Nutrition Basics: Really Understanding Carbohydrates


Ah… carbohydrates.  Probably the most controversial and possibly the most misunderstood of the macronutrients.  So let’s see if we can get to the bottom of this vilified nutrient…

What are carbohydrates?

Carbohydrates are organic compounds (ie, they contain carbon) that are soluble in water (think of how sugar dissolves in water, but oil does not) that do not contain nitrogen (which protein does).

Based on their structure, carbohydrates are classified as follows:

  • Simple carbohydrates:
    • Monosaccharides (aka, simple sugars): this is the simplest form in that they are made of one molecule.  There are only three different ones and they are glucose, fructose (sometimes called fruit sugar) and galactose (sometimes called milk sugar).
    • Disaccharides: these are made of two monosaccharides hooked together.  The three main disaccharides are:
      • Lactose: made of glucose and galactose
      • Sucrose: made of glucose and fructose
      • Maltose: made of 2 glucose molecules
  • Complex carbohydrates:
    • Oligosaccharides: carbohydrates made of 3 – 10 monosaccharide units.  These have become better known recently as people have started talking about FODMAPs (which stands for Fermentable Oligo-, Di-, Monosaccharides and Polyols), which some people may have trouble digesting.
    • Polysaccharides: carbohydrates made of more than 10 monosaccharides (usually glucose).  The two categories in food are:
      • Starch: polysaccharides that humans can digest
      • Fiber: polysaccharides that humans cannot digest, but some animals and bacteria can

The reason for giving you all this information is because I want you to understand something important: all carbohydrates are broken down to glucose, fructose and galactose.  Period.

What impact do carbohydrates have in the body?

When we consume carbohydrates, our body works to break down (ie, digest) them into smaller and smaller pieces until finally, in the small intestine, they are monosaccharides that can be absorbed into the body.

If a person is missing the enzymes required to break the bonds that hold the monosaccharide molecules together, the carb will pass through the digestive system whole because only monosaccharides can be absorbed into the body.  This is what happens to people with lactose intolerance… they do not have the enzyme that breaks lactose into glucose and galactose.  This is also what happens with fiber… humans don’t have the enzymes necessary to break the bonds in the polysaccharide.  As such, fiber passes through our digestive system until it gets to our colon where our good bacteria do the happy dance and thank us for feeding them. 🙂

Once absorbed, glucose, fructose and galactose travel directly to the liver.  Here, fructose and galactose are both converted into glucose.  Glucose is then:

  • Kept in the liver to be:
    • Used as energy,
    • Stored as glycogen for future energy needs,
    • Converted into fat (triglycerides) and released into the blood stream (this is why high sugar diets can be linked to high blood triglycerides), or
    • Converted to NADPH where it is involved with detoxification of drugs, alcohol, etc;
  • Released into the blood stream where it can be used by other tissues such as:
    • Muscles where it can be used for energy or stored as glycogen for future energy needs,
    • The kidneys and the brain where it is used for energy,
    • Fat tissue… as glucose where it is used in the process of creating fatty acids or as the triglycerides for storage, and
    • Other cells where it is necessary for the creation of RNA and DNA.

So, in a nut shell… the body primarily uses glucose for energy or stores it to use as energy later or, secondarily, converts it to be used in the creation of genetic material or to help with detoxification.

And just to be clear, unlike fat and protein which are the building blocks of the human body, carbohydrates are not.

How do carbohydrates impact insulin?

The topic of blood sugar control is bigger than this post, so check out my series on understanding blood sugar control.

What foods contain carbohydrates?

Alright, I’m about to get on my soap box, so y’all just get ready…

It drives me c-r-a-z-y when people say they “don’t eat carbs” because they try not to eat “starchy” carbs that come from grains (bread, pasta, cereal, rice, oatmeal, etc.) and potatoes or whatever else they have decided to put in this category.  So let me be sure that you do not say such ridiculous things by telling you that…


Please, please, please share this post with any- and everyone who has ever told you that they don’t eat carbs, but still eat fruits, veggies or legumes.  Please.  You can use the picture at the top of this other post as an example. Okay, now that it’s off my chest, I will step off of my soapbox and we can move on…

Most plant foods are high in carbohydrate.  This is because, unlike for humans, carbohydrate is the primary component of cellular structure for plants.  The exception here is with nuts and seeds, which are plant-based foods that tend to have a higher concentration of fats and proteins than carbohydrates.  That’s not to say there is no carbohydrate in them, because there certainly is; it’s just not the dominant macronutrient as it is in fruit, veg, legumes, and grains.  (For example, almonds are about 77% fat, 15% protein, and 8% carbohydrate.)

