Sugar And Carbohydrate Metabolism Made Easy To Understand

Sugar And Carbohydrate Metabolism Made Easy To Understand


Let’s take a few minutes to discuss the basics of how your body metabolizes carbohydrates in general, and specifically sugar. In order to understand why certain dietary choices are good or bad for you, it’s important to understand the basics and how it affects your body. This post is designed for people with little-to-no science background to help simplify how carb metabolism works in our body. I’m going to intentionally glaze over some of the more detail oriented aspects and focus on the larger picture.

The purpose of food, including sugars, fats and proteins is to sustain the body. One of the major components of this sustenance is to maintain a basal blood sugar level that allows your body to function. All your vital organs – including your brain, heart, kidneys etc as well as your skeletal muscle – depend on glucose as it’s primary source of energy. This energy, which occurs at the cellular level drives everything in your body from head to toe.

What are sugars and carbohydrates?

When someone talks about ‘macro’ in dietary terms, they mean protein, fat and carbohydrates. Carbohydrates are one of the macronutrients essential to life. You’ll hear people talk about “glucose”, “simple” and “complex” sugars and these are terms used to describe different types of carbs.

The basic unit of sugar in simple terms is called glucose. This glucose can come as one molecule, as two molecules linked together or as a chain. Monosaccharides, or 1 unit of sugar, include glucose, galactose, fructose and ribose among others. Disaccharides, or a 2 unit chain, includes sucrose and lactose. Common table sugar generally contains sucrose, or a mix of glucose and fructose. When three or more glucose molecules are linked together, this is generally what is considered a ‘complex’ carb.

Are Simple Sugars Bad?

Now these simple sugars are generally considered worse for you. The reason that simple sugars are worse for you than complex sugars is because they are more readily absorbed. It’s easier for the GI tract to absorb simple sugars, so they can drive up your blood sugar more quickly. And your GI tract doesn’t communicate well with your liver about this. It doesn’t shut off absorption very well. So the more sugar you eat, the more you will absorb. And we’ll talk about where that extra sugar goes in a minute.

However, it is worth noting that it may not be that simple. There is some evidence to suggest that complex carbohydrates may raise your blood sugar as much as ingesting pure glucose. This lead to the development the glycemic index, and I’ll talk more about that at the end as well.

How are carbs/sugars metabolized?

The basic premise is that glucose is ingested into your stomach and absorbed in your small intestine. This glucose travels into your blood to your liver, where there are receptors that detect your newly ingested sugar. Your liver’s job is to decide if you have enough sugar circulating in your blood, if you have too much or if you need to produce more. Along with your pancreas, it is constantly monitoring how much sugar is available.

Every cell in every organ if your body uses this sugar to produce something called adenosine triphosphate or ATP, which is the energy currency at the cellular level. All metabolic activity in the body is ATP dependent. The process by which glucose is converted to ATP occurs through the TCA cycle (aka kreb cycle or citric acid cycle) in conjunction with the electron transport chain. If you’re interested, I would encourage you to look those up; they are beyond the scope of this post but terms worth learning.

Usually after eating, your blood sugar will go up. The more carbohydrates there are in your meal, the quicker your blood sugar will go up and the higher it will go. For example, 50g of sugar will drive your blood sugar up faster than 25g of sugar. Another way to think of it is that a couple candy bars will drive your blood sugar up much faster than a handful of carrots because there is more net carbohydrates in the candy bars. Also, the more your diet is composed of carbohydrates, as opposed to fats and protein, the more your blood sugar will go up and the higher your average blood sugar will be.

Without diving too deeply, I will briefly mention glucagon and insulin. These two hormones, produced by the pancreas, are the body’s primary metabolic referee’s of glucose regulation and they act as opposing forces. When blood sugar goes down, more glucagon is released telling your body to increase available blood sugar however possible. When blood sugar goes up, more insulin is released telling your body to utilize or store blood sugar. Your body is always balancing them to try to maintain a normal blood sugar.

What if you increase your carb intake and/or eat too many carbs?

