Muscle or Fat?: Part 2

If you made it through all of last week's post, you probably came away with a few "huh?" moments or better yet, some "a ha" moments. In the event neither of those happened and it was a bit overwhelming, let me give you the bullet points below:

  • Most dietary fat is stored (at least short-term) as body fat
  • Most dietary carbohydrate is stored as muscle/liver glycogen and/or burned for energy via muscles and nervous sytem
  • You need REALLY high carbohydrate levels to start turning carbohydrates into fat, around 500-700 g per day
  • If you eat more fat you store more fat and burn more fat - whether you gain or lose body fat is determined by calories
  • If you eat more carbohydrates you store and burn more carbohydrates, but also store more dietary fat as body fat - whether you gain or lose body fat is determined by calories
  • Carbohydrates are our bodys preferred fuel source for muscle and nervous system and we have a finite storage capacity
  • Fat cells (you have billions of them) have a near limitless storage capacity, whether it's from expanding cells or creating new ones
  • Fat is metabolically active - fat stores tell your hypothalamus how much stored energy you have which affects a host of things: menses, testosterone, thyorid, catecholamines, insulin sensitivity to name a few
  • Muscle tissue is more metabolically active - but it takes more effort and energy to create new muscle tissue

As it stands right now, you're pretty much caught up. But what do we do with this information?  How does knowing the physiology behind fat and glycogen storage guide us to better health and body composition?

Glad you asked.

Muscle and Fat Cells: Is "Sensitive" Good?

You've probably hear of insulin sensitivity.  Essentially this means how sensitive a cell is to the affects of insulin; the more sensitive a cell is, the less insulin is needed to exert it's effects.

However, how much body fat, muscle, activity and exercise you do affects this, not to mention genetic factors, sex and environment. 

People can genetically vary 20 fold in how insulin sensitive they are and although this can be influenced by diet, activity and life style some people just have it harder or easier than others.  We probably all know someone who seems to lean out and/or gain muscle with ease.  And we all know someone, despite best efforts, who doesn't get quite the same results.

This doesn't mean a person without good genetics can't have a great physique, it just means it takes more effort, time and consistency. So, no excuses.


If a muscle cell is sensitive to insulin then it takes a relatively small amount to get glucose into the cell. This is good because an insulin insensitive muscle cell needs more insulin over time to get glucose into the cell, which negatively affects other tissue like fat cells, liver and pancreas.  A diet chronically high in carbohydrates, saturated fats, low fiber and of course, excess calories can contribute to this, as well as little to no activity or exercise.

Since muscle tissue is your primary glucose disposal site, having sensitive muscle tissue gives you excellent clearance of blood glucose which is going to help your butt and your heart.  Dr Ben House talks about "pulling" energy through the system; having larger amounts of muscle mass that can pull energy out of the blood stream is important to health.

Weight training improves chronic insulin sensitivity and if you gain muscle, you simply have more tissue to dispose of glucose.  Makes sense why men can eat more carbs than women; more muscle mass = more glucose storage.   Interestingly, women have BETTER insulin sensitivity than men but lower capacity to store those carbs. During weight training, you can pull glucose into the muscle cell without insulin which is why activity AND weight training is so imperative for those with impaired insulin sensitivity among other things.


If you read the above about muscle, you probably think that you want fat cells that are really insulin sensitive too.  Lets chat about that.

Insulin does work to drive glucose and fatty acids into fat cells but fat cells do NOT need insulin to store dietary fat.  A protein call ASP (acylation stimulating protein) causes fat cells to store fatty acids without insulin present and in research, looks like it can stimulate insulin all by itself.

What this tells us is that even if you zero carbs and have super low insulin, over-eating calories, namely fat in this case, will still cause excess fat storage.

