Life is energy – 2

Which fuel type is our body best suited for? That was the open question from the last post. Just like my friends’ Lexus Hybrid is designed to run on petrol and electric, what is the human body designed to use and when? Just like the hybrid car is designed to switch fuel sources on a steep uphill when it requires extra power, and cruise along on electric power on a flat country road, is our body also designed to use different sources to fuel it’s energy needs? Most definitely !!

For an individual weighing 70.5 kg, let us look at the stored potential sources of fuel. First up – protein.

Fuel comparison Protein

Source: Devlin, T., ed. Textbook of Biochemistry with Clinical Correlations. Wiley & Sons, Inc. 2011

*Based on an individual weighing 155 pounds (approx. 70.5kg)

The muscle mass of this individual has typically about 6000 grams of body protein, which means 24,000 calories of stored fuel (remember proteins and carbohydrates have 4 calories per gram). WOW, that is a lot of stored energy! But where is that protein stored? In our muscles, right? And our organs, glands, bones, and other precious tissue that we do not want to break down (catabolize) for fuel. (More on this when we get to weight loss—specifically, why we want to aim for fat loss or “waist loss” rather than “weight loss.” We don’t just want to lose weight; we want to lose fat and hang onto as much muscle mass and other lean tissue as we can.) Proteins are essential nutrients of our body. The building blocks of body tissue, so why would you want to break that down for energy? Proteins are made of essential amino acids which we must obtain from food …therefore ESSENTIAL!! Anyways, protein is not what we want to use as our primary fuel. It has too many valuable jobs to do. Bottom line: those 24,000 calories are awfully tempting, but as a fuel source, but they’re out.

Let’s move on to carbohydrate. (Abbreviated as CHO in the chart below)

fuel-comparison proteins and carbs

Source: Devlin, T., ed. Textbook of Biochemistry with Clinical Correlations. Wiley & Sons, Inc. 2011

Our bodies digest the food we eat by mixing it with fluids (acids and enzymes) in the stomach. When the stomach digests food, the carbohydrate (sugars and starches) in the food breaks down into another type of sugar, called glucose. The stomach and small intestines absorb the glucose and then release it into the bloodstream (blood sugar? anyone?). Once in the bloodstream, glucose can be used immediately for energy or stored in our bodies, to be used later. This is the 20 grams (or 80 calories) of fuel you see above. It’s not much. 80 calories isn’t anything to write home about, so let’s move on to the other form of stored carbohydrate in our bodies: glycogen.

Glycogen is to humans what starch is to plants. It is the form in which we store carbohydrate. (We store it as glycogen, and a potato, for example, stores it as starch.) Since our blood can only hold so much glucose at any given time (even for a type 2 diabetic with sky-high blood sugar), any excess has to be removed quickly and our bodies have to find somewhere else to stick it. This “somewhere else” is our liver and our muscles. Looking at the chart above, the liver can only hold about 70 grams of carbohydrate as glycogen, for about 280 calories’ worth. That’s still not much. So this liver glycogen, like the glucose in the blood, and like the protein in the muscles, doesn’t seem like such a great fuel for the body to rely on.

But the muscles—now we’re getting somewhere. Even a relatively non-muscular person still has a fair bit of muscle mass. The hypothetical 70.5 kg person represented in this chart stores about 120 grams of carbohydrate in their muscle glycogen, for around 480 calories. Not too bad, but nothing to write home about, either. Plus, another mark against relying on muscle glycogen as a fuel for the whole body is that it can only be used to power activity in the muscles in which it’s stored. It does the rest of the body no good. Glycogen stored in, say, your biceps, can’t be released into the bloodstream when your blood sugar is getting a little low.

Only liver glycogen can do that. In fact, that’s pretty much liver glycogen’s ‘reason for being’ — to keep you from getting woozy and feeling faint if you find yourself needing to go a few hours without eating any carbohydrate, or any anything, for that matter.

