Macronutrients 101: Metabolism Overview

Now that we have taken a look at the three key macronutrients- carbohydrates, lipids, and proteins, it is important to know when our body utilizes these fuels. In subsequent posts I will go into more detail with regards to different metabolic pathways but for now I thought that giving you a brief overview would be useful.

First off, how does our body store energy? There is a molecule called adenosine triphosphate (ATP) which is the energy currency within a cell. Literally, you can think of it as money. In the process of breaking dowm macronutrients into energy, a phosphate group gets attached to adenosine diphosphate (ADP) to convert it into ATP. Kind of like you depositing money in the bank from doing your job. Furthermore, when energy is required ATP will let go of one phosphate group to become ADP and cause a reaction to occur. This is similar to you giving  money to someone for doing a service for you. Remember that all macronutrients can get converted into ATP, the trick is how fast this process will occur and oxygen availability. Additionally, fat metabolism requires adequate oxygen supply while carbohydrates could be burned without oxygen.

ATP

When at rest or undergoing very light physical activity, our body’s energy demands are low. Therefore, there’s plenty of time to generate ATP from macronutrients. The most efficient way to do so would be by burning fat as it yields more ATP per gram and oxygen is readily available. At the same time, the body can conserve its glucose stores in case it needs a quick burst of energy for a “fight or flight” response.

Once the body’s energy demand go up like during jogging or aerobic exercise, burning fat alone is simply not fast enough. There is still enough oxygen to continue fat metabolism but now glucose must also be metabolized to meet the increased ATP requirements. Hence, our bodies will begin to break down our body’s glycogen stores into glucose. Unfortunately, our glycogen stores are limited and can sustain aerobic activity for a limited amount of time. This effect can be observed with marathon runners as they “hit the wall” if they run of glycogen.

With heavy exercise or sprinting, our oxygen levels drop. Without oxygen, we cannot metabolize fat and must solely rely on burning glucose. Even so, our bodies will burn this glucose in an inefficient anaerobic pathways that results in lactose production. As a result we feel cramps in our muscles and can only sustain this intense exercise for a short duration.

It is important to note that the body doesn’t really use protein as fuel. Doing so is inefficient, slow and yields toxic by-products like urea that must be removed from the body. The only time our bodies will burn protein is during times of extreme starvation. 

Do these rules apply to all organs? Well technically what I’ve just described is particular to muscles. Our nervous system can only burn glucose or ketone bodies (by product of fat produced during starvation). Alternatively, our adipose tissue will preferentially burn fat. All in all, muscles, brain and adipose tissue will make up most of our required daily calories.

So what’s the deal with the macronutrient ratio? Although the research is still controversial, I would assume that it would depend on your activity level. If you lead a sedentary lifestyle, then stay clear of carbs as they will get stored as fat. However, if you lead a healthy active lifestyle then don’t shy away from carbs 😉 Just make sure they’re the good unrefined kind!

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Macronutrients 101: Protein Structure

Now that I’ve discussed what proteins do, let’s ask the million dollar question! Where in the food chain do proteins come from?

If you think they come from animals than you’re absolutely wrong. Those animals actually got their proteins from plants. Yes plants

Proteins unlike other macronutrients contain nitrogen atoms. The issue is that nitrogen is naturally found as a dimer with an extremely stable triple bond. Neither plants nor animals are able to break this bond. However, nitrogen-fixing soil bacteria possess the specific enzymes needed to break it down. These commensal bacteria reside at the roots of legume plants where they exchange the accessible nitrogen for glucose. The plants than utilize the ammonia to form amino acids and in turn proteins.

So what are these amino acids? Well, amino acids are monomeric units that make up proteins. There’re 20 different amino acids found in human proteins and they all have the same generic structure. There is a central (alpha) carbon atom that has an amine group (base), a carboxyl group (acid), and a variable side chain group.

The amine and carboxyl groups of two separate amino acids can undergo an acid-base reaction to form a peptide bond. Through this simple reaction, amino acids can form long chains called polypeptides. Now if you think about carbohydrates, large dietary polysaccharides like starch are only made up of glucose. However, having 20 different amino acids allows polypeptides to be constructed in multitude of diverse patterns.

The polypeptide sequence is dictated by our genes. One gene is a DNA sequence that will code for an amino acid sequence of a polypeptide. Within the polypeptide, amino acids will interact with each other to give it some 3D “secondary” structure. The two most common structures are alpha-helices or beta-sheets. These form through interactions within the invariable regions of the amino acid.

The variable groups of amino acids will then interact with each other to give the protein additional “tertiary” structural features. For example, two cysteine amino acids can form a disulfide bridge between its two sulfur atoms. Hence, this feature will create a fold within the polypeptide. All of these structural features make the proteins unique and determine their physiological function.

Lastly, a protein can be made up of multiple polypeptide chains and can include prosthetic (non-amino acid) groups. A common example of such protein is hemoglobin in red blood cells. This protein is made up of four polypeptide chains and heme “iron” containing prosthetic groups. Below is a ribbon diagram of what hemoglobin looks like based on x-ray crystallography predictions.

Beautiful ain’t it? These gorgeous proteins can be produced by our bodies, all from tiny little amino acids. Proteins are the essence of our bodies and control most of our physiological functions. In fact, I’ve taken an undergraduate course that was purely on proteins. There’s so much to learn about them! One thing is for certain is that we really don’t need much dietary protein, especially animal protein. 😉

Macronutrients 101: Protein Function

Throughout times we have gone from low-fat to low-carb to now high-protein fad diets. You hear people saying “I need x grams of protein pre day” or see guys at the gym chugging protein shakes. It seems like the world revolves around “getting enough protein” and “having protein at every meal”. However, what exactly is protein and why do our bodies need it?

