Carb Trafficking, Body Fat Storage, and Insulin Resistance

SOME BACKGROUND

After taking a bit of a hiatus over the summer from writing posts, I’m now back with a vengeance delivering this detailed post on carb trafficking and insulin resistance.  Whenever applicable, I’m going to end my posts with a Children’s Corner section reflecting the experience of my wife Shally (a pediatrician), who works on the front lines of treating children with obesity and insulin resistant disorders.

Insulin resistance is an incredibly complex concept which is influenced by countless factors including diet (micro and macronutrient intake), exercise/activity, sleep, stress, genes, etc.  I want to use a simple traffic diagram I pulled from my book which has helped patients and lecture attendees tremendously in understanding how their daily habits influence their body composition via the actions of insulin.  I am not going to dive too deep in this post about the biochemistry of insulin resistance since I want this to be easily understandable for readers of all levels.  I will be referring back to the carb traffic diagram in future posts as I focus on lifestyle factors that influence insulin so you understand how seemingly subjective factors like chronic frustration and anger or even minor sleep deprivation may be preventing your body from burning fat.

LET’S TALK TRAFFIC

In the diagram below I’ve illustrated carbohydrates as a car (think CARbohydrates) with 3 major parking lots where carbs are stored: Muscle, Fat, and Liver.  Let’s explore each of these parking lots:

1. Muscle Parking Lot: This parking lot can hold roughly between 300-500 grams of carbohydrates (depending on your muscle mass) which are stored in a form called glycogen (storage form of glucose).  The more muscle/strength you have, the more parking lots you have available to store carbohydrates.  The way carbohydrates are stored inside the muscle is via the hormone insulin.  You eat carbs, your pancreas releases insulin and the insulin is like a parking pass that lets carbs into your muscle parking lots.  A side note is that muscle can also store fat in the form of triglycerides (aka intramuscular triglycerides or IMTGs), but let’s put that on the shelf for now and just focus on glycogen.

2. Fat Parking Lot: Like muscle, your fat cells also require an insulin parking pass from carbs before they are allowed to enter.  There are 2 major differences though.  First, carbohydrates are not stored as glycogen but are instead converted to a storage form of fat called triglycerides.   So carbs are stored as glucose packages called glycogen in muscle and in fat they are stored in fat packages known as triglycerides.  You might recognize triglycerides as one of the results reported on a standard cholesterol profile.  When fat cells swell up with triglycerides, your body gains fat.  So excess dietary carbohydrates trigger insulin release and insulin shoves those extra carbohydrate cars into fat cells where they are stored as triglycerides.  That’s right, carbohydrates (sugar, rice, breads, etc.) are actually converted into fat!   The other big difference is recall that I mentioned that muscle has limited parking spaces (~300-500 grams).  Unfortunately your fat cells have unlimited parking spaces as evidenced by the magnitude of obesity human beings are able to achieve.

3. Liver Parking Lot: The liver is a bit more complicated, but just remember that it stores glucose in the form of glycogen like muscle (aka liver glycogen), but carbohydrate cars can get inside liver without an insulin parking pass.  It has less parking space than muscle with about 100 grams of parking space.  However, your liver is kind of like a hybrid between fat and muscle because it can also form triglycerides, which as you recall is how carbohydrates are stored in fat cells. Basically once your liver is filled to capacity with glycogen, it switches on fat synthesis and starts pumping out triglycerides.  The fancy term for this is “de novo lipogenesis” or DNL.

INSULIN RESISTANCE AND EXCESS BODY FAT

I call the process described above of carbs zooming between different parking lots, “carb trafficking,” but the technical term is “fuel partitioning.”  In an ideal situation illustrated in the image above, ingested carbohydrates are sensibly trafficked between the different parking lots  with most of it flowing towards muscle.   However, when you become insulin resistant (illustrated in the diagram below), it is usually your muscles first that stop accepting the insulin parking pass. As a result those carbohydrate cars are rerouted to fat parking lots which are open 24/7 with unlimited capacity, or they zoom on over to the liver where they are stored as glycogen if there is space.  If there’s no liver glycogen parking space left, then your liver switches on fat synthesis (de novo lipogenesis) and your liver pumps out triglycerides and may also become filled with fat (aka “fatty liver”).  All these effects result in some of the hallmark features of insulin resistance outlined below and which define the condition, Metabolic Syndrome, which I discuss here.

• Increased body fat, especially abdominal fat
• Increased triglycerides formed and released by the liver, which by the way has the secondary effect of lowering HDL (good cholesterol) and increasing the formation of dangerous small, dense LDL (more on this in a future post)
• Fatty liver due to a carbohydrate-overwhelmed liver that desperately switches on fat synthesis
• Elevated blood glucose.  If your carbohydrate intake overwhelms your parking capacity, especially that of muscle, your blood glucose levels will rise.  We need our muscles to be sensitive to insulin and we need to frequently create space in our muscles through increased activity and exercise to effectively control our blood glucose levels.

You can see from the above image that the carb car is diverted away from the muscle parking lot due to a combination of insulin resistance (insulin parking pass not accepted), inactivity (no demand for energy), and high carb intake (no parking space left).   If you don’t stay intermittently active, you can’t create space in your muscles to accommodate the large amount of carbs you are consuming.

If your muscles are insulin resistant, even a small amount of excess carbs will be turned away from muscle and redirected to your fat and liver.  My book and online program explain in detail how to determine your level of insulin resistance and how to make lifestyle changes so you don’t keep overwhelming your muscle parking lots.

Unfortunately, most of my patients are fighting an uphill battle since they are showing signs of insulin resistance and are eating a quantity of carbohydrates that far outstrips their meager activity levels.  Read my post here on getting sufficient activity levels.  Most of my Silicon Valley engineers sit at a desk all day and as a result have very little muscle/strength.

This means they’ve got small muscle parking lots that easily get filled up, pushing overflow carb traffic to fat and liver.  A lack of sufficient muscle  is also why most of my female patients who don’t do some form of weight training struggle with weight loss more than men.  It takes very little carbohydrates to fill up their muscles, so more carbs keep getting pushed to fat storage.

So this is the foundation for most of the chronic health conditions I see in my clinic (obesity, diabetes, heart disease, etc.).  The good news is that even after years of having a serious metabolic traffic crisis, making some simple improvements in nutrition, activity, and sleep/stress management can lead to remarkable changes within a short period of time, restoring a more normal flow of traffic.

CHILDREN’S CORNER

Unfortunately my wife Shally, a pediatrician, is seeing a similar traffic problem in young children.  Kids are more sedentary than ever and they are overwhelming their tiny little inactive muscles with floods of excess carbohydrates and sugar leading to the dangerous insulin resistant conditions I listed above.

Obesity, high triglycerides, adult onset diabetes in kids, and fatty liver are becoming a common signature for modern day childhood.  Fortunately children have great metabolic resilience and often even modest improvements to diet (get rid of the junk!) and increasing activity levels (let your kids play more often!) can restore normal insulin sensitivity.