Remember: carbohydrates supply only 4 calories per gram (in our car analogy it was 4 km/l). We have to keep fueling the fire (refilling the tank) constantly. We will need to have a snack every two or three hours (which is what BigFood wants you to do BTW and you will see endless sponsored research telling you the benefits of snacking). I’d rather use a fuel I don’t have to top off quite so often, wouldn’t you? You wouldn’t have to stop often and you wouldn’t get those hypoglycemic symptoms when your tank starts getting low, because your body would be running on a fuel that you didn’t have to constantly put more of in the tank.

Let’s look at fat.

fuel-comparison - all

Source: Devlin, T., ed. Textbook of Biochemistry with Clinical Correlations. Wiley & Sons, Inc. 2011


Do you see what I see? OMG

The hypothetical person, weighing 70.5 kg, stores about 15,000 grams of fat in his adipose (fat) tissue, for a whopping 135,000 calories!! (9 cals per gram for fat) NOW WE’RE TALKING, PEOPLE. And he is of normal weight, not fat, not obese. A normal bloke. This is like a 45 day store of energy for the body. Did you ever wonder how some of our great leaders could go on hunger strikes for days and live to tell the tale?!! (those were the days of our good old fat- burning lean politicians. Can you imagine the jokers today going on hunger strikes after being fuelled by the Sodas, BigMacs and Donuts!! They won’t last 30 minutes!! That’s why hunger strikes are so out-of-fashion!)

Anyway, THAT is some serious fuel storage. The human body has an almost unlimited capacity to store fat and accumulate adipose tissue. Taking this into account, it almost seems as if nature (or evolution, or the big voice in the sky, or Bramha or whatever you happen to believe in) evolved/created/designed our bodies to run on fat, because that is the type of fuel our gas tanks are designed to hold the most of.

The single most fundamental – and simple – way to improve mitochondrial function is to turn away from relying on sugar-burning and transform yourself into a fat-burning beast. See, mitochondria burn fatty acids cleaner than they burn carbohydrates and generate way more ATP as we have seen in my previous post.

We have seen so far that fatty acids are the most preferred fuel of the mitochondria, fatty acids are available in abundance in our bodies, they are the long lasting and produce way more energy per molecule than any other fuel source. Remember this next time you hear some poor, misguided soul say that carbohydrates are the body’s “preferred fuel source.”

How do our bodies know which fuel to use?

Is there a way to prime our bodies so that they’ll run more on fat than on carbohydrate? (Yes.)

Is this fat/carbohydrate debate like the Apple OS/Android thing? (No.)

Is fuel partitioning absolute? If I’m running on fat, does that mean I’m not using any carbohydrate at all? (No.)

Can my entire body run on fat? (No.)

Does my body need some glucose? (Yes.)

HOWEVER – and the big CAPITAL HOWEVER – let me take a moment here to tell everyone that being a sugar- or fat-burner is not a binary thing. It’s not a yes/no, on/off scenario.

Unlike the lovely Hybrid car , it is not the case that the human body—every single cell in every single organ, tissue, and system—is either fueled by glucose OR fueled by fat.

Different parts of the body are running on different fuels concurrently, and the proportions of different fuel types that are used can change, based on the type of activity a tissue is engaged in.

Here are a few examples:

Red blood cells: RBCs have no mitochondria. Therefore, they must use glucose exclusively.Remember, mitochondria are the “powerhouses” of most cells. The energy factories, if you will. And when fats are used as fuel, they are “burned” in the mitochondria. So if a cell has no mitochondria, it can’t use fat, right? (If your stove isn’t a gas stove, then it can’t use gas. It has to use electricity or some other form of energy. Simple enough.) And since we’ve already said fat is our primo, go-to fuel (maximum ATP per molecule), why would there be cells in our bodies that can’t use it? You must love your creator, the ultimate guru: In producing energy, the mitochondria use up a lot of oxygen. But what is the job of RBCs? They transport oxygen (via the bloodstream) to the rest of the body, right? Well, what good would it do if RBCs used up all the oxygen they’re supposed to deliver to the rest of the body? It would be like using the services of the DHL man who keeps all the packages for himself so that none of the cargo gets delivered. If you think you would be dead if your shipment of books from Amazon never got delivered, imagine how dead you’d be if your heart (or gonads) stopped getting oxygen.