It's important to understand that in fat cells, insulin does more than just help store calories as body fat.  It also suppresses the release of fat (triglycerides) into the blood stream.  While this might sound like a bad thing, consider how Type 1 diabetics, not producing insulin, can get dangerously high levels of fat in the blood, which cannot all be burned as energy and thus turned into ketones.  Excess ketones, in very high amounts, can cause keto-acidosis which is life threatening. When you pair this with the inability for insulin to drive glucose into the cell, you now have high levels of blood glucose AND fatty acids.  In addition, insulin suppresses the production and release of new glucose from the liver. Left unchecked, dietary carbohydrates are not disposed of, resulting in high blood glucose levels, fat cells cause dangerously high levels of blood triglycerides (and ketones) all while the liver is still producing more glucose.  That's bad.

If you think you don't need insulin to help regulate things to actually help keep you alive, you're wrong.  It's vital for your health and ability to dispose and regulate energy.

Liver and Pancreas

As noted above, dietary carbohydrates cause insulin release to help lower blood gluose back to safe levels.  Since the pancreas produces insulin, the pancreas also has insulin sensitivity.  Essentially, through negative feedback loop, the pancreas releases insulin to clear blood glucose, senses high levels of insulin and stops producing more.  All good.

Your liver can store about 100g glycogen (stored glucose).  Nothing crazy compared to muscle cells but it acts as a regulator of blood glucose. Not only does it store glycogen but it can create more glucose.  Think of this as your blood glucose regulator; blood glucose falls and the liver can release some into the blood stream to keep you going, if carbohydrates are scarce, the liver can create new glucose from other sources.

Insulin suppresses the creation of new glucose and the release of it into the bloodstream.  Again, it's helping to regulate your energy systems.  WHY DO WE THINK IT"S EVIL?!

Confounding Factors

One last note about fat cell insulin sensitivity; when dieting, fat cells temporarily actually become more insulin resistant.  This is because with lower levels of insulin and total energy, the fat cells don't have the same level of fat-breakdown suppression.  Hormone Sensitive Lipase (HSL) is the primary enzyme responsible for breaking down body fat into the blood stream.  Insulin suppresses this, so with lowered insulin levels, more fatty acids are broken down for energy.

High levels of blood fatty acids cause insulin resistance as well (what?!) so your low-carb, high-fat diet is NOT making you more insulin sensitive.  In addition, as blood fatty acid concentrations go up, HSL is suppressed.  So high levels of blood fatty acids also shut down the release of fat for fuel.  As always, total calories are going to determine how much fat is gained or lost, no matter the pathway.

Keep in mind that the benefits of lowering insulin, lowering calories and breaking down stored body fat also has downsides.  With lowered insulin, sex hormones drop, muscle protein breakdown increases, protein synthesis is impaired (muscle building potential), thyroid is suppressed and ultimately, metabolic rate drops.

This explains why not only is it harder to build muscle when calories are below maintenance but sex drive goes down, energy goes down and your fat loss slows down. No matter the energy source, fat or carbohydrates, being in a negative energy balance has deleterious effects.  Now, we want to optimize this by burning as much fat as we can, keeping all of our muscle, ingesting enough carbs to optimize training and do it all in such a balance that we don't crash and burn after a couple weeks.

In Part III next week I'll go into how we can tackle all of that.  In the meantime, sum everything up below and re-read Part I if you need a refresher.

Condensing All of This

We now have our final piece of the puzzle before going practical with this.

Understanding that energy storage doesn't mean fat storage is important; we not only have multiple sites of energy disposal but different means of storing and releasing energy. To sum it all up before we jump into our practical section next week, lets go down the bullet list:

  • Muscle insulin sensitivity is crucial for disposing of blood glucose in a healthy manner, as well as having necessary glycogen levels stored to fuel high intensity activity
  • Fat cells need a certain amount of insulin sensitivity but VERY insulin sensitive fat cells are primed for fat storage.
  • Fat cells with low insulin sensitivity can be good for short-term fat loss but really insulin resistant fat cells can lead to unchecked blood triglyceride levels
  • Pancreas and liver botn need insulin to regulate total insulin release as well as blood glucose levels
  • You don't need insulin to store dietary fat as body fat
  • As usual, total calories play a huge role in how much fat is stored or burned