Nutrition: Insulin sensitivity

I recently came across the following video on YouTube, from one of the many fitness guru's that post their videos on YouTube.  In the video, the presenter talks about the effects of high intake of carbohydrates versus low intake of carbohydrates.  While much of what is discussed is factually correct and is actually a useful source of information on the topic, there is an element of untruth surrounding the topic of insulin sensitivity at around the 2 minute mark.  The presenter describes insulin sensitivity with relation to consumption of carbohydrates as follows:

The gentleman that eats the pasta and feels very energetic, most likely has good insulin sensitivity.  The gentleman that eats the pasta and feels like he has to go to sleep, most likely does not.

Video in question

While true, that you can use sugar consumption and the body's reaction as a benchmark for insulin sensitivity, the untruth is in the order of the statements.  Since the vast majority of healthy people exhibit tiredness after eating carbohydrates, this statement suggests that it is abnormal or those healthy people are exhibiting signs of insulin insensitivity; or type II diabetes as described by our presenter.  Don't fear, feeling tired after eating "a big bowl of pasta" is a perfectly normal hormonal response to an increase in blood sugar.  Let me explain why...

Tryptophan
This is an essential amino-acid that cannot be synthesised by the body and thus must be obtained through food. The significance of this amino-acid in relation to this topic, is that this particular amino-acid is used by the brain in order to produce the neurotransmitter, serotonin.  If you are thinking that serotonin sounds familiar, then that's because you may have heard it used to describe tiredness; we will discuss this in a moment.  tryptophan has to compete against a whole array of other amino-acids in the blood stream, in order to get absorbed by the brain.  Under normal circumstances, these other amino-acids act like a barrier and prevent absorption.  However, if other amino-acid levels are reduced, then concentration of tryptophan increases and thus more tryptophan is able to enter the brain, with the effect of an increase in serotonin production.  Generally, foods rich in carbohydrates are also a primary source of tryptophan.

Serotonin
This is a neurotransmitter that is used by the pineal gland, just below the brain, to secrete a endocrine hormone called melatonin.  The more serotonin released by the brain, the more melatonin that is secreted from the pineal gland.  Let's see what melatonin does...

Melatonin
This is an endocrine hormone that is released into the blood stream to regulate the body's sleep-wake cycle, among a whole host of other antioxidant properties.  It lowers body temperature and causes drowsiness, by suppressing nervous activity to the brain.

Insulin
Insulin, another hormone secreted by the pancreas, has many functions.  One of its primary functions is to allow liver, muscles and fat-cells to absorb glucose from the blood stream.  It also allows these cells, if required, to absorb amino acids, excluding tryptophan, from the blood and has various anabolic effects.  So what?

The missing links
So how does insulin, tryptophan, serotonin and melatonin relate to this topic?  Well, the answer lies in the effects of insulin on amino acid levels in the blood as opposed to glucose levels.  When the amino acid levels in the blood are reduced, tryptophan is left behind in higher concentration.  This allows tryptophan to enter the brains blood barrier and thus results in the production of serotonin and thus melatonin.  Good insulin sensitivity is in fact indicative of high levels of melatonin and thus lethargy and tiredness after uptake of carbohydrates.  If you feel more energetic having just eaten "a big bowl of pasta", then this would indicate blood glucose saturation and low levels of insulin in the blood: insulin insensitivity.  The density of tryptophan would actually be reduced, even though there is an uptake of tryptophan from the carbohydrates, since the glucose molecules are larger and more prominent than most other molecules in the blood stream.

So lets correct that original statement:

The gentleman that eats the pasta and feels like he has to go to sleep, most likely has good insulin sensitivity.  The gentleman that eats the pasta and feels very energetic, most likely does not.