Dairy products also contain carbohydrate, with the amount depending on whether or not the food has been fermented or aged.  (For more information on milk and dairy products, including how they impact the body, check out this post.)

And perhaps it goes without saying, but just so that everyone is aware, all sweeteners are simple carbohydrates and, therefore, a form of sugar… table sugar, brown sugar, honey, molasses, agave, cane juice and maple syrup.  (I won’t get into artificial sweeteners or those that are produced in a lab like high fructose corn syrup here.  They are fake and you should just avoid them.)

Are there good carbs and bad carbs?

The short answer is yes, but perhaps not for the reasons you think.

Too much glucose in our blood stream at one time is really bad for us.  This primarily happens when we eat too many easily digestible carbohydrates (like monosaccharides, disaccharides and overly processed polysaccharides – like white bread), especially when eaten separately from other foods that can slow digestion and absorption (which is part of what fat, protein and fiber do for us).

Beyond all of the bad things that happen, it seems that the body doesn’t actually register the calories in carbohydrates when they are taken in without fat or protein.  (Think about how you can drink a giant soda, but it does nothing for your hunger… ie, need for calories.  I’ve actually experienced this when I have cereal with skim milk for breakfast.  I need like 3 bowls to feel satisfied!)  And there is plenty of evidence that surges of blood sugar is addicting, which means we want to make it happen more often and the easiest way to do this is with carbohydrates that are easy to digest and absorb.

I’m not trying to vilify simple carbohydrates, as I know exactly how yummy dessert can be!  But I do want you to understand the impact… eating foods which are packed with simple carbs (like cookies and cakes) and have very little fiber, fat or protein to slow the digestion and absorption is hard on the body.

As I mentioned in my post about eating veggies, I personally do not think that limiting fruit or vegetables (even carrots) is necessary for most people.  The natural carbohydrates come packaged with enough water and fruit that it doesn’t give the same blood sugar spike as desserts or white bread.

So, clear as mud?  Probably not.  So to summarize, it’s not that carbohydrates are structurally all that different and, therefore, are not inherently good or bad.  The problem is that which ones we choose to eat and how we eat them can have a huge negative impact on the body.

What happens if we eat more carbohydrates than we can use?

As mentioned above in the section about the impact of carbohydrates in the body, if we eat more carbohydrates than we need to use for energy, genetic material or NADPH, most of the excess will get stored to be used as energy later… either in the form of glycogen or as fat.  Some of it, however, will bind with proteins and lipids in the blood and cell walls which leads to problems.

Do we actually need carbohydrates?

In short, yes we do, but  maybe not as many as we’ve been told.  As I mentioned in the post about fat, the entire body (including the brain) can use fat for energy.  And since energy production is the primary use of carbohydrates in the body, clearly that is not a reason to consume them because we can get energy from fat.

However, glucose is a necessary component in the creation of RNA and DNA, it allows the process of fatty acid creation to proceed, and it’s involved in the liver’s detoxification pathways.  Those are critical parts of a normal functioning body for which there appears to be no substitute.

As well, many vitamins and minerals are found only in plant-based foods.  So giving up all carbs would mean that we would eliminate really important micronutrients from our diets.

So how many you need, are comfortable eating and in what form you eat them will ultimately be up to you and how carbohydrates make you look and feel!


— Gropper SS, Smith JL. Advanced Nutrition and Human Metabolism. 6th ed. Belmont, CA: Wadsworth Cengage Learning; 2013.
— Hall J. Guyton and Hall Textbook of Medical Physiology. 12th ed. Philadelphia, PA: Saunders Elsevier; 2011.
— Harvard T. C. Chan School of Public Health. Added sugar in the diet. In Nutrition Source.  Accessed on March 24, 2015.
— Lewis JG. Here’s what happens to your brain when you give up sugar for Lent. In Health + Medicine. Accessed on March 24, 2015.
— SELF Nutrition Data. Nuts, almonds [Includes USDA commodity food A256, A264]. In Nutrition Facts. Accessed on March 25, 2015.

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