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.


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!

2 thoughts on “Macronutrients 101: Metabolism Overview

  1. Great post Nadiya. How does the body prioritize where energy goes? Because obviously the brain and heart get first priority, but what about everything else?

    • Thanx Paul 🙂 That is a very interesting question! From what I know in terms of major organs, the brain consumes 20%, liver about 25%, kidney 10%, heart 10%, and muscle about 20% of our basal metabolic rate calories. Those are approximate values…

      During starvation less energy will be consumed by muscle and they will in fact get broken down into proteins to be used as energy. The basal metabolic rate in general will decrease for sure. However, I haven’t read anything about proportions of energy going to each organ changing (other than muscle).

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