This from this website:
I think it makes sense.
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.
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
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
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
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.
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
Lubert Stryer Biochemistry Fourth Edition
The Metabolism of Weight Loss J. Mericle M.D.
Thanks for your attention.