The liver and other organs of your body can become satisfied with the amount of glucose available.  However, your body will continue absorbing sugar if it’s in your gut. There aren’t a lot of mechanisms that turn absorption off. The organs that manage blood glucose – namely the liver and pancreas – begin releasing the hormone insulin and other enzymes that promote glucose storage.  And so where does your body store sugar?

A small amount of that sugar gets deposited into muscle and put into chains called glycogen. This glycogen in skeletal muscle is a reserve for when your muscles need it. When you start working out, this is the backup system to ensure that your muscle has enough energy. This is what we’ll call the short-term storage.

Unfortunately, your muscle only needs so much short-term storage and the rest of that extra sugar gets put into long-term storage. Long-term storage in the human body means fat or adipose tissue. That means that when your body has too much sugar circulating in it’s blood, it deposits this as fat in long-term storage.  This process is called lipogenesis. It involves a converting a metabolic product called acetyl-CoA into fatty acid chains. This is why if you eat sugary foods in excess over long periods of time, you will probably gain weight as your body stores it.

What if you decrease your carb intake?

So, interestingly enough, if you decrease your carb or sugar intake, your body will have to find ways to keep your blood sugar up so that there is energy available for your body. This happens in a number of ways, but we’ll talk about how it specifically happens as far as fat goes. If you use excess sugar and put it into chains and store it as fat. Conversely, that fat can be broken down and  turned back into sugar. (Technically, the fatty acid breakdown creates acetyl-Coa which is pumped directly into the TCA cycle to produce the ATP I mentioned earlier. For the purpose of this video, the distinction is not important).

Thus, if you are at a carb deficit for the day or for your workout, your body has started to recruit fat cells (and protein sources)  to bring your blood sugar back up. This process is regulated by the hormone glucagon, which is essentially the opposite of insulin. It’s also true that the bigger your carb deficit, the more fat you will burn. It’s worth noting that glucagon also facilitates the breakdown of protein, especially skeletal muscle, to help make more glucose as well.

During the first 24 hours of low carb or no carbohydrate intake, your body is primarily breaking down glycogen and protein for energy, with lesser recruitment of fat. After 24 hours or so, more fat cells are recruited for breakdown and this can become a more significant energy source. Breakdown of fats for energy, when compared to glucose, is not very efficient. This process is called ketogenesis and that is where the term ‘keto diet’ comes from.

The Glycemic Index (GI)

Glycemic Index Explained:

This index compares different carbohydrates and evaluates them based on how much they increase blood glucose after ingestion. The higher the glycemic index, the more it increases your blood sugar. Thus, the lower the glycemic index the better. There are three categories, high (>70), medium (56-69) and low (<55). The usefulness of this index is it allows easy comparison of different types of carbohydrates, including simple and complex. However, it’s usefulness in the real world is met with some skepticism. This mainly stems from the lack of long-term data about it’s clinical utility and use.

You may also hear about the glycemic load. The glycemic load adjusts for both quantity and quality whereas the glycemic index only adjusts for quality. While the usefulness to physicians and the population at large remains debated, you can still use it to guide your food choices when the information is available. The rule is simple, the lower the glycemic index or glycemic load, the better.


The general advice here is to closely moderate your dietary intake of carbohydrates, and especially try to avoid refined carbohydrates. To determine your carbohydrate needs, you can calculate your total daily energy expenditure. This will give you an estimate of how many calories you should eat per day based on your height, weight, age, exercise level and goals. The 2010 American Dietary Guidelines recommends an adult diet consist of 45-65% carbohydrates, 10-35% protein and 20-35% fat.

Total Daily Energy Expenditure Explained:

My personal recommendation for macronutrient makeup is don’t let your carbs be any more than 40% of your total daily caloric intake; protein and fats will make up the other 60% to 80%. If you’re trying to lose weight, you can decrease your daily carb intake to 20%.

Finally, I want to reinforce the notion that the number one determinant of weight loss is that calories burned must exceed calories consumed. This is more important for weight loss or weight maintenance than the macronutrient breakdown of your diet.  

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