Cardiac/heart cells: Your heart is one of the most aerobically active of all your cherished parts and pieces. Think about it: your heart is using energy 24/7/365. IT. NEVER. STOPS. (Well, that is, until it stops for good. But other than that, it is a muscle that is contracting and relaxing every minute, every day of your life.) It never gets a rest. It needs to be fueled all the time, no matter what. In order to make sure this happens, your heart is loaded with mitochondria. Oxygen users like crazy! So since the heart has all these mitochondrial fuel generators available, it might as well use them to generate some fuel—and what did we see ? The nutrient that gives us the most fuel per gram is fat. So the heart uses fat like a champ. But don’t just take my word for it. According to people way smarter than I am, “Between meals, cardiac muscle cells meet 90% of their ATP demands by oxidizing fatty acids. Although these proportions may fall to about 60% depending on the nutritional status and the intensity of contractions, fatty acids may be considered the major fuel consumed by cardiac muscle.” “The heart has virtually no glycogen reserves. Fatty acids are the heart’s main source of fuel, although ketone bodies as well as lactate can serve as fuel for heart muscle. The point is, the heart runs on at least two different types of fuel better than it runs on carbohydrate.)

Brain cells: The brain requires glucose. No two ways about it. Even the most ardent low-carber can’t deny that the brain needs glucose. However, it doesn’t need as much glucose as we tend to think it does, as long as the glucose debt is made up for by fuel coming from an alternative source, like ketones. Fatty acids are generally not used as fuel for the brain. Glucose and ketones are the main players in the upper level.

EnterocytesThese are the cells that line the small intestine. And what is one of their big jobs? To move nutrients (such as glucose) from the lumen of the intestine into the bloodstream, yes? (Yes.) So what good would it do if these cells used that glucose to fuel themselves? The rest of the body wouldn’t get its requisite share. So instead of glucose, the main fuels for these intestinal cells are the amino acids L-glutamine and L-glutamate.

Let us take a look at different activity levels.

Generally speaking, when your heart rate is lower, and your exercise is less intense, it is likely being fueled more by fat than by carbohydrate. (Another way of saying this is that anaerobic activity is fueled more by carbs, and aerobic more by fat. Note: aerobic is the stuff you can do for a long time at a sufficiently slow pace. Anaerobic is the stuff that makes your muscles hurt after just a short time, assuming you’re doing it right).

chihuahuaThat means, the intense stuff require a lot of glycolysis and/or glycogen breakdown – i.e., a lot of glucose. Intense activity—CrossFit, intense lifting, sprinting, running from a chasing chihuahua, for example—that is fueled more by carbohydrate than by fat. This isn’t a binary all-or-nothing deal, just a balancing act where, in this case, the balance leans toward carbs.

43139373-couch-potato-cartoonWhat about the non-intense stuff? What about a nice, comfortable stroll through the park? What about all the stuff I’ve said before we don’t think of as “exercise” or “burning calories,” but which does use energy? Think about it: pretty much anything we do requires at least some energy, even just sitting in your chair reading this blog. (Think about all the postural muscles in your back and neck working hard just to keep you upright the entire time you’re there on your rear end.) All that kind of sitting-around-doing-nothing type activity is mostly fueled by fat ( Yes Guys, tell your wife to read this blog!! ). Again, this isn’t a binary all-or-nothing deal, just a balancing act where, in this case, the balance leans toward fats.

According to the smarties“Fatty acids are the main source of energy in skeletal muscle during rest and mild-intensity exercise. As exercise intensity increases, glucose oxidation surpasses fatty acid oxidation.”

The body is a hybrid engine that can and does run on a variety of fuels concurrently.

We have now seen that the type of cells/tissue performing the activity and the kind of activity being performed largely determine which types of fuel our bodies prefer to use.

Why is it that in spite of the abundance of fat deposits we are not able to utilize it for our energy needs and keep this source of fuel locked away.

To unlock this mystery, we need to look at another key factor which determines the type of fuel our bodies use and that is the interplay of our hormones. 

Next up in the ‘energy’ series, we will start exploring the hormonal milieu under which different activities take place which in turn determine the type of fuel our bodies use.




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