Proteins are the final products of our genes. Our DNA is nothing but a series of instructions for our bodies to manufacture proteins. The DNA sequence tells the order in which monomer amino acids are linked up to form a protein. This unique sequence of monomers will determine the overall shape of the protein and in turn its function.

There is a plethora of various protein functions but I though I’d go over some of the most common ones.

1) Structure

Long thin proteins like collagen, elastin, and keratin make up much of our connective tissues such bones, cartilage, and skin.

2) Transport

Proteins can be involved in systemic and cellular transport. Albumin plays a role in systemic transport as a carrier of water insoluble molecules such hormones in a water rich environment like blood. Similarly, hemoglobin is a systemic carrier of oxygen in the body. In terms of cellular transport, some protein are able to embed within the fatty cellular membranes. These proteins can then act as channels to allow certain molecules to pass through them into or out of the cell.

3) Molecular reactions

Enzymes are a type of proteins that allow for specific reactions to occur by reducing the reaction’s threshold energy. You can think of it as enzymes as something that will give molecules a “push over the fence” without which they wouldn’t be able to cross over.

4) Signal transduction

Proteins are involved in signalling pathways both as the signals and the receptors that receive these signals. These signalling pathways are vital for our bodily functions and range from embryonic development to metabolism.

5) Defense

An antibody or immunoglobin is a protein created by out white blood cells.

6) Movement

Of course, who could forget that proteins are important for our muscles.

As you can tell, the functions of proteins are highly diverse. Yet, if proteins are naturally made by our bodies then why do we need to consume them? Well it’s not necessarily the protein that we need but its building blocks, the amino acids. Once our own proteins serve their purpose our body breaks them back down to amino acids. Our kidneys are not perfect and a little bit of amino acids end up in urine. To replenish these amino acids we must get them from dietary protein sources.  All of the excess amino acids will not magically become muscles but will instead be burned for energy. Hence, don’t be fooled by all that whey protein garbage! To build muscle you need excess calories NOT bucket-loads of protein.

There we have it, the function of proteins. Technically, proteins aren’t an efficient energy source and you’re better off burning carbs or fats. In fact, protein metabolism produces a toxic byproduct called uric acid which is hard on our kidneys. However, I’ll save that for a future post 😉

Macronutrients 101: Cholesterol

In my previous posts on lipids, I have mentioned cholesterol. I thought it’d be important to dedicate a whole post to cholesterol because a lot of people don’t have a solid understanding of what it is. I hear people talking about cholesterol and LDL, using the two terms interchangeably, or automatically assuming that cholesterol clogs arteries.  Well let’s dispel some of these myths 😉

Cholesterol is a macronutrient required for metabolism and cell structure in animals but not plants. Hence, only animal food products contain cholesterol. The four ring steroid structure of cholesterol makes it insoluble in water and is the reason it’s considered to be a fat. However, it is structurally and functionally different from triglycerides.

In our society, cholesterol has a horrible reputation as people fail to understand that it is absolutely essential for our bodies. In fact, our cells are capable of making cholesterol out of acetyl-CoA, a component of glucose metabolism. This same building block is involved in fatty acid synthesis but the two pathways involve two different sets of enzymes. Now before anyone gets excited, cholesterol synthesis is a tightly regulated process and glucose doesn’t magically turn into unnecessary cholesterol.

Well then, what does cholesterol do?

As I’ve previously explained, the cell membrane is made up of phospholipids. In order to make phospholipid bilayers more fluid, there are cholesterol molecules embedded within it. This allows for various protein channels to be found in the membrane which can transport other proteins or molecules in or out of the cell.

Not only is cholesterol found in cell membranes but it also acts as the starting material for manufacturing various hormones. These hormones include sex hormones (estrogen, progesterone, testosterone), cortisol, and aldosterone. Note how all of these hormones have the same four ring steroid backbone.

Furthermore, cholesterol is a precursor for vitamin D and is used by the liver to make bile acids. These bile acids are crucial for helping us digest fats.

As with almost anything health related, balance is key. Although cholesterol is vital for our bodies, too much of it in the bloodstream has profound health consequences. Hence, our bodies have devised sophisticated strategies to keep plasma cholesterol levels intact. These mechanisms include:

* Synthesizing new cholesterol

* Reducing cholesterol absorption in the intestines

* Pumping cholesterol into or out of a cell

* Transporting cholesterol to the intestines for excretion

Now you may wondering, what LDL and HDL are? Low-density and high-density lipoproteins are carriers of cholesterol. Remember that cholesterol is insoluble in water and since our blood is water based, cholesterol needs a transportation shuttle. LDL is what transports cholesterol to cells that require it. Alternatively, HDL is where cholesterol is packaged for excretion. In this way you can think of LDL as “bad” since it raises blood cholesterol levels while HDL is “good” because it remove cholesterol from the body.

ldl vs hdl

Does cholesterol cause heart disease? Yes, in the bloodstream LDL oxidation triggers an immune response that leads to plaque formation.

Does dietary cholesterol cause heart disease? I wish I had a concrete answer… The thing is that dietary cholesterol absorption is complex and there’re high interpersonal variations in percentage of dietary cholesterol absorbed. By no means would I recommend someone to eat foods high in cholesterol. Personally, I think more research needs to be done and this topic deserves a follow up in-depth post of its own 😉