anabolism) … and the metabolism of energy utilization (catabolism) as separate biochemical entities. The relevance to the above question is now apparent -are we talking about the metabolism of energy utilization or the metabolism of energy storage?
Won’t fasting slow your metabolic rate?
What does the body do when we fast? In order to understand this we need to take a brief look at how the body regulates our metabolism so that we can go days without food -the hormonal system of insulin and glucagon. Insulin is the hormone that facilitates entry of glucose into our cells. Glucagon is the hormone that facilitates the utilization of the stored energy (glucose, glycogen etc). When we eat, insulin is secreted into our bloodstream to allow entry of the glucose into the liver, muscle and fat where insulin also acts to promote storage of this glucose as glycogen in the liver and muscle and as triacylglycerols (fat) in the fat cells. About two to four hours after eating the insulin levels drop and the glucagon levels rise. This reverses the effects of insulin and mobilizes glucose by degrading the glycogen in the liver and muscle and the triacylglycerols in fat.
The “teeter-totter” of human energy metabolism
As one goes up, the other goes down. As insulin levels rise, glucagon levels decrease. As insulin levels decrease, glucagon levels rise. It is very important to understand that human biochemistry does not like to store energy at the same time it is utilizing it. Or, if one looks at ATP/ADP/AMP which is the “currency” of energy metabolism we find the same. As ATP (energy rich donor) levels rise the AMP and ADP levels decrease. When the energy charge is high the levels of ADP and AMP are low. When the energy charge is low, the levels of ADP and AMP are high. Not that you need to understand ATP/ADP/AMP metabolism but they are mentioned only to reinforce the concept that this is a yin/yang or teeter-totter effect. As one goes up the other goes down.
The Answer
Now you can answer the question. What happens when we starve? Insulin levels go down, glucagon levels go up. This is BASIC human biochemistry. When glucagon levels go up what happens -the human body utilizes, not stores energy. The “metabolism” / “metabolic rate” of energy utilization goes up! It does not go down as there is NO way for it to go down. What does go down? The “metabolism” or “metabolic rate” of energy storage.
Eating Multiple Small Meals
Now that you understand some of the hormonal control of your metabolism, it is easy to see why eating multiple small meals is a recipe for morbid obesity. There is no way to eat multiple meals without elevating your serum insulin levels. You end up spending your whole day / life in the insulin “fed” state. This effectively prevents utilization of stored energy. What happens when you don’t utilize what you have stored … you get fat!
Reference:
Lubert Stryer Biochemistry Fourth Edition
The Metabolism of Weight Loss J. Mericle M.D.
Thanks for your attention.
Copyright © John Mericle M.D. 2000-2009 All Rights Reserved
Dr Mericle:
Thank you for writing such an excellent article.
This sounds like a silly question, but I think there is a side to it that some people will find illuminating (I know I will). That is, when you lose weight, where does it go? Everyone talks about calories in/calories out. Sure, that’s useful. But if you “ride the scales” everyday when trying to pull off pounds, one may start to wonder, What about pounds in/pounds out? So, on a pound for pound basis, when food and liquid is consumed, and equal amount must be released for weight to remain constant. Basically, my question amounts to, in the metabolism of triaglycerides and glucogen, is there a proportion of liquid byproduct (water) to solid waste that must be expelled by the body? My question ignores the effects of sweat, as I think that must be dehydration weight loss. But please correct me if I’m wrong. Thanks for reading this!
Mike
Hello Mike … Thank you for the interesting question. The answer is that the weight is metabolized finally ending up mostly as water and CO2. From an earlier article (actually a rage on the dangers of sports nutrition) I put up this question from Stryer Biochemistry that might shed some light on where your weight goes. This is also interesting especially to the long distance endurance athlete because of the preservation of water when you are burning fat.
question number two … the “camel’s hump” From Stryer Biochemistry
“Compare the H2O yield from the complete oxidation of 1 gram of glucose with that of 1 gram of tripalmitoylglycerol. Relate these values to the evolutionary selection of the contents in a camel’s hump.”
“One gram of glucose (molecular weight 180.2) is equal to 5.55 mmol, and one gram of tripalmitoylglycerol (molecular weight 807.3) is equal to 1.24 mmol. The reaction stoichiometries indicate that 6 mol of H2O are produced per mole of glucose oxidized, and 49 mol of H2O per mol of tripalmitoylglycerol oxidized. Hence, the H2O yields per gram of fuel are 33.3 mmol (0.6g) for glucose, and 60.8 mmol (1.09g) for tripalmitoylglycerol. Thus, complete oxidation of this fat gives 1.82 times as much water as does glucose. Another advantage of triacylglycerols is that they can be stored in essentially anhydrous form whereas glucose is stored as glycogen, a highly hydrated polymer (p. 605). A hump consisting mainly of glycogen would be an intolerable burden -far more than the straw that broke the camel’s back!”
Thank you, Doctor. Fascinating. I appreciate the response a great deal. I’ll read the article